On the determination of reference levels for quality assurance of flattening filter free photon beams in radiation therapy Alessandro Clivio, Maria Francesca Belosi, Luca Cozzi, Giorgia Nicolini, Eugenio Vanetti, Grégory Bolard, Pascal Fenoglietto, Harald Krauss, and Antonella Fogliata Citation: Medical Physics 41, 021713 (2014); doi: 10.1118/1.4861817 View online: http://dx.doi.org/10.1118/1.4861817 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/41/2?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Definition of parameters for quality assurance of flattening filter free (FFF) photon beams in radiation therapy Med. Phys. 39, 6455 (2012); 10.1118/1.4754799 Comparison of action levels for patient-specific quality assurance of intensity modulated radiation therapy and volumetric modulated arc therapy treatments Med. Phys. 39, 4378 (2012); 10.1118/1.4729738 Validation of a virtual source model for Monte Carlo dose calculations of a flattening filter free linac Med. Phys. 39, 3262 (2012); 10.1118/1.4709601 An EPID based method for efficient and precise asymmetric jaw alignment quality assurance Med. Phys. 36, 5488 (2009); 10.1118/1.3253463 Impact of multileaf collimator leaf positioning accuracy on intensity modulation radiation therapy quality assurance ion chamber measurements Med. Phys. 32, 1440 (2005); 10.1118/1.1901843
On the determination of reference levels for quality assurance of flattening filter free photon beams in radiation therapy Alessandro Clivio, Maria Francesca Belosi, Luca Cozzi, Giorgia Nicolini,a) and Eugenio Vanetti Oncology Institute of Southern Switzerland, Medical Physics Unit, Bellinzona CH-6500, Switzerland
Grégory Bolard Clinique de Genolier, Radiation Oncology department, Genolier CH-1272, Switzerland
Pascal Fenoglietto Département de Cancérologie Radiothérapie, CRLC Val d’Aurelle-Paul Lamarque, Montpellier F-34090, France
Harald Krauss Kaiser-Franz-Josef-Spital, Institut für Radioonkologie, Vienna A-1100, Austria
Antonella Fogliata Oncology Institute of Southern Switzerland, Medical Physics Unit, Bellinzona CH-6500, Switzerland
(Received 24 October 2013; revised 4 December 2013; accepted for publication 23 December 2013; published 15 January 2014) Purpose: New definitions for some dosimetric parameters for use in quality assurance of flattening filter free (FFF) beams generated by medical linear accelerators have been suggested. The present study aims to validate these suggestions and to propose possible reference levels. Methods: The main characteristics of FFF photon beams were described in terms of: field size, penumbra, unflatness, slope, and peak-position parameters. Data were collected for 6 and 10 MVFFF beams from three different Varian TrueBeam Linacs. Measurements were performed with a 2Darray (Starcheck system from PTW-Freiburg) and with the portal dosimetry method GLAaS utilizing the build-in portal imager of TrueBeam. Data were also compared to ion chamber measurements. A cross check validation has been performed on a FFF beam of 6 MV generated by a Varian Clinac-iX upgraded to FFF capability. Results: All the parameters suggested to characterize the FFF beams resulted easily measurable and little variation was observed among different Linacs. Referring to two reference field sizes of 10 × 10 and 20 × 20 cm2 , at SDD = 100 cm and d = dmax, from the portal dosimetry data, the following results (averaging X and Y profiles) were obtained. Field size: 9.95 ± 0.02 and 19.98 ± 0.03 cm for 6 MV-FFF (9.94 ± 0.02 and 19.98 ± 0.03 cm for 10 MV-FFF). Penumbra: 2.7 ± 0.3 and 2.9 ± 0.3 mm for 6 MV-FFF (3.1 ± 0.2 and 3.3 ± 0.3 for 10 MV-FFF). Unflatness: 1.11 ± 0.01 and 1.25 ± 0.01 for 6 MV-FFF (1.21 ± 0.01 and 1.50 ± 0.01 for 10 MVFFF). Slope: 0.320 ± 0.020%/mm and 0.43 ± 0.015%/mm for 6 MV-FFF (0.657 ± 0.023%/mm and 0.795 ± 0.017%/mm for 10 MV-FFF). Peak Position −0.2 ± 0.2 and −0.4 ± 0.2 mm for 6 MV-FFF (−0.3 ± 0.2 and 0.7 ± 0.3 mm for 10 MV-FFF). Results would depend upon measurement depth. With thresholds set to at least 95% confidence level from the measured data and to account for possible variations between detectors and methods and experimental settings, a tolerance set of: 1 mm for field size and penumbra, 0.04 for unflatness, 0.1%/mm for slope, and 1 mm for peak position could be proposed from our data. Conclusions: The parameters proposed for the characterization and routine control of stability of profiles of FFF beams appear to be a viable solution with a strong similarity to the conventional parameters used for flattened beams. The results from three different TrueBeams and the crossvalidation against a Clinac-iX suggested the possible generalization of the methods and the possibility to use common tolerances for the parameters. The data showed also the reproducibility of beam characteristics among different systems (of the same vendor) and the resulting parameter values could therefore be possibly generalized. © 2014 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4861817] Key words: flattening filter free beams, TrueBeam, GLAaS algorithm, quality assurance 1. INTRODUCTION The clinical introduction of general purpose linear accelerators equipped with photon beams generated without a flat021713-1
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tening filter, the ‘‘flattening filter free’’ (FFF) beams, is consolidating and several studies reported about their application to consistent groups of patients.1–3 Physical and dosimetric characteristics of FFF beams have been extensively
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© 2014 Am. Assoc. Phys. Med.
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investigated4–11 and data from commissioning of FFF-capable linear accelerators are available.8, 12–14 Nevertheless, standardized and consolidated definitions about beam parameters to be used for routine quality assurance (QA) are not yet available from international consensus. This is particularly relevant for quantities such as field size, penumbra, or flatness, all related to the properties of the transverse profiles of the beams. In order to establish an operational set of definitions that could be used by any center, irrespective of the beam energy and manufacturer, a study was recently published15 where literature was investigated and new definitions were proposed. The aim of that study was to determine simple procedures to normalize FFF profiles in a simple and robust way allowing to use, as much as possible, equal or similar definitions as for conventional flattened beams. Second, two additional definitions were proposed to characterize the specificity of FFF beams: the slope and the unflatness of the profiles. A last definition was proposed to identify the position of the peak of the profile which is another signature of the FFF beams and that, if not positioned at the center of the geometrical beam axis (CAX) might have severe dosimetric implications on the irradiation of patients. The present report aims to provide some initial experimental evidence to (i) the measurability of the definitions proposed by Fogliata;15 (ii) the stability over a medium time of the FFF beams in terms of the five parameters on one machine and the reproducibility over a short time for other machines; and (iii) the coherence of results when the definitions are tested and applied to different machines in different centers and measured with different equipments. As a second aim of the study, a set of possible tolerances have been computed from the 95% confidence interval of all the aggregated measurements available for analysis. These tolerances have been defined out of the initial experience of a cooperative group of users of the FFF beams for a specific single vendor Linac. This implies that obvious limitations exist and that the proposed values might be conditionally applied also by other clinics in their routine quality assurance procedures to determine if the concepts can have a general applicability. These
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potential thresholds could be further relaxed or tightened if future data would prove the necessity and, more important, some broader consensus validation and or recommendations from national and international bodies will be necessary. 2. METHODS The parameters identified as useful in the characterization of unflattened beam profiles and different from the conventional flatness, profiles and penumbra were introduced and defined by Fogliata.15 The definitions are summarized here while details can be found in the original study. Profile normalization: The use of a “shoulder” point as “normalization” point of a FFF profile with respect to a corresponding flattened beam was proposed. To establish an operational procedure, the pre-tabulated normalization values were applied as from Table I of the original publication. Dosimetric field size: The distance between the 50% dose levels in the normalized FFF profile. Penumbra: The distance between the 20% and the 80% dose levels in the field edge of the normalized FFF profiles. Unflatness: The ratio between the dose at the central axis and the dose at a fixed off-axis point: Unflatness = Dosecentral.axis / DoseX.off-axis ; the off-axis point was chosen to be located at the 80% of the field size (FS) for FS ≥ 10 cm and 60% for FS < 10 cm. Slope: As a descriptor of the profile gradient, Slope = (y1 − y2 ) / (x1 − x2 ) where (x1 , y1 ) and (x2 , y2 ) are the coordinates, as dose value y at position x, on each half-profile at 2/3 and 1/3 of the off-axis positions. Equivalence of the left and right slope is a further verification of the symmetry of the beams. Since the shape of the FFF profile depends on the energy, the constancy of the slope is also an indirect measurement of the constancy of the beam energy. For small FS, FFF and flattened beams present quite the same shape, and conventional parameters could be used.8, 12–14 Peak position: The presence of a pronounced peak in the central axis (CAX) is the signature of FFF beams. Deviations from CAX would mean a mistuning of the beam steering
TABLE I. Analysis of profiles measured with the Starcheck system in the beam X and Y directions. Results are presented as averages between left and right field side of the profiles with respect to central beam axis. Data were measured at SDD = 100 cm. 6 MV FFF
10 MV FFF
10 × 10 cm2
20 × 20 cm2
10 × 10 cm2
20 × 20 cm2
X
Y
X
Y
X
Y
X
Y
Field size (cm)
9.93 ± 0.02 [9.89;10.00]
10.00 ± 0.03 [9.91;10.16]
19.99 ± 0.04 [19.96;20.12]
20.09 ± 0.04 [19.94;20.21]
9.94 ± 0.02 [9.90;10.01]
10.01 ± 0.02 [9.92;10.09]
20.04 ± 0.04 [20.00;20.16]
20.13 ± 0.04 [19.99;20.25]
Penumbra (mm)
4.9 ± 0.1 [4.3;5.1]
4.0 ± 0.3 [3.4;4.5]
5.1 ± 0.2 [4.5;5.5]
4.0 ± 0.2 [3.7;4.7]
5.1 ± 0.1 [4.5;5.3]
4.5 ± 0.1 [4.0;4.8]
5.9 ± 0.2 [5.1;6.2]
5.0 ± 0.1 [4.7;5.4]
1.10 ± 0.01 [1.09;1.12]
1.10 ± 0.01 [1.09;1.11]
1.22 ± 0.01 [1.22;1.23]
1.22 ± 0.01 [1.21;1.23]
1.19 ± 0.02 [1.17;1.22]
1.20 ± 0.02 [1.17;1.22]
1.42 ± 0.01 [1.40;1.44]
1.42 ± 0.01 [1.40;1.44]
Slope (%/mm)
0.295 ± 0.021 [0.262;0.326]
0.285 ± 0.007 [0.262;0.309]
0.378 ± 0.021 [0.343;0.413]
0.371 ± 0.011 [0.341;0.392]
0.588 ± 0.021 [0.550;0.619]
0.588 ± 0.021 [0.550;0.619]
0.686 ± 0.039 [0.600;0.741]
0.685 ± 0.030 [0.620;0.766]
Peak position (mm)
−1.2 ± 0.2 [−1.7;−0.8]
0.5 ± 0.2 [0.0;1.0]
1.7 ± 0.2 [1.0;2.5]
2.2 ± 0.3 [1.1;3.2]
−0.3 ± 0.2 [−0.6;0.1]
−0.3 ± 0.2 [−0.6;0.1]
2.1 ± 0.4 [0.2;2.7]
1.8 ± 0.4 [0.5;3.1]
Unflatness
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parameters and should be promptly identified. The “peak position” parameter is defined as the off-axis position of the intersection point of the left and right slopes: Peak position = (IL − IR ) / (SR − SL ) where IL and IR are the left and right intercepts, respectively; SL and SR are the left and right slopes, respectively. In the present study, we focused only on QA parameters that were either newly proposed for FFF beams (e.g., unflatness) or that required specific manipulation of the measured data prior to their numerical quantification (e.g., penumbra, computed with the usual definition only after the FFF normalization is applied). Other parameters such as symmetry, beam output, beam quality, and energy should be part of any comprehensive quality assurance program of photon beams but were not subject to investigation here since the definitions proposed and used in the reference study did not differ from what used for the conventional flattened beams. Validation of the QA parameters specific for FFF beams was performed on three different TrueBeam linear accelerators (Varian Medical Systems, Palo Alto, USA) installed in the three institutions (A, B, and C), all equipped with 6 and 10 MV unflattened beams. Measurements were performed using two systems available for routine quality controls in the centers. A commercial 2D-array (the StarCheck system from PTWFreiburg, Germany) was used in Institute A only for one year, performing weekly measurements of all parameters. Experiments were performed with source to detector distance SDD = 100 cm and two field sizes of nominal dimension of 10 × 10 and 20 × 20 cm2 . These fields were chosen for consistency with the current QA protocols of the institute and in the agreement with the local national recommendations. The StarCheck system is made by a series of 527 air-filled ion chambers of 0.08 cm3 of volume aligned along the two main axes and the two diagonals with a spatial separation of 3 mm. The chambers are embedded in solid water material at a water-equivalent depth of 5.8 mm. The tests performed over one year with the 2D-array allowed to determine the long term stability of the beams and the reproducibility of the tests and of the parameters proposed. A second group of measurements was performed, in institutes A, B, and C, using the electronic portal dosimetry methods by means of the GLAaS methodology that was adapted to FFF beams and described in Nicolini et al.16 The detector used for the dosimetry is the electronic portal imager made of aSi detectors with a 0.392 mm/pxl spatial resolution, positioned at 100 or 150 cm from the beam source. Different experimental configurations were tested in all the three centers. For SDD = 100 cm, three FS were investigated: 5 × 5, 10 × 10, and 20 × 20 cm2 . For SDD = 150 cm, three FS were tested: 5 × 5, 10 × 10, and 15 × 15 cm2 . All measurements were performed with detector reading calibrated in absorbed dose to water at the depth of maximum dose deposition: d = dmax . All institutes performed 20 repeated measurements for all the experiments in a time frame of 1–2 months. The data collected from these experiments allowed to ascertain the short term reproducibility of the tests and parameters on a group of similar machines (same model). Medical Physics, Vol. 41, No. 2, February 2014
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All measurements in these first two groups were repeated for both energies 6 and 10 MV-FFF. A final set of measurements was performed as a cross validation of the methods on a fourth linear accelerator. This was a Clinac-iX from Institute D updated to flattening filter free mode (released for clinical operation) for the 6 MV beam. These tests were performed using a LA48 linear array detector from PTW-Freiburg. The LA48 is made of 48 liquid ion chambers (with a volume of 4 × 4 × 0.5 mm3 ) with a centreto-centre separation of 8 mm. In the penumbra region, where a high resolution detector is preferable, results refer to a SRS diode. Scope of the cross-validation was the assessment of the reliability of the QA parameters when applied to different types of Linacs (but manufactured from the same vendor). It is a first step towards full generalization of the parameters to any FFF type photon beam. Formally binding tolerance levels to each of the five parameters investigated should be defined within a consensusbased process. Nevertheless, from the data of the study we are presenting values as operational tolerance, assuming that they could be derived from the concept of confidence interval, set at 95% as used in the AAPM TG-119 (Ref. 17) (i.e., mean ± 1.96 standard deviation). 3. RESULTS AND DISCUSSION Figure 1 shows an example of the data acquired and used for the analysis. In this case, a 20 × 20 cm2 field for both energies is reported: in the first panel the display of the 2D beam profile at dmax from the GLAaS portal dosimetry measurements; in the second and third panels the X and Y lateral profiles measured with a 0.125 cm3 ion chamber and the 2Darray at institute A and the GLAaS method at institutes A, B, and C. As a general observation, valid for all fields and experiments performed, there is a very good qualitative (and quantitative) agreement among the three institutes with the GLAaS approach which is also highly consistent with the ion chamber measurements. The noticeably different shapes of the profiles obtained between GLAaS and the 2D-array (both from the same beams at Institute A) can be re-conducted at least to two concurrent potential sources: (i) the different depth of measurement and (ii) the different sensitivity of the detectors in the array mainly due to scattering conditions. To appraise qualitatively and quantitatively the stability over time of the FFF beams with respect to the five parameters suggested for routine quality assurance, Figs. 2 (6 MV-FFF) and 3 (10 MV-FFF) show a single institute (A) experience on the long term measurements. Data are plotted as a function of time (weeks of measurement) for measurements performed with either the 2D-array or with the GLAaS portal dosimetry method. The graphs show, for the FS = 10 × 10 cm2 and SDD = 100 cm, the deviation of each of the parameters from the reference value. The reference was chosen as the first week (25th) when both dosimetry methods were implemented in the routine verification of the FFF beams in the institute. Data are separated for X and Y profiles. For unflatness and slope the results for the left and right parts of the profiles are presented separately as well. Similar qualitative results
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F IG . 1. Display of the 2D profile for 6 and 10 MV-FFF beams for a test field of 20 × 20 cm2 measured with the GLAaS method (a). Profiles in the X (crossplane) direction (b) and in the Y (inplane) direction (c) measured with the GLAaS method in the three centers and with the 2D array, the ion chamber (IC).
were obtained for the other field sizes and are therefore not shown. Penumbra: During the entire year extremely stable results within a fraction of mm were obtained, and only limited number of measurements were out of the ‘‘almost-zero’’ deviation Medical Physics, Vol. 41, No. 2, February 2014
but all within 0.1 mm at 6 MV-FFF and 0.8 mm at 10 MV-FFF as visible in Figs. 2 and 3. Field size: Generally very good and stable results were obtained for this parameter with all measurements (except one case for the 2D-array, where the distance from reference was
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F IG . 2. Deviations from reference of the various QA parameters investigated to characterize the FFF beams: unflatness, slope, peak position, field size, and penumbra. Data are presented for the 6 MV-FFF beam measured at center A with the 2D array and the GLAaS method. Data were measured at SDD = 100 cm. Medical Physics, Vol. 41, No. 2, February 2014
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F IG . 3. Deviations from reference of the various QA parameters investigated to characterize the FFF beams: unflatness, slope, peak position, field size, and penumbra. Data are presented for the 10 MV-FFF beam measured at center A with the 2D array and the GLAaS method. Data were measured at SDD = 100 cm. Medical Physics, Vol. 41, No. 2, February 2014
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TABLE II. Analysis of the profiles measured with the GLAaS method in the beam X and Y directions. Data are presented as the average of the findings from the three centers. Data were measured at SDD = 100 cm. 6 MV FFF
10 MV FFF
10 × 10 cm2
20 × 20 cm2
10 × 10 cm2
20 × 20 cm2
X
Y
X
Y
X
Y
X
Y
Field size (cm)
10.00 ± 0.02 [9.93;10.03]
9.90 ± 0.02 [9.88;9.92]
20.03 ± 0.02 [19.96;20.06]
19.93 ± 0.03 [19.83;19.99]
9.98 ± 0.03 [9.92;10.03]
9.89 ± 0.02 [9.84;9.92]
20.03 ± 0.02 [19.98;20.06]
19.92 ± 0.03 [19.84;19.96]
Penumbra (mm)
2.4 ± 0.3 [1.7;3.0]
3.0 ± 0.2 [2.8;3.7]
2.5 ± 0.3 [1.6;3.0]
3.4 ± 0.4 [2.8;4.1]
2.7 ± 0.2 [2.0;2.9]
3.5 ± 0.2 [3.3;4.1]
2.8 ± 0.2 [2.4;3.4]
3.9 ± 0.4 [3.6;4.7]
1.11 ± 0.01 [1.09;1.12]
1.11 ± 0.01 [1.09;1.13]
1.25 ± 0.01 [1.24;1.27]
1.26 ± 0.01 [1.24;1.28]
1.21 ± 0.01 [1.19;1.23]
1.21 ± 0.01 [1.19;1.23]
1.50 ± 0.01 [1.48;1.52]
1.50 ± 0.01 [1.48;1.52]
Slope (%/mm)
0.313 ± 0.011 [0.294;0.329]
0.327 ± 0.032 [0.286;0.369]
0.430 ± 0.011 [0.417;0.448]
0.430 ± 0.019 [0.400;0.463]
0.652 ± 0.020 [0.610;0.690]
0.663 ± 0.027 [0.605;0.716]
0.798 ± 0.014 [0.772;.0.822]
0.792 ± 0.021 [0.758;0.830]
Peak position (mm)
−0.3 ± 0.2 [−0.7; −0.1]
−0.1 ± 0.2 [−0.5;0.5]
−0.9 ± 0.1 [−1.2; −0.7]
0.0 ± 0.2 [−0.4;0.4]
−0.6 ± 0.1 [−0.8; −0.3]
−0.1 ± 0.3 [−0.7;0.4]
−1.3 ± 0.3 [−1.8; −0.8]
−0.1 ± 0.3 [−0.7; 0.5]
Unflatness
1.4 mm) falling within 1 mm from reference. Narrower distribution was achieved with the portal dosimetry method, where the detector resolution is finer. Slope: The deviation from reference resulted relatively wide although most of the measurements fall within ±2.5%, even for the 10 MV-FFF case which is the more critical considering its more peaked profile. The 2D-array data, in the case of 10 MV-FFF in Fig. 3, showed a systematic separation between the results computed for the left- and right-side of the profiles. This might suggest that the slope index is highly sensitive to small asymmetries in the beams as well as (or more likely) to the positioning of the 2D-array with respect to the beam. In fact, results from portal dosimetry measurements suggested no systematic difference between left and right sides of the profiles. Peak position: All measurements resulted within 0.5 mm for both energies. The fine resolution of the portal dosimetry system seems to have a favorable impact also on the spread of the peak position. Unflatness: Also measurements of the unflatness resulted highly stable over time with all entries falling within 0.8%– 0.9% of the reference. Table I presents the detailed analysis of the data from the 2D-array during a 1 year-long time period. Data are presented for both energies, two fields and separated for the X and Y profiles. For each parameter the mean, the standard deviation and the range are provided.
Table II presents, similarly to Table I, the results for the same fields from the GLAaS based portal dosimetry process over the short-term time period. In this case, the data are averaged over the three participant institutes (collecting a total of 60 repeated measurements, 20 per institute). For field size and unflatness, both dosimetry methods lead to highly consistent results. For penumbra, the difference between 2D-array and GLAaS results can be primarily attributed to the different spatial resolution of the detectors. The difference in the slope is not statistically significant but it is systematic and confirms the qualitative trend shown in Fig. 1. The measurement of the peak position resulted, as expected, influenced by the spatial resolution of the detectors used. For all data, very narrow distributions of values were obtained leading to small standard deviations for all parameters. Table III presents for the same fields the values of the parameters derived, for comparison and cross-validation, from ion chamber from a single measurement acquired at the time of Linac commissioning. These data refer to Institute A. The cross-validation against data from Table I and II confirms the robustness of the parameters vs. the measurement, the fact that penumbra and peak position are the most sensitive parameters to the spatial resolution, and that the absolute value of the slope should be carefully associated to the experimental set-up.
TABLE III. Analysis of the profiles measured with the ion chamber in a water phantom from Institute A. Data were measured at SDD = 100 cm and d = dmax. 6 MV FFF 10 × 10 X Field size (cm) Penumbra (mm) Unflatness Slope (%/mm) Peak position (mm)
10.0 4.9 1.09 0.307 − 0.1
10 MV FFF 20 × 20
cm2 Y 10.0 5.1 1.09 0.308 0.1
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X 20.1 5.0 1.24 0.435 − 0.1
10 × 10
cm2 Y 20.1 5.3 1.24 0.435 0.0
X 10.0 5.4 1.19 0.645 − 0.2
20 × 20 cm2
cm2 Y 10.0 5.8 1.19 0.647 0.0
X 20.1 5.6 1.456 0.819 − 0.1
Y 20.1 6.0 1.47 0.820 0.1
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TABLE IV. Analysis of profiles measured with the LA48 (SRS diode for penumbras) system in the Institute D on a 6 MV-FFF beam from a Clinac iX at commissioning and over a period of four months for the 10 × 10 and 20 × 20 cm2 fields and the corresponding deviation from 2D-array (Institute A) and the GLAaS portal dosimetry (averages of Institutes A, B, and C). Values are presented as averages between X and Y directions.
Field size (cm) Penumbra (mm) Unflatness Slope (%/mm) Peak position (mm)
10 × 10 cm2
2D-array
GLAaS
20 × 20 cm2
2D-array
GLAaS
10.06 ± 0.08 2.8 ± 0.3 1.09 ± 0.03 0.310 ± 0.004 −0.3 ± 0.5
0.09 − 1.7 − 0.1 0.06 0.
0.11 − 0.1 − 0.02 − 0.01 − 0.1
20.05 ± 0.10 3.1 ± 0.3 1.24 ± 0.04 0.407 ± 0.003 −0.2 ± 0.5
0.01 − 1.4 0.02 0.029 − 2.2
0.07 0.1 − 0.01 − 0.027 0.3
Tables S-I and S-II, available electronically only, present summaries of the quantitative analysis for all investigated fields and separated per each institute.18 The determination of the 95% confidence level (C.L.) interval of acceptable findings from the combined data of Tables I and II would lead to the following possible tolerances. Field size: 1 mm; penumbra: 1 mm; unflatness: 0.04; slope: 0.1%/mm; peak position: 1 mm. The cross-validation performed with the data from the Clinac-iX 6 MV-FFF beam of Institute D is presented in Table IV as unflatness, slope, and peak position for the 10 × 10 and 20 × 20 cm2 fields from weekly repeated measurements. The deviation from the corresponding averages from TrueBeam data is reported for comparison. The observed deviations fall, for all parameters, within the 95% C.L. and therefore within the suggested interval of acceptable values. While the absolute values of some of the results might be affected by the instrumentation used, the here presented data showed a good stability over time of all parameters and a consistent equivalence of the results originating from different beams. This is to be mitigated when data from the 2D-array are considered where the spread of the results for some parameters (e.g., the slope or the peak position) is somehow larger. These facts confirm that the proposed definitions are well posed and that can effectively monitor the characteristics of FFF beams in a standardized routine procedure. The spatial resolution of the detectors used might impact on the determination of the reference values. As a consequence, to determine references and to eventually proceed for machine re-tuning, it would be advisable to use systems with the highest spatial resolution. For simple periodic stability checks, any detector could be used. From the aggregated analysis of all the data collected in the various experiments, it was possible to extract confidence intervals based on 95% C.L. of each parameter. These values could be used as an operational tolerance level for the acceptance of periodic quality control checks, at least until more general consensus will be established. One pillar of the entire structure of the newly proposed parameters is the possibility to “normalize” FFF profiles in such a way to make them “comparable” to flattened profiles for at least some of the basic aspects (i.e., penumbra and field size definitions). Fogliata et al.15 proposed a conceptual normalization procedure based on the determination of the third derivative of the beam profiles. From an operational point of view this is obviously not an immediate and trivial task. For Medical Physics, Vol. 41, No. 2, February 2014
this reason, comprehensive fit of a variety of beams and field sizes was performed and an analytical expression to determine the normalization factor or even pre-defined tabulated values was proposed. In the present study, we opted to use in all experiments the pre-tabulated normalization values which constitute the simplest approach not requiring the preliminary determination of the third derivatives of the beam profiles. Nothing prevents individual users to perform this calculation by numerical methods or to make use of the analytical formula. One limit of the study summarized in this report is that no sensitivity tests were performed, in other words, no efforts were made to identify which are the minimum real deviations from ideal conditions that can be detected. For this reason, the present study is more a “measurability” test of the new proposed parameters rather than an assessment of their robustness in detecting pathologic situations. Nevertheless, to perform these tests it would be necessary to intentionally detune the steering of the FFF photon beams. Since all Linacs included in the study were operational in the clinics, this was not performed to avoid un-necessary burden to the normal hospital workflow. It is foreseen to perform these tests in a further investigational phase on a dedicated system. A second limit that should be solved in further investigations is the applicability of these definitions to FFF beams generated by Linacs of different vendors. The data presented in this study demonstrate that FFF beams from different Linac series and generations from the same vendor, deeply different in the hardware and in the beam generation mechanisms such as the TrueBeam and the Clinac, are coherent and consistent. Therefore it would be advisable to prove the legitimate assumption that the parameters proposed here could be effectively applied to FFF beams of any source and be used for a general purpose quality assurance paradigm. 4. CONCLUSION The concepts and definitions of the parameters usable in a quality control process to characterize and monitor the profiles of a FFF beam were defined in an earlier study15 where new definitions were proposed for the main signatures of these. With the present study, a first attempt to validate these new parameters from an experimental point of view has been presented. Data from multiple linear accelerators of different types (TrueBeam and Clinac) all capable to deliver FFF beams were collected using different instrumentations. Both
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short-term series and long-term series of experiments were performed to appraise the measurability of the new parameters as well as the stability of the FFF beams. The results presented here demonstrate that field size, penumbra, unflatness, slope, and peak position can be reliably measured with a variety of instruments. ACKNOWLEDGMENT The corresponding author states that Dr. L. Cozzi acts as a scientific advisor to Varian Medical Systems and is Head of Research and Technological Development to Oncology Institute of Southern Switzerland, IOSI, Bellinzona.
a) Author
to whom correspondence should be addressed. Electronic mail: [email protected]; Telephone: 0041-91-8119184. 1 M. Scorsetti et al., “Feasibility and early clinical assessment of flattening filter free (FFF) based stereotactic body radiotherapy (SBRT) treatments,” Radiat. Oncol. 6, 1–8 (2011). 2 S. Lang, B. Shrestha, S. Graydon, F. Cavelaars, C. Linsenmeier, J. Hrbacek, S. Klöck, G. Studer, and O. Riesterer, “Clinical application of flattening filter free beams for extracranial stereotactic radiotherapy,” Radiother. Oncol. 106, 255–259 (2013). 3 M. Scorsetti, S. Arcangeli, A. Tozzi, T. Comito, F. Alongi, P. Navarria, P. Mancosu, G. Reggiori, A. Fogliata, G. Torzilli, S. Tomatis, and L. Cozzi, “Is stereotactic body radiation therapy an attractive option for unresectable liver metastases? A preliminary report from a phase 2 trial,” Int. J. Radiat. Oncol., Biol., Phys. 86, 336–342 (2013). 4 D. Georg, T. Knöös, and B. McClean, “Current status and future perspective of flattening filter free photon beams,” Med. Phys. 38, 1280–1293 (2011). 5 J. Cashmore, “The characterization of unflattened photon beams from a 6 MV linear accelerator,” Phys. Med. Biol. 53, 1933–1946 (2008). 6 G. Kragl, S. af Wetterstedt, B. Knäusl, M. Lind, P. McCavana, T. Knöös, B. McClean, and D. Georg, “Dosimetric characteristics of 6 and 10MV unflattened photon beams,” Radiother. Oncol. 93, 141–146 (2009). 7 J. Hrbacek, S. Lang, and S. Klöck, “Commissioning of photon beams of a flattening filter-free linear accelerator and the accuracy of beam model-
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ing using an anisotropic analytical algorithm,” Int. J. Radiat. Oncol., Biol., Phys. 80, 1228–1237 (2011). 8 D. Georg, G. Kragl, S. af Wetterstedt, P. McCavana, and B. McClean, “Photon beam quality variations of a flattening filter free linear accelerator,” Med. Phys. 37, 49–53 (2010). 9 O. N. Vassiliev, U. Titt, F. Pönisch, S. F. Kry, R. Mohan, and M. T. Gillin, “Dosimetric properties of photon beams from a flattening filter free clinical accelerator,” Phys. Med. Biol. 51, 1907–1917 (2006). 10 M. Dalaryd, G. Kragl, C. Ceberg, D. Georg, B. McClean, S. af Wetterstedt, E. Wieslander, and T. Knöös, “A Monte Carlo study of a flattening filterfree linear accelerator verified with measurements,” Phys. Med. Biol. 55, 7333–7343 (2010). 11 Y. Wang, M. K. Khan, J. Y. Ting, and S. B. Easterling, “Surface dose investigation of the flattening filter-free photon beams,” Int. J. Radiat. Oncol., Biol., Phys. 83, e281–e285 (2012). 12 Z. Chang, Q. Wu, J. Adamson, L. Ren, J. Bowsher, H. Yan, A. Thomas, and F.,F. Yin, “Commissioning and dosimetric characteristics of TrueBeam system: Composite data of three TrueBeam machines,” Med. Phys. 39, 6981– 7018 (2012). 13 C. Glide-Hurst, M. Bellon, C. Foster, C. Altunbas, M. Speiser, M. Altman, D. Westerly, N. Wen, B. Zhao, M. Miften, I. Chetty, and T. Solberg, “Commissioning of the Varian TrueBeam linear accelerator: A multi institutional study,” Med. Phys. 40, 031719 (15pp.) (2013). 14 G. Beyer, “Commissioning measurements for photon beam data on three TrueBeam linear accelerators and comparison with Trilogy and Clinac 2100 linear accelerators,” J. Appl. Clin. Med. Phys. 14, 273–288 (2013). 15 A. Fogliata, R. Garcia, T. Knöös, G. Nicolini, A. Clivio, E. Vanetti, C. Khamphan, and L. Cozzi, “Definition of parameters forquality assurance of flattening filter free (FFF) photon beams in radiation therapy,” Med. Phys. 39, 6455–6464 (2012). 16 G. Nicolini, A. Clivio, E. Vanetti, H. Krauss, P. Fenoglietto, L. Cozzi, and A. Fogliata, “Evaluation of an aSi-EPID with flattening filter free beams: Applicability to the GLAaS algorithm for portal dosimetry and first experience for pre-treatment QA of RapidArc,” Med. Phys. 40, 111719 (9pp.) (2013). 17 G. G. Ezzell, J. W. Burmeister, N. Dogan, T. J. LoSasso, J. G. Mechalakos, D. Mihailidis, A. Molineu, J. R. Palta, C. R. Ramsey, B. J. Salter, J. Shi, P. Xia, N. J. Yue, and Y. Xiao, “IMRT commissioning: Multiple institution planning and dosimetry comparisons, a report from AAPM Task Group 119,” Med. Phys. 36, 5359–5373 (2009). 18 See supplementary material at http://dx.doi.org/10.1118/1.4861817 for Tables S-I and S-II.
On the determination of reference levels for quality assurance of flattening filter free photon beams in radiation therapy Alessandro Clivio, Maria Francesca Belosi, Luca Cozzi, Giorgia Nicolini,a) and Eugenio Vanetti Oncology Institute of Southern Switzerland, Medical Physics Unit, Bellinzona CH-6500, Switzerland
Grégory Bolard Clinique de Genolier, Radiation Oncology department, Genolier CH-1272, Switzerland
Pascal Fenoglietto Département de Cancérologie Radiothérapie, CRLC Val d’Aurelle-Paul Lamarque, Montpellier F-34090, France
Harald Krauss Kaiser-Franz-Josef-Spital, Institut für Radioonkologie, Vienna A-1100, Austria
Antonella Fogliata Oncology Institute of Southern Switzerland, Medical Physics Unit, Bellinzona CH-6500, Switzerland
(Received 24 October 2013; revised 4 December 2013; accepted for publication 23 December 2013; published 15 January 2014) Purpose: New definitions for some dosimetric parameters for use in quality assurance of flattening filter free (FFF) beams generated by medical linear accelerators have been suggested. The present study aims to validate these suggestions and to propose possible reference levels. Methods: The main characteristics of FFF photon beams were described in terms of: field size, penumbra, unflatness, slope, and peak-position parameters. Data were collected for 6 and 10 MVFFF beams from three different Varian TrueBeam Linacs. Measurements were performed with a 2Darray (Starcheck system from PTW-Freiburg) and with the portal dosimetry method GLAaS utilizing the build-in portal imager of TrueBeam. Data were also compared to ion chamber measurements. A cross check validation has been performed on a FFF beam of 6 MV generated by a Varian Clinac-iX upgraded to FFF capability. Results: All the parameters suggested to characterize the FFF beams resulted easily measurable and little variation was observed among different Linacs. Referring to two reference field sizes of 10 × 10 and 20 × 20 cm2 , at SDD = 100 cm and d = dmax, from the portal dosimetry data, the following results (averaging X and Y profiles) were obtained. Field size: 9.95 ± 0.02 and 19.98 ± 0.03 cm for 6 MV-FFF (9.94 ± 0.02 and 19.98 ± 0.03 cm for 10 MV-FFF). Penumbra: 2.7 ± 0.3 and 2.9 ± 0.3 mm for 6 MV-FFF (3.1 ± 0.2 and 3.3 ± 0.3 for 10 MV-FFF). Unflatness: 1.11 ± 0.01 and 1.25 ± 0.01 for 6 MV-FFF (1.21 ± 0.01 and 1.50 ± 0.01 for 10 MVFFF). Slope: 0.320 ± 0.020%/mm and 0.43 ± 0.015%/mm for 6 MV-FFF (0.657 ± 0.023%/mm and 0.795 ± 0.017%/mm for 10 MV-FFF). Peak Position −0.2 ± 0.2 and −0.4 ± 0.2 mm for 6 MV-FFF (−0.3 ± 0.2 and 0.7 ± 0.3 mm for 10 MV-FFF). Results would depend upon measurement depth. With thresholds set to at least 95% confidence level from the measured data and to account for possible variations between detectors and methods and experimental settings, a tolerance set of: 1 mm for field size and penumbra, 0.04 for unflatness, 0.1%/mm for slope, and 1 mm for peak position could be proposed from our data. Conclusions: The parameters proposed for the characterization and routine control of stability of profiles of FFF beams appear to be a viable solution with a strong similarity to the conventional parameters used for flattened beams. The results from three different TrueBeams and the crossvalidation against a Clinac-iX suggested the possible generalization of the methods and the possibility to use common tolerances for the parameters. The data showed also the reproducibility of beam characteristics among different systems (of the same vendor) and the resulting parameter values could therefore be possibly generalized. © 2014 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4861817] Key words: flattening filter free beams, TrueBeam, GLAaS algorithm, quality assurance 1. INTRODUCTION The clinical introduction of general purpose linear accelerators equipped with photon beams generated without a flat021713-1
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tening filter, the ‘‘flattening filter free’’ (FFF) beams, is consolidating and several studies reported about their application to consistent groups of patients.1–3 Physical and dosimetric characteristics of FFF beams have been extensively
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© 2014 Am. Assoc. Phys. Med.
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investigated4–11 and data from commissioning of FFF-capable linear accelerators are available.8, 12–14 Nevertheless, standardized and consolidated definitions about beam parameters to be used for routine quality assurance (QA) are not yet available from international consensus. This is particularly relevant for quantities such as field size, penumbra, or flatness, all related to the properties of the transverse profiles of the beams. In order to establish an operational set of definitions that could be used by any center, irrespective of the beam energy and manufacturer, a study was recently published15 where literature was investigated and new definitions were proposed. The aim of that study was to determine simple procedures to normalize FFF profiles in a simple and robust way allowing to use, as much as possible, equal or similar definitions as for conventional flattened beams. Second, two additional definitions were proposed to characterize the specificity of FFF beams: the slope and the unflatness of the profiles. A last definition was proposed to identify the position of the peak of the profile which is another signature of the FFF beams and that, if not positioned at the center of the geometrical beam axis (CAX) might have severe dosimetric implications on the irradiation of patients. The present report aims to provide some initial experimental evidence to (i) the measurability of the definitions proposed by Fogliata;15 (ii) the stability over a medium time of the FFF beams in terms of the five parameters on one machine and the reproducibility over a short time for other machines; and (iii) the coherence of results when the definitions are tested and applied to different machines in different centers and measured with different equipments. As a second aim of the study, a set of possible tolerances have been computed from the 95% confidence interval of all the aggregated measurements available for analysis. These tolerances have been defined out of the initial experience of a cooperative group of users of the FFF beams for a specific single vendor Linac. This implies that obvious limitations exist and that the proposed values might be conditionally applied also by other clinics in their routine quality assurance procedures to determine if the concepts can have a general applicability. These
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potential thresholds could be further relaxed or tightened if future data would prove the necessity and, more important, some broader consensus validation and or recommendations from national and international bodies will be necessary. 2. METHODS The parameters identified as useful in the characterization of unflattened beam profiles and different from the conventional flatness, profiles and penumbra were introduced and defined by Fogliata.15 The definitions are summarized here while details can be found in the original study. Profile normalization: The use of a “shoulder” point as “normalization” point of a FFF profile with respect to a corresponding flattened beam was proposed. To establish an operational procedure, the pre-tabulated normalization values were applied as from Table I of the original publication. Dosimetric field size: The distance between the 50% dose levels in the normalized FFF profile. Penumbra: The distance between the 20% and the 80% dose levels in the field edge of the normalized FFF profiles. Unflatness: The ratio between the dose at the central axis and the dose at a fixed off-axis point: Unflatness = Dosecentral.axis / DoseX.off-axis ; the off-axis point was chosen to be located at the 80% of the field size (FS) for FS ≥ 10 cm and 60% for FS < 10 cm. Slope: As a descriptor of the profile gradient, Slope = (y1 − y2 ) / (x1 − x2 ) where (x1 , y1 ) and (x2 , y2 ) are the coordinates, as dose value y at position x, on each half-profile at 2/3 and 1/3 of the off-axis positions. Equivalence of the left and right slope is a further verification of the symmetry of the beams. Since the shape of the FFF profile depends on the energy, the constancy of the slope is also an indirect measurement of the constancy of the beam energy. For small FS, FFF and flattened beams present quite the same shape, and conventional parameters could be used.8, 12–14 Peak position: The presence of a pronounced peak in the central axis (CAX) is the signature of FFF beams. Deviations from CAX would mean a mistuning of the beam steering
TABLE I. Analysis of profiles measured with the Starcheck system in the beam X and Y directions. Results are presented as averages between left and right field side of the profiles with respect to central beam axis. Data were measured at SDD = 100 cm. 6 MV FFF
10 MV FFF
10 × 10 cm2
20 × 20 cm2
10 × 10 cm2
20 × 20 cm2
X
Y
X
Y
X
Y
X
Y
Field size (cm)
9.93 ± 0.02 [9.89;10.00]
10.00 ± 0.03 [9.91;10.16]
19.99 ± 0.04 [19.96;20.12]
20.09 ± 0.04 [19.94;20.21]
9.94 ± 0.02 [9.90;10.01]
10.01 ± 0.02 [9.92;10.09]
20.04 ± 0.04 [20.00;20.16]
20.13 ± 0.04 [19.99;20.25]
Penumbra (mm)
4.9 ± 0.1 [4.3;5.1]
4.0 ± 0.3 [3.4;4.5]
5.1 ± 0.2 [4.5;5.5]
4.0 ± 0.2 [3.7;4.7]
5.1 ± 0.1 [4.5;5.3]
4.5 ± 0.1 [4.0;4.8]
5.9 ± 0.2 [5.1;6.2]
5.0 ± 0.1 [4.7;5.4]
1.10 ± 0.01 [1.09;1.12]
1.10 ± 0.01 [1.09;1.11]
1.22 ± 0.01 [1.22;1.23]
1.22 ± 0.01 [1.21;1.23]
1.19 ± 0.02 [1.17;1.22]
1.20 ± 0.02 [1.17;1.22]
1.42 ± 0.01 [1.40;1.44]
1.42 ± 0.01 [1.40;1.44]
Slope (%/mm)
0.295 ± 0.021 [0.262;0.326]
0.285 ± 0.007 [0.262;0.309]
0.378 ± 0.021 [0.343;0.413]
0.371 ± 0.011 [0.341;0.392]
0.588 ± 0.021 [0.550;0.619]
0.588 ± 0.021 [0.550;0.619]
0.686 ± 0.039 [0.600;0.741]
0.685 ± 0.030 [0.620;0.766]
Peak position (mm)
−1.2 ± 0.2 [−1.7;−0.8]
0.5 ± 0.2 [0.0;1.0]
1.7 ± 0.2 [1.0;2.5]
2.2 ± 0.3 [1.1;3.2]
−0.3 ± 0.2 [−0.6;0.1]
−0.3 ± 0.2 [−0.6;0.1]
2.1 ± 0.4 [0.2;2.7]
1.8 ± 0.4 [0.5;3.1]
Unflatness
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parameters and should be promptly identified. The “peak position” parameter is defined as the off-axis position of the intersection point of the left and right slopes: Peak position = (IL − IR ) / (SR − SL ) where IL and IR are the left and right intercepts, respectively; SL and SR are the left and right slopes, respectively. In the present study, we focused only on QA parameters that were either newly proposed for FFF beams (e.g., unflatness) or that required specific manipulation of the measured data prior to their numerical quantification (e.g., penumbra, computed with the usual definition only after the FFF normalization is applied). Other parameters such as symmetry, beam output, beam quality, and energy should be part of any comprehensive quality assurance program of photon beams but were not subject to investigation here since the definitions proposed and used in the reference study did not differ from what used for the conventional flattened beams. Validation of the QA parameters specific for FFF beams was performed on three different TrueBeam linear accelerators (Varian Medical Systems, Palo Alto, USA) installed in the three institutions (A, B, and C), all equipped with 6 and 10 MV unflattened beams. Measurements were performed using two systems available for routine quality controls in the centers. A commercial 2D-array (the StarCheck system from PTWFreiburg, Germany) was used in Institute A only for one year, performing weekly measurements of all parameters. Experiments were performed with source to detector distance SDD = 100 cm and two field sizes of nominal dimension of 10 × 10 and 20 × 20 cm2 . These fields were chosen for consistency with the current QA protocols of the institute and in the agreement with the local national recommendations. The StarCheck system is made by a series of 527 air-filled ion chambers of 0.08 cm3 of volume aligned along the two main axes and the two diagonals with a spatial separation of 3 mm. The chambers are embedded in solid water material at a water-equivalent depth of 5.8 mm. The tests performed over one year with the 2D-array allowed to determine the long term stability of the beams and the reproducibility of the tests and of the parameters proposed. A second group of measurements was performed, in institutes A, B, and C, using the electronic portal dosimetry methods by means of the GLAaS methodology that was adapted to FFF beams and described in Nicolini et al.16 The detector used for the dosimetry is the electronic portal imager made of aSi detectors with a 0.392 mm/pxl spatial resolution, positioned at 100 or 150 cm from the beam source. Different experimental configurations were tested in all the three centers. For SDD = 100 cm, three FS were investigated: 5 × 5, 10 × 10, and 20 × 20 cm2 . For SDD = 150 cm, three FS were tested: 5 × 5, 10 × 10, and 15 × 15 cm2 . All measurements were performed with detector reading calibrated in absorbed dose to water at the depth of maximum dose deposition: d = dmax . All institutes performed 20 repeated measurements for all the experiments in a time frame of 1–2 months. The data collected from these experiments allowed to ascertain the short term reproducibility of the tests and parameters on a group of similar machines (same model). Medical Physics, Vol. 41, No. 2, February 2014
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All measurements in these first two groups were repeated for both energies 6 and 10 MV-FFF. A final set of measurements was performed as a cross validation of the methods on a fourth linear accelerator. This was a Clinac-iX from Institute D updated to flattening filter free mode (released for clinical operation) for the 6 MV beam. These tests were performed using a LA48 linear array detector from PTW-Freiburg. The LA48 is made of 48 liquid ion chambers (with a volume of 4 × 4 × 0.5 mm3 ) with a centreto-centre separation of 8 mm. In the penumbra region, where a high resolution detector is preferable, results refer to a SRS diode. Scope of the cross-validation was the assessment of the reliability of the QA parameters when applied to different types of Linacs (but manufactured from the same vendor). It is a first step towards full generalization of the parameters to any FFF type photon beam. Formally binding tolerance levels to each of the five parameters investigated should be defined within a consensusbased process. Nevertheless, from the data of the study we are presenting values as operational tolerance, assuming that they could be derived from the concept of confidence interval, set at 95% as used in the AAPM TG-119 (Ref. 17) (i.e., mean ± 1.96 standard deviation). 3. RESULTS AND DISCUSSION Figure 1 shows an example of the data acquired and used for the analysis. In this case, a 20 × 20 cm2 field for both energies is reported: in the first panel the display of the 2D beam profile at dmax from the GLAaS portal dosimetry measurements; in the second and third panels the X and Y lateral profiles measured with a 0.125 cm3 ion chamber and the 2Darray at institute A and the GLAaS method at institutes A, B, and C. As a general observation, valid for all fields and experiments performed, there is a very good qualitative (and quantitative) agreement among the three institutes with the GLAaS approach which is also highly consistent with the ion chamber measurements. The noticeably different shapes of the profiles obtained between GLAaS and the 2D-array (both from the same beams at Institute A) can be re-conducted at least to two concurrent potential sources: (i) the different depth of measurement and (ii) the different sensitivity of the detectors in the array mainly due to scattering conditions. To appraise qualitatively and quantitatively the stability over time of the FFF beams with respect to the five parameters suggested for routine quality assurance, Figs. 2 (6 MV-FFF) and 3 (10 MV-FFF) show a single institute (A) experience on the long term measurements. Data are plotted as a function of time (weeks of measurement) for measurements performed with either the 2D-array or with the GLAaS portal dosimetry method. The graphs show, for the FS = 10 × 10 cm2 and SDD = 100 cm, the deviation of each of the parameters from the reference value. The reference was chosen as the first week (25th) when both dosimetry methods were implemented in the routine verification of the FFF beams in the institute. Data are separated for X and Y profiles. For unflatness and slope the results for the left and right parts of the profiles are presented separately as well. Similar qualitative results
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F IG . 1. Display of the 2D profile for 6 and 10 MV-FFF beams for a test field of 20 × 20 cm2 measured with the GLAaS method (a). Profiles in the X (crossplane) direction (b) and in the Y (inplane) direction (c) measured with the GLAaS method in the three centers and with the 2D array, the ion chamber (IC).
were obtained for the other field sizes and are therefore not shown. Penumbra: During the entire year extremely stable results within a fraction of mm were obtained, and only limited number of measurements were out of the ‘‘almost-zero’’ deviation Medical Physics, Vol. 41, No. 2, February 2014
but all within 0.1 mm at 6 MV-FFF and 0.8 mm at 10 MV-FFF as visible in Figs. 2 and 3. Field size: Generally very good and stable results were obtained for this parameter with all measurements (except one case for the 2D-array, where the distance from reference was
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F IG . 2. Deviations from reference of the various QA parameters investigated to characterize the FFF beams: unflatness, slope, peak position, field size, and penumbra. Data are presented for the 6 MV-FFF beam measured at center A with the 2D array and the GLAaS method. Data were measured at SDD = 100 cm. Medical Physics, Vol. 41, No. 2, February 2014
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F IG . 3. Deviations from reference of the various QA parameters investigated to characterize the FFF beams: unflatness, slope, peak position, field size, and penumbra. Data are presented for the 10 MV-FFF beam measured at center A with the 2D array and the GLAaS method. Data were measured at SDD = 100 cm. Medical Physics, Vol. 41, No. 2, February 2014
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TABLE II. Analysis of the profiles measured with the GLAaS method in the beam X and Y directions. Data are presented as the average of the findings from the three centers. Data were measured at SDD = 100 cm. 6 MV FFF
10 MV FFF
10 × 10 cm2
20 × 20 cm2
10 × 10 cm2
20 × 20 cm2
X
Y
X
Y
X
Y
X
Y
Field size (cm)
10.00 ± 0.02 [9.93;10.03]
9.90 ± 0.02 [9.88;9.92]
20.03 ± 0.02 [19.96;20.06]
19.93 ± 0.03 [19.83;19.99]
9.98 ± 0.03 [9.92;10.03]
9.89 ± 0.02 [9.84;9.92]
20.03 ± 0.02 [19.98;20.06]
19.92 ± 0.03 [19.84;19.96]
Penumbra (mm)
2.4 ± 0.3 [1.7;3.0]
3.0 ± 0.2 [2.8;3.7]
2.5 ± 0.3 [1.6;3.0]
3.4 ± 0.4 [2.8;4.1]
2.7 ± 0.2 [2.0;2.9]
3.5 ± 0.2 [3.3;4.1]
2.8 ± 0.2 [2.4;3.4]
3.9 ± 0.4 [3.6;4.7]
1.11 ± 0.01 [1.09;1.12]
1.11 ± 0.01 [1.09;1.13]
1.25 ± 0.01 [1.24;1.27]
1.26 ± 0.01 [1.24;1.28]
1.21 ± 0.01 [1.19;1.23]
1.21 ± 0.01 [1.19;1.23]
1.50 ± 0.01 [1.48;1.52]
1.50 ± 0.01 [1.48;1.52]
Slope (%/mm)
0.313 ± 0.011 [0.294;0.329]
0.327 ± 0.032 [0.286;0.369]
0.430 ± 0.011 [0.417;0.448]
0.430 ± 0.019 [0.400;0.463]
0.652 ± 0.020 [0.610;0.690]
0.663 ± 0.027 [0.605;0.716]
0.798 ± 0.014 [0.772;.0.822]
0.792 ± 0.021 [0.758;0.830]
Peak position (mm)
−0.3 ± 0.2 [−0.7; −0.1]
−0.1 ± 0.2 [−0.5;0.5]
−0.9 ± 0.1 [−1.2; −0.7]
0.0 ± 0.2 [−0.4;0.4]
−0.6 ± 0.1 [−0.8; −0.3]
−0.1 ± 0.3 [−0.7;0.4]
−1.3 ± 0.3 [−1.8; −0.8]
−0.1 ± 0.3 [−0.7; 0.5]
Unflatness
1.4 mm) falling within 1 mm from reference. Narrower distribution was achieved with the portal dosimetry method, where the detector resolution is finer. Slope: The deviation from reference resulted relatively wide although most of the measurements fall within ±2.5%, even for the 10 MV-FFF case which is the more critical considering its more peaked profile. The 2D-array data, in the case of 10 MV-FFF in Fig. 3, showed a systematic separation between the results computed for the left- and right-side of the profiles. This might suggest that the slope index is highly sensitive to small asymmetries in the beams as well as (or more likely) to the positioning of the 2D-array with respect to the beam. In fact, results from portal dosimetry measurements suggested no systematic difference between left and right sides of the profiles. Peak position: All measurements resulted within 0.5 mm for both energies. The fine resolution of the portal dosimetry system seems to have a favorable impact also on the spread of the peak position. Unflatness: Also measurements of the unflatness resulted highly stable over time with all entries falling within 0.8%– 0.9% of the reference. Table I presents the detailed analysis of the data from the 2D-array during a 1 year-long time period. Data are presented for both energies, two fields and separated for the X and Y profiles. For each parameter the mean, the standard deviation and the range are provided.
Table II presents, similarly to Table I, the results for the same fields from the GLAaS based portal dosimetry process over the short-term time period. In this case, the data are averaged over the three participant institutes (collecting a total of 60 repeated measurements, 20 per institute). For field size and unflatness, both dosimetry methods lead to highly consistent results. For penumbra, the difference between 2D-array and GLAaS results can be primarily attributed to the different spatial resolution of the detectors. The difference in the slope is not statistically significant but it is systematic and confirms the qualitative trend shown in Fig. 1. The measurement of the peak position resulted, as expected, influenced by the spatial resolution of the detectors used. For all data, very narrow distributions of values were obtained leading to small standard deviations for all parameters. Table III presents for the same fields the values of the parameters derived, for comparison and cross-validation, from ion chamber from a single measurement acquired at the time of Linac commissioning. These data refer to Institute A. The cross-validation against data from Table I and II confirms the robustness of the parameters vs. the measurement, the fact that penumbra and peak position are the most sensitive parameters to the spatial resolution, and that the absolute value of the slope should be carefully associated to the experimental set-up.
TABLE III. Analysis of the profiles measured with the ion chamber in a water phantom from Institute A. Data were measured at SDD = 100 cm and d = dmax. 6 MV FFF 10 × 10 X Field size (cm) Penumbra (mm) Unflatness Slope (%/mm) Peak position (mm)
10.0 4.9 1.09 0.307 − 0.1
10 MV FFF 20 × 20
cm2 Y 10.0 5.1 1.09 0.308 0.1
Medical Physics, Vol. 41, No. 2, February 2014
X 20.1 5.0 1.24 0.435 − 0.1
10 × 10
cm2 Y 20.1 5.3 1.24 0.435 0.0
X 10.0 5.4 1.19 0.645 − 0.2
20 × 20 cm2
cm2 Y 10.0 5.8 1.19 0.647 0.0
X 20.1 5.6 1.456 0.819 − 0.1
Y 20.1 6.0 1.47 0.820 0.1
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TABLE IV. Analysis of profiles measured with the LA48 (SRS diode for penumbras) system in the Institute D on a 6 MV-FFF beam from a Clinac iX at commissioning and over a period of four months for the 10 × 10 and 20 × 20 cm2 fields and the corresponding deviation from 2D-array (Institute A) and the GLAaS portal dosimetry (averages of Institutes A, B, and C). Values are presented as averages between X and Y directions.
Field size (cm) Penumbra (mm) Unflatness Slope (%/mm) Peak position (mm)
10 × 10 cm2
2D-array
GLAaS
20 × 20 cm2
2D-array
GLAaS
10.06 ± 0.08 2.8 ± 0.3 1.09 ± 0.03 0.310 ± 0.004 −0.3 ± 0.5
0.09 − 1.7 − 0.1 0.06 0.
0.11 − 0.1 − 0.02 − 0.01 − 0.1
20.05 ± 0.10 3.1 ± 0.3 1.24 ± 0.04 0.407 ± 0.003 −0.2 ± 0.5
0.01 − 1.4 0.02 0.029 − 2.2
0.07 0.1 − 0.01 − 0.027 0.3
Tables S-I and S-II, available electronically only, present summaries of the quantitative analysis for all investigated fields and separated per each institute.18 The determination of the 95% confidence level (C.L.) interval of acceptable findings from the combined data of Tables I and II would lead to the following possible tolerances. Field size: 1 mm; penumbra: 1 mm; unflatness: 0.04; slope: 0.1%/mm; peak position: 1 mm. The cross-validation performed with the data from the Clinac-iX 6 MV-FFF beam of Institute D is presented in Table IV as unflatness, slope, and peak position for the 10 × 10 and 20 × 20 cm2 fields from weekly repeated measurements. The deviation from the corresponding averages from TrueBeam data is reported for comparison. The observed deviations fall, for all parameters, within the 95% C.L. and therefore within the suggested interval of acceptable values. While the absolute values of some of the results might be affected by the instrumentation used, the here presented data showed a good stability over time of all parameters and a consistent equivalence of the results originating from different beams. This is to be mitigated when data from the 2D-array are considered where the spread of the results for some parameters (e.g., the slope or the peak position) is somehow larger. These facts confirm that the proposed definitions are well posed and that can effectively monitor the characteristics of FFF beams in a standardized routine procedure. The spatial resolution of the detectors used might impact on the determination of the reference values. As a consequence, to determine references and to eventually proceed for machine re-tuning, it would be advisable to use systems with the highest spatial resolution. For simple periodic stability checks, any detector could be used. From the aggregated analysis of all the data collected in the various experiments, it was possible to extract confidence intervals based on 95% C.L. of each parameter. These values could be used as an operational tolerance level for the acceptance of periodic quality control checks, at least until more general consensus will be established. One pillar of the entire structure of the newly proposed parameters is the possibility to “normalize” FFF profiles in such a way to make them “comparable” to flattened profiles for at least some of the basic aspects (i.e., penumbra and field size definitions). Fogliata et al.15 proposed a conceptual normalization procedure based on the determination of the third derivative of the beam profiles. From an operational point of view this is obviously not an immediate and trivial task. For Medical Physics, Vol. 41, No. 2, February 2014
this reason, comprehensive fit of a variety of beams and field sizes was performed and an analytical expression to determine the normalization factor or even pre-defined tabulated values was proposed. In the present study, we opted to use in all experiments the pre-tabulated normalization values which constitute the simplest approach not requiring the preliminary determination of the third derivatives of the beam profiles. Nothing prevents individual users to perform this calculation by numerical methods or to make use of the analytical formula. One limit of the study summarized in this report is that no sensitivity tests were performed, in other words, no efforts were made to identify which are the minimum real deviations from ideal conditions that can be detected. For this reason, the present study is more a “measurability” test of the new proposed parameters rather than an assessment of their robustness in detecting pathologic situations. Nevertheless, to perform these tests it would be necessary to intentionally detune the steering of the FFF photon beams. Since all Linacs included in the study were operational in the clinics, this was not performed to avoid un-necessary burden to the normal hospital workflow. It is foreseen to perform these tests in a further investigational phase on a dedicated system. A second limit that should be solved in further investigations is the applicability of these definitions to FFF beams generated by Linacs of different vendors. The data presented in this study demonstrate that FFF beams from different Linac series and generations from the same vendor, deeply different in the hardware and in the beam generation mechanisms such as the TrueBeam and the Clinac, are coherent and consistent. Therefore it would be advisable to prove the legitimate assumption that the parameters proposed here could be effectively applied to FFF beams of any source and be used for a general purpose quality assurance paradigm. 4. CONCLUSION The concepts and definitions of the parameters usable in a quality control process to characterize and monitor the profiles of a FFF beam were defined in an earlier study15 where new definitions were proposed for the main signatures of these. With the present study, a first attempt to validate these new parameters from an experimental point of view has been presented. Data from multiple linear accelerators of different types (TrueBeam and Clinac) all capable to deliver FFF beams were collected using different instrumentations. Both
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short-term series and long-term series of experiments were performed to appraise the measurability of the new parameters as well as the stability of the FFF beams. The results presented here demonstrate that field size, penumbra, unflatness, slope, and peak position can be reliably measured with a variety of instruments. ACKNOWLEDGMENT The corresponding author states that Dr. L. Cozzi acts as a scientific advisor to Varian Medical Systems and is Head of Research and Technological Development to Oncology Institute of Southern Switzerland, IOSI, Bellinzona.
a) Author
to whom correspondence should be addressed. Electronic mail: [email protected]; Telephone: 0041-91-8119184. 1 M. Scorsetti et al., “Feasibility and early clinical assessment of flattening filter free (FFF) based stereotactic body radiotherapy (SBRT) treatments,” Radiat. Oncol. 6, 1–8 (2011). 2 S. Lang, B. Shrestha, S. Graydon, F. Cavelaars, C. Linsenmeier, J. Hrbacek, S. Klöck, G. Studer, and O. Riesterer, “Clinical application of flattening filter free beams for extracranial stereotactic radiotherapy,” Radiother. Oncol. 106, 255–259 (2013). 3 M. Scorsetti, S. Arcangeli, A. Tozzi, T. Comito, F. Alongi, P. Navarria, P. Mancosu, G. Reggiori, A. Fogliata, G. Torzilli, S. Tomatis, and L. Cozzi, “Is stereotactic body radiation therapy an attractive option for unresectable liver metastases? A preliminary report from a phase 2 trial,” Int. J. Radiat. Oncol., Biol., Phys. 86, 336–342 (2013). 4 D. Georg, T. Knöös, and B. McClean, “Current status and future perspective of flattening filter free photon beams,” Med. Phys. 38, 1280–1293 (2011). 5 J. Cashmore, “The characterization of unflattened photon beams from a 6 MV linear accelerator,” Phys. Med. Biol. 53, 1933–1946 (2008). 6 G. Kragl, S. af Wetterstedt, B. Knäusl, M. Lind, P. McCavana, T. Knöös, B. McClean, and D. Georg, “Dosimetric characteristics of 6 and 10MV unflattened photon beams,” Radiother. Oncol. 93, 141–146 (2009). 7 J. Hrbacek, S. Lang, and S. Klöck, “Commissioning of photon beams of a flattening filter-free linear accelerator and the accuracy of beam model-
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ing using an anisotropic analytical algorithm,” Int. J. Radiat. Oncol., Biol., Phys. 80, 1228–1237 (2011). 8 D. Georg, G. Kragl, S. af Wetterstedt, P. McCavana, and B. McClean, “Photon beam quality variations of a flattening filter free linear accelerator,” Med. Phys. 37, 49–53 (2010). 9 O. N. Vassiliev, U. Titt, F. Pönisch, S. F. Kry, R. Mohan, and M. T. Gillin, “Dosimetric properties of photon beams from a flattening filter free clinical accelerator,” Phys. Med. Biol. 51, 1907–1917 (2006). 10 M. Dalaryd, G. Kragl, C. Ceberg, D. Georg, B. McClean, S. af Wetterstedt, E. Wieslander, and T. Knöös, “A Monte Carlo study of a flattening filterfree linear accelerator verified with measurements,” Phys. Med. Biol. 55, 7333–7343 (2010). 11 Y. Wang, M. K. Khan, J. Y. Ting, and S. B. Easterling, “Surface dose investigation of the flattening filter-free photon beams,” Int. J. Radiat. Oncol., Biol., Phys. 83, e281–e285 (2012). 12 Z. Chang, Q. Wu, J. Adamson, L. Ren, J. Bowsher, H. Yan, A. Thomas, and F.,F. Yin, “Commissioning and dosimetric characteristics of TrueBeam system: Composite data of three TrueBeam machines,” Med. Phys. 39, 6981– 7018 (2012). 13 C. Glide-Hurst, M. Bellon, C. Foster, C. Altunbas, M. Speiser, M. Altman, D. Westerly, N. Wen, B. Zhao, M. Miften, I. Chetty, and T. Solberg, “Commissioning of the Varian TrueBeam linear accelerator: A multi institutional study,” Med. Phys. 40, 031719 (15pp.) (2013). 14 G. Beyer, “Commissioning measurements for photon beam data on three TrueBeam linear accelerators and comparison with Trilogy and Clinac 2100 linear accelerators,” J. Appl. Clin. Med. Phys. 14, 273–288 (2013). 15 A. Fogliata, R. Garcia, T. Knöös, G. Nicolini, A. Clivio, E. Vanetti, C. Khamphan, and L. Cozzi, “Definition of parameters forquality assurance of flattening filter free (FFF) photon beams in radiation therapy,” Med. Phys. 39, 6455–6464 (2012). 16 G. Nicolini, A. Clivio, E. Vanetti, H. Krauss, P. Fenoglietto, L. Cozzi, and A. Fogliata, “Evaluation of an aSi-EPID with flattening filter free beams: Applicability to the GLAaS algorithm for portal dosimetry and first experience for pre-treatment QA of RapidArc,” Med. Phys. 40, 111719 (9pp.) (2013). 17 G. G. Ezzell, J. W. Burmeister, N. Dogan, T. J. LoSasso, J. G. Mechalakos, D. Mihailidis, A. Molineu, J. R. Palta, C. R. Ramsey, B. J. Salter, J. Shi, P. Xia, N. J. Yue, and Y. Xiao, “IMRT commissioning: Multiple institution planning and dosimetry comparisons, a report from AAPM Task Group 119,” Med. Phys. 36, 5359–5373 (2009). 18 See supplementary material at http://dx.doi.org/10.1118/1.4861817 for Tables S-I and S-II.
