Radiotherapy Elsevier RADION
and Oncology,
17 (1990) 141-151
141
00661
Quality assurance in radiotherapy 1. Entrance dose measurements, G. Leunens,
J. Van Dam,
Department of Radiotherapy,
A. Dutreix
by in vivo dosimetry. a reliable procedure and E. van der Schueren
University Hospital St. Rafagl, Leuven. Belgium
(Received 21 April 1989, revision received 20 June 1989, accepted
Key words: Silicon diodes; In vivo dosimetry;
21 August 1989)
Quality assurance
Summary Entrance dose measurements were performed with semiconductor detectors on patients treated for head and neck and brain tumors with a 6 MV X-ray beam. A total number of 554 treatment set-ups were measured. The results showed a gaussian distribution with a mean value of 97.8% and a standard deviation of 2.8%. A systematic error of 2.2% on the mean value was shown to be due to a systematic deficiency in the algorithm used in the planning system and to a systematic error in the application of the dosimetry protocol. Two treatment techniques were identified leading to an erroneous dose delivery. Finally, large deviations (more than 2 S.D.) of the measured dose from the expected dose were detected in 3% of the measured treatment set-ups, the sources of the errors could in all cases be identified and eliminated in the further treatment sessions. This study demonstrated the reliability of the use of semiconductor detectors for in vivo dosimetry and its usefulness as part of a departmental quality assurance program.
Introduction Shukovsky [ 191, and Herring and Compton [7] demonstrated the narrow relationship between probability of local tumor control or normal tissue injury and the total absorbed dose. Therefore, the uncertainty associated with dose delivery should -Address for
correspondence: G. Leunens, Department of Radiotherapy, University Hospital St. Rafael, Capucijnenvoer 33, 3000 Leuven, Belgium.
0167-8140/90/$03.50
0 1990 Elsevier Science Publishers
be less than & 3.5%, expressed as one relative standard deviation [ 121. From calculations of Goitein [ 51 the 5% accuracy requirement as proposed by the ICRU [ 81 should be considered as a 1.5 S.D. [12]. Each step involved in the planning or accomplishment of the treatment can contribute to the total uncertainty in the absorbed dose delivered to the patient [ 1,4,9,20]. An ultimate check of absorbed dose is only possible by means of in vivo dosimetry. This technique is useful as an overall
B.V. (Biomedical
Division)
142 check of basic dosimetry, treatment unit parameters, planning and calculation methods and the daily set-up of the patient. In vivo dosimetry has already a long history. In 1932, Sievert performed routinely patient dose measurements with small ionisation chambers. Thermoluminescence dosemeters for in vivo dosimetry were introduced in routine therapy in the 1960s [2,3,11,18]. Today, there is a growing interest for semiconductor in vivo dosimetry, mainly under the impulse of the work of Rikner [6,16,17]. The main advantage of using semiconductor detectors is that there is no delay between irradiation and results, allowing a direct check of all treatment parameters when an error in dose delivery is detected. This makes it possible to identify the sources of the error, and to correct them immediately. In this study, the calibration method and the physical characteristics of the semiconductor detectors are described and the results of entrance dose measurements performed with semiconductor detectors on patients, treated for head and neck and brain malignancies, are discussed.
Material and methods The use of semiconductor detector in vivo dosimetry for quality assurance in external beam radiotherapy has been evaluated for several months. Entrance dose measurements have been performed weekly on patients treated for head and neck and brain malignancies on a 6 MV linear accelerator. The 6 MV X-ray beam was produced by a Mevatron 6700 Siemens linear accelerator. The treatment unit is supplied with an automatic verification system (Mevamatic-Siemens) to check treatment set-up parameters such as field size, wedges, gantry and collimator rotation. The treatment couch parameters are not automatically checked.
I. Material 1.a. Semiconductor detectors P-type EDP-10 semiconductor detectors were used in connection with a DPD-6 electrometer (Therados). P-type silicon diodes are more resistant to radiation damage than n-type silicon diodes; the sensitivity drop of the diode response with accumulated dose is less pronounced and the signal of p-type silicon diodes shows a linear dependence on dose rate [ 161. The build-up cap material of the EDP-10 diodes is made of 0.75 mm steel and 4.05 mm PVC, the water-equivalent thickness of the cap is about 10 mm. When used for entrance dose measurements, the build-up cap of the diodes has to be relevant for the depth of dose maximum for the beam energy used in order to achieve conditions of electronic equilibrium. EDP-10 silicon diodes can only be used for 6 MV X-ray beams with depth of dose maximum of 15 mm of water, when relevant corrections are applied for the lack of electronic equilibrium. 1.b. Ionisation chamber The ionisation chamber used for calibration of the semiconductor detectors was a 0.6 cm3 NE 2505/3B thimble chamber connected to a 2500/3 Ionex dosemeter (Nuclear Enterprises). II. Determination of absorbed dose with semiconductor detectors 1I.a. Calibrationprocedure A calibration is necessary in order to determine the calibration factor for each individual diode. The calibration was performed with the diodes positioned on the surface of a polystyrene phantom at the center of a 10 x 10 cm2 field at S.S.D. 100 cm. The signal of the diode was compared to the absorbed dose determined with the ionisation chamber, positioned at the field center, at the depth of dose maximum: 15 mm, defined as the entrance dose. The calibration factor (F,,) was then determined as the ratio of the absorbed dose deter-
143 mined with the ionisation chamber (IC) and the semiconductor (SC) signal in reference conditions. absorbed Fcal =
dose IC
CF lOI--
1
oo--
signal SC
Therefore, the measured dose in reference conditions is equal to: signal x F_,.
0 99 --
0 98 --
1I.b. Correctionfactors The influence of collimator opening, S.S.D., the presence of wedges and trays on the semiconductor signal was investigated and the respective correction factors (CFcO1, CFsSD, CFwedge, CFtray) were determined. They were normalised to a 10 x 10cm2 field at S.S.D. 100cm. The results are shown in Figs. 1 and 2 and Tables I and II. After determination of the correction fac-
80
85
90 95 100 SSD IN cm
105
110
115
120
Fig. 2. Correction factors as a function of S.S.D. The correction factors are determined as the ratio of the reading of the ionisation chamber and the reading of the semiconductor detector for a 10 x 10 cm* collimator opening at the given S.S.D. and normalised to the same ratio ofthe reference field. Curves 1 to 4 correspond to four different diodes.
tors (CF), the semiconductor signal can be converted in measured dose for the different treatment conditions met in clinical practice.
CF 101 -.
Measured
dose = signal x Fca, x
(Cl;coi x CF,s,
1.00 --
0.99 --
TABLE Correction Wedge
0 98 -.
5
10
15
20
25
30
“C” IN CM
Fig. 1. Correction factors as a function of field size. The correction factors are determined as the ratio of the reading of the ionisation chamber and the reading of the semiconductor detector for the given field size at S.S.D. 100 cm normalised to the same ratio for the reference field (10 x 10 cm2 at S.S.D. 100 cm). “C-is the side of the square field at 100 cm. Curves 1 to 4 correspond to four different diodes.
15” 30” 45”
x C&edge x CFtraJ
I factors for wedges. Diode identification 1
2
3
4
1.015 1.017 1.025
1.009 1.011 1.017
1.007 1.010 1.015
1.007 1.010 1.015
The correction factors (CF) are determined as the ratio of the reading of the ionisation chamber and the reading of the semiconductor detector for the wedged field (15”, 30” and 45” wedge) divided by the same ratio for the open field R R 25 wedge/ ;;” no wedge. The correction R SC SC mined for four different diodes.
factors are deter-
144 TABLE
II
Correction “C” (in cm)
5 10 15 20 25 30
factors for trays. Diode identification 1
2
3
4
1 1 1 0.995 0.995 0.995
1 1 0.995 0.995 0.995 0.990
1 1 1 0.995 0.995 0.990
1 1 1 0.995 0.995 0.990
The correction factors are determined as the ratio of the reading of the ionisation chamber and the reading of the semiconductor detector for a given field size with the tray in the beam divided by the same ratio for the open field (see Table I). “C” is the side of the square field at 100 cm. The correction factors are determined for four different diodes.
S.S.D. decreases, the number of contaminating electrons and low energy photons able to reach the semiconductor is larger and the ratio of the ionisation chamber reading and the semiconductor reading decreases. When beam modifying devices such as wedges and trays are used, not only the beam contamination but also the dose rate is modified and a correction factor has to be applied. The correction factors for wedges or trays placed in the beam, are listed in Tables I and II, respectively. The variation of the wedge correction factor with field size was less than 0.5% and has been neglected. The correction factor for the tray varied between 0 and 1% depending on field size and it has been applied. III. Determination of the expected dose
The signal variation in the range of temperatures and dose rates used in clinical practice was not significant for open fields and therefore the relative correction factors have been neglected. As the silicon diodes were always used on a surface perpendicular to the beam axis, we have checked that the correction factors for the directional dependence of the diode signal could be neglected. 11.~. Discussion As the signal of the detector is proportional to the electron fluence at the depth of the sensitive part of the semiconductor, the ratio of the ionisation chamber reading and the semiconductor reading depends on the collimator size (Fig. 1). For large collimator sizes, the contamination of the primary beam due to electrons and low energy photons born in the machine head, increases the surface dose and decreases the depth of the maximum dose. As a result, the thickness of the build-up cap of the diodes, which was sufficient to assure electronic equilibrium for large field sizes, was not sufficient for the smaller ones and a different correction factor has been applied depending on the collimator size. The variation of the correction factor as a function of S.S.D. is shown in Fig. 2. When the
The expected dose (defined as the dose at the depth of dose maximum) was calculated from the prescribed tumor dose with the algorithms of the treatment planning system. IV. Measurement procedure on patients The measurements were performed on a fixed day once a week. One of the authors (G. Leunens) was responsible for the positioning of the semiconductor detectors and the reading of the semiconductor signal. The treatment set-ups were performed as usual by the technicians and after completion, the diodes were positioned on the skin of the patients in the center of the irradiation field. After irradiation, the measured dose was calculated immediately as a percentage value of the expected dose. When discrepancies of k 5 y0 were encountered, a check of the treatment set-up was performed with the patient still on the treatment couch in treatment position. About 50 fields were measured each week. The supplementary set-up time due to the achievement of the measurement procedure was about 20 min on a whole treatment day (about 0.5 min per treatment set-up) and was acceptable as compared to the total set-up time.
145 Results The results were plotted in histograms as the frequency distribution of the ratio of the measured dose and the expected dose (MD/ED) in percentage. N was the number of treatment set-ups measured, the mean value (X) and one standard deviation (S .D.) were calculated. The frequency distribution of all the measures has been studied first. In sections II and III separate groups of patients have been investigated to detect the groups for which the uncertainty was larger or for which a systematic error appeared. I. Overall results of the entrance dose measurements
Entrance dose measurements were performed on a total number of 554 treatment set-ups (23 1 treatment fields). The 68 patients included in the study were patients treated for head and neck and brain malignancies. The results of the entrance dose measurements, obtained with semiconductor detectors, showed a distribution with one relative standard deviation of 2.75 y0 and a mean value of 97.8% (Fig. 3). The discrepancy between the 100 i
80 t
m
0
SD= 2.75%
:
.-_
MD*ED
Fig. 3. Overall results. The overall results of all the patients are plotted in the histogram as the frequency distribution (total number N = 554) ofthe ratio ofmeasured and expected dose (MD/ED) as a percentage. The overall uncertainty, expressed as 1 SD. is 2.75% and the mean value (X) is 97.8”/,. The discrepancy between the measured and the expected mean value is 2.2%.
measured and the expected mean value was thus 2.2%. The comparison with a gaussian distribution showed a dissymmetry of the histogram. 1.a. The systematic error on the mean value The 2.2 y0 systematic error on the mean value was investigated. It was due to inaccuracies in the algorithm used for the dose calculation with the treatment planning system (1) and to an inaccuracy on the absorbed dose determination with the ionisation chamber in the region of the maximum build-up depth (2). (1) Scatter defects due to the use of shielding blocks or when part of the field is in air, were not taken into account by the computer software. Phantom measurements were performed to study the importance of the scatter defects in the 6 MV X-ray beam. The overestimation on the expected dose in the field center varied between 0.5 and l%, depending on the extent of the shielding blocks and the missing tissue. The expected dose was corrected and recalculated with new algorithms taking into account scatter defects (Bridier, pers. commun.). (2) The Dutch dosimetry protocol [ 131, includes the application of a displacement correction factor for the ionisation chamber as a function of the Quality Index of the beam (0.9875 in the 6 MV X-ray beam of quality index = 0.67). The displacement correction factor has to be applied when measurements are performed on the exponential part of the depth-dose curves in order to correct for displacement between the effective measurement point and the center of the ionisation chamber. This factor should not have been applied at depth of dose maximum due to the shape of the depth-dose curve. The application of the displacement correction factor at depth of dose maximum has led to an underestimation of the measured dose of 1.25%. This error should have been avoided by calibrating the diodes with the ionisation chamber positioned at the reference depth of 5 cm. However, the estimation of the entrance dose is necessary for the determination of the patient transmission: the ratio of the exit
146
1;
A. / N = 554
z 60.3 P f l&l--
1
1
1
= 100.6%
SD=
2.95 %
B.lNz474
1
Fig. 4. Overall results corrected for the systematic error. (A) This histogram shows the overall results after correction of the systematic error due to inaccuracies in the calculation algorithm and in the absorbed dose determination with the ionisation chamber in the region of depth of dose maximum. The mean value (X) is 100.6% and the S.D. is 2.95%. (B) The black histogram (N = 474, X = 99.69x, S.D. = 2.3%) shows the frequency distribution without the patient groups, treated with techniques in which larger deviations are detected. Discrepancies between measured and expected dose of more than 2 SD. are considered as large deviations.
and entrance doses (part 2 of present paper, in preparation). The results of the measurements were corrected for both these errors (Fig. 4, X = 100.6%, S.D. = 2.95 %). All the following histograms have been calculated from corrected values. 1.b. Large deviations of the measured dose from the expected dose
Although the standard deviation was reasonably small, large errors have been detected in 3 y0 of the measured treatment set-ups. A large error was defined as a discrepancy between measured and expected dose larger than 5 %, in agreement with the ICRU recommendations (report 24). In 17 of the 554 (3%) measured treatment set-ups such large errors were detected (sections II.c., 111.~. and 1II.d.). I.c. Comparison between the measurements
per-
formed on open and wedged&Ids
The distribution observed for wedged fields was not larger than for open fields. A total number of
290 treatment set-ups without wedges and 264 treatment set-ups with wedges have been measured. The standard deviations were 3.34 and 2.3 1 %, respectively. This was in contradiction with the results of the entrance dose measurements obtained with silicon diodes by Nilsson et al. [ 141, who had found a larger spread for wedged fields. They studied the influence of the displacement of the diodes in wedged fields: a displacement of + 1.5 cm in a 45 ’ wedged field led to a variation of the measured dose of k 10% compared to the dose measured on the central axis. This demonstrated clearly that the positioning of the diodes in wedged fields is very critical and that interpretation of the results is impossible when the diodes are misaligned. This was the reason why much care has been especially taken for the correct positioning of the diodes, which was reflected in the small S.D. observed for the entrance dose measurements performed on wedged fields.
1.d. Reliability of the entrance dose measurements The reliability of the results was checked by phantom measurements simulating the treatment conditions including wedges, shielding blocks etc. Measurements have been performed with both silicon diodes and ionisation chamber. A total number of 87 different treatment set-ups have been simulated on a phantom for measurement. The agreement between the dose determined with the semiconductor and the dose measured with the ionisation chamber was always better dan 1% and was within 0.5% in 78% of the simulated set-ups. II. Results of the entrance dose measurements
on patients treatedfor head and neck malignancies
1I.a. Treatment technique The primary tumor and adjacent nodal regions were irradiated with two lateral fields with an isocentric technique. The lower cervical and the supraclavicular nodes were irradiated with a cervical anterior field at S.S.D. 100 cm.
147 Exception was made for patients with an extremely short neck or when the primary tumor was situated in the junction zone of the lateral fields and the cervical anterior field. Two extended lateral fields were then used (isocentric technique) and the treatment couch was rotated to avoid the homolateral shoulder and to include the lower cervical and supraclavicular nodes in the margins of the lateral fields. All patients were immobilized with individual plastic masks. 1I.b. Overall results A total number of 413 entrance dose measurements has been performed on 47 patients (179 treatment fields) treated for head and neck malignancies (Fig. 5, black and white). The mean value was 100.6% and the S.D. was 2.44%. Large errors were detected in 2.2% (9/413) of the measurements. 11.~. Results of lateral fields
without treatment couch rotation and of cervical anterior fields
364 measurements have been performed on lateral fields without treatment couch rotation and on cervical anterior fields. The mean value was
t/.; 90
95
1001 .
165
-
%
MD/ED
Fig. 5. Patients treated for head and neck malignancies alone. (A) The histogram of this patient group (N = 413) shows a frequency distribution of MD/ED with a mean value (X) of 100.6% and a S.D. of 2.44%. The distribution is asymmetric with a positive top. (B)The black histogram shows the frequency distribution without the patients, treated with large lateral fields and a treatment couch rotation, responsible for the asymmetry of the total histogram (N = 364, X = 99.76% and S.D. = 2.3%).
99.76%
and the S.D. was 2.3% (Fig. 5, black). Large deviations on lateral treatment fields have been detected in five measured treatment set-ups. The overdosage of 9 % was due to a positioning mistake of the patient on the treatment couch. The underdosages of 6% and 7 % were due to inaccuracies in the external contour of the patient, for instance, the contour diameter used for the treatment planning was 8.5 cm, the measured patient diameter being 6 cm. An underdosage of 6% was measured during a treatment session where repeated automatic treatment interruption of the linear accelerator occurred due to unsteady dose rate. The positioning and contour mistakes were immediately rectified after detection and the next entrance dose measurements showed good agreement between delivered and expected dose. Although the results of the entrance dose measurements performed on cervical anterior fields were good (N = 64, X = 99.93x, S.D. = 2.5%) it has to be stressed that the positioning of the silicon diodes was often difficult because several of these patients had a tracheostomy which was situated near the field center. One large deviation was detected leading to an overdosage of 12 % . This error was due to a mistake in the set-up of the treatment parameters: an isocentric technique was used while the prescribed technique was a beam at fixed S.S.D. (100 cm). The error in technique was rectified. 1I.d. Lateralfields with treatment couch rotation A total number of 49 entrance dose measurements has been performed on patients treated with lateral fields and a treatment couch rotation. The frequency distribution of the ratio MD/ED was plotted in white on Fig. 5 and separately on Fig. 6. The mean value was 103 %, the S.D. was 2.3 %. The systematic overdosage was due to nonstandard geometric conditions including the rotation of the treatment couch, and the irregular irradiated surface. This patient group was responsible for the asymmetric distribution of the overall
148 15 --
N = 49 x
90
95
8,’
100
105 *
: 102.92 %
r SD=
-_
1,:::
2.3
%
110
%
MD/ED
Fig. 6. Lateral fields with treatment couch rotation. The histogram shows the frequency distribution (N = 49) of MD/ED for lateral fields with treatment couch rotation. The mean value (X) is 102.92 y0 and the SD. is 2.3 %. The deviation on the mean value is due to non-standard geometric treatment conditions, enhancing the entrance dose.
results of the entrance dose measurements performed on patients treated for head and neck malignancies (Fig. 5, black and white). The overall frequency distribution was the sum of two gaussian distributions (Fig. 5, black, and Fig. 6). III. Results for brain tumors andpancranial
irradi-
ation
1II.a. Treatment technique Patients with brain tumors were treated with two or more beams using an isocentric technique. Large shielding blocks were often placed in the beam to protect healthy brain tissue as much as possible. Pancranial irradiation was performed with two lateral beams. Both isocentric and fixed S. S.D. beam techniques were used simultaneously in the department. A large proportion of the field was often outside the skull, leading to scatter defects due to missing tissue. All patients were immobilized with individual plastic masks.
5 f
90
95
1Ool .
r165
MD/ED
110
135
Fig. 7. Overall results for brain tumors and pancranial irradiation. (A) The histogram shows a frequency distribution (N = 141) with a mean value (X) of 100.6% and a S.D. of 4.09%. The overall uncertainty is larger than the uncertainty in the head and neck patient group. (B) The black histogram refers to the brain tumors without pancranial irradiations (N = 111, X = 99.98x, S.D. = 2.32%).
irradiation were plotted in Fig. 7. The total number of the measurements performed was 141, the mean value was 100.6% and the SD. was 4.09%. The frequency spread of the results was broader than for head and neck malignancies. The systematic error due to inaccuracies in the algorithm used for dose calculation was more important for this patient group because important shielding blocks were often used to protect the healthy brain tissue and because of scatter defects. Phantom measurements simulating the treatment conditions have been performed and showed an overestimation of the entrance dose at the field center of l-1.5% compared to 0.5% in head and neck treatments. Large errors have been detected in 5.7% (8/ 14 1) of the measured treatment set-ups, instead of 2.2% in the head and neck set-ups.
1II.b. Overall results of the entrance dose measurements
The ratios MD/ED for patients treated for brain tumors and for patients treated with pancranial
%
111.~. Results for brain tumors alone A total number of 111 entrance dose measurements has been performed with semiconductor
149 detectors on patients treated for brain tumors. The frequency distribution of the ratio MD/ED was plotted in Fig. 7 (black). The mean value was 99.98x, the SD. was 2.32% instead of 4% for the overall estimation of the uncertainty for all patients irradiated on the brain, including patients treated with pancranial irradiation. Large errors have been detected in 3 of the 111 treatment set-ups measured. The 8 % over&age was due to an inaccurate positioning of the patient on the treatment couch, decreasing the S.S.D. with 2 cm. The 6% underdosages were due to the use of very large shielding blocks, covering about 45% of the total field area. The margins of the shielding blocks were rather close to the field center, decreasing the entrance dose measured on the field center due to the penumbra of the shielding blocks. This was confirmed with phantom measurements simulating the treatment conditions. 1II.d. Results on patients treated with pancranial irradiation
Figure 8 showed the results of entrance dose measurements performed on patients treated with pancranial irradiation. A total number of 30 treatment set-ups has been measured. No gaussian distribution was found, the mean value was 102.9%.
Fig. 8. Results for pancranial irradiation alone. (A) The histogram shows the frequency distribution of MD/ED for both isocentric and fixed S.S.D. techniques (N = 30). Large deviations are detected: 13/30 results show deviations larger than f 3%. (B) The black histogram shows the distribution for the pancranial irradiations performed with isocentric techniques. In this patient group only 4/16 deviations are larger than k 3 % instead of 9/14 in the patient group treated with a fixed S.S.D.
Large deviations have been detected in the treatment set-ups using fixed S.S.D. Inaccurate settings of the S.S.D. were leading to an overdosage of 6 % and underdosages of 5 and 11% . The overdosage of 35% was due to a mistake in treatment set-up: the right lateral field was irradiated at a S.S.D. of 100 cm, the gantry was then rotated as for an isocentric irradiation to treat the left lateral field (the irradiation was performed at a S.S.D. of 86 cm instead of the prescribed 100 cm). The results for the patients treated with pancranial irradiations, showed a smaller deviation of the ratio MD/ED with an isocentric technique (Fig. 8, black) than with a fixed S. S.D. technique (Fig. 8, white). For fixed S.S.D., 9/14 measures showed errors larger than 3% and 3/14 larger than 5 y0 as compared with 3/ 16 and l/16, respectively, for isocentric techniques. This patient group was responsible for the larger uncertainty on the overall results of the entrance dose measurements on brain patients as compared to the results of the head and neck patient group.
Conclusions It has been shown that in vivo dosimetry performed with semiconductor detectors is a reliable method for patient dose control (section 1.d.). A high precision can be obtained when the correction factors for each parameter of influence on the diode response are carefully determined and applied to convert the semiconductor signal in absorbed dose (see section 1.a.). Much care has to be taken for the correct positioning of the semiconductor detectors at the field center, especially when measurements are performed in wedged fields (section I.c.). The main advantages of using semiconductor detectors for in vivo dosimetry is that results are available in real time allowing the detection of the sources of error due to the treatment set-up. Semiconductor in vivo dosimetry is also less time consuming as compared to TLD in vivo dosimetry. The standard deviation on the results of the
150 entrance dose measurements was reasonably small (2.75%) as compared with the results of Nilsson et al. [ 141: 4% at 6 MV for open fields and 6% for wedged fields. This was due to several reasons: the excellent stability of the treatment machine, the automatic verification system, the use of individual plastic masks for immobilisation, the good accuracy in daily treatment set-up and the achievement of the measurements by one selected person. The check of the overall treatment system led to the identification of two small, systematic errors: an inaccuracy in the algorithm used for dose calculation with the treatment planning system and an error in the determination of the absorbed dose with the ionisation chamber in the region of depth of maximum dose (section 1.a.). These inaccuracies have been corrected, improving the quality of the treatment chain. Two treatment techniques with large uncertainties have been identified: (1) The patients treated with large lateral fields and a treatment couch rotation for whom a systematic overdosage occurred due to non-standard geometric conditions affecting the dose calculation (section 1I.d.). (2) The patients treated with pancranial irradiation for whom a large frequency distribution was found due to the confusion between isocentric and fixed S.S.D. techniques in daily treatment set-up. This could probably be prevented with a uniform technique within a department for similar treatment fields. The results of the measurements performed on treatment set-ups using an isocentric technique were better than the results of the fixed S.S.D. beam measurements, proving the superiority in the precision in dose delivery that could be achieved with isocentric treatment setups (section 1II.d.). Large errors have been detected in 3% of the measured treatment set-ups. The sources of the errors could in all cases be identified and corrected (see Results), avoiding large under- or overdosages. The detection of such errors is espe-
cially critical for individual patients. These errors are directly related to treatment failures and previously “unexpected” complications. The present work has shown that periodic in vivodosimetryis useful as a check of the quality of the whole treatment chain. The quality of several treatment techniques can be evaluated and improved when needed, sytematic errors can be detected and corrected. However, the check of the dose delivered to a given patient can be performed only by systematic in vivodosimetry.Large errors can be detected and the sources of error can be identified and corrected decreasing the uncertainty on the total dose delivered to each individual patient. In vivo dosimetry has been proved to be very useful as part of a departmental quality assurance program.
Acknowledgments We wish to thank A. Rijnders for his support and J. Verstraete for his enthusiastic cooperation in this study. The authors greatly appreciate the secretarial work by Mrs. L. Minnen and the assistance in computer programming by Mr. J. Van Kelecom. This work was supported by grants from the Belgian Work Against Cancer.
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13 Mijnheer, B. J., Aalbers, A. H.L., Visser, A. G. and Wittkamper, F. W. Consistency and simplicity in the determination of absorbed dose to water in high-energy photon beams: A new code of practice. Radiother. Oncol. 7: 371-384, 1986. 14 Nilsson, B., Ruden, B. I. and Sorcini, B. Characteristics of silicon diodes as patient dosemeters in external beam therapy. Radiother. Oncol. 11: 279-288, 1988. 15 Noel, A. and Aletti, P. Les mesures in vivo systtmatiques. A propos de 800 controles. Proceeding of XXVIieme Congrts CFPH Vittel: 124-136, 1987. 16 Rikner, G. Silicon diodes as detectors in relative dosimetry of photon, electron and proton radiation fields. Thesis, Uppsala University, Sweden, 1983. 17 Rikner, G. and Grusell, E. General specifications for silicon semiconductors for use in radiation dosimetry. Phys. Med. Biol. 32: 1109-1117, 1987. 18 RudCn, B. I. Evaluation of the clinical use of TLD. Acta Radiol. Ther. Phys. Biol. 15: 447-464, 1976. 19 Shukovsky, L. J. Dose, time, volume relationships in squamous cell carcinoma of the supraglottic larynx. Am. J. Roentgenol. 108: 27-29, 1970. 20 Svensson, H. Quality assurance in radiation therapy: physical aspects. Int. J. Radiat. Oncol. Biol. Phys. 10: 59-65. 1984.
and Oncology,
17 (1990) 141-151
141
00661
Quality assurance in radiotherapy 1. Entrance dose measurements, G. Leunens,
J. Van Dam,
Department of Radiotherapy,
A. Dutreix
by in vivo dosimetry. a reliable procedure and E. van der Schueren
University Hospital St. Rafagl, Leuven. Belgium
(Received 21 April 1989, revision received 20 June 1989, accepted
Key words: Silicon diodes; In vivo dosimetry;
21 August 1989)
Quality assurance
Summary Entrance dose measurements were performed with semiconductor detectors on patients treated for head and neck and brain tumors with a 6 MV X-ray beam. A total number of 554 treatment set-ups were measured. The results showed a gaussian distribution with a mean value of 97.8% and a standard deviation of 2.8%. A systematic error of 2.2% on the mean value was shown to be due to a systematic deficiency in the algorithm used in the planning system and to a systematic error in the application of the dosimetry protocol. Two treatment techniques were identified leading to an erroneous dose delivery. Finally, large deviations (more than 2 S.D.) of the measured dose from the expected dose were detected in 3% of the measured treatment set-ups, the sources of the errors could in all cases be identified and eliminated in the further treatment sessions. This study demonstrated the reliability of the use of semiconductor detectors for in vivo dosimetry and its usefulness as part of a departmental quality assurance program.
Introduction Shukovsky [ 191, and Herring and Compton [7] demonstrated the narrow relationship between probability of local tumor control or normal tissue injury and the total absorbed dose. Therefore, the uncertainty associated with dose delivery should -Address for
correspondence: G. Leunens, Department of Radiotherapy, University Hospital St. Rafael, Capucijnenvoer 33, 3000 Leuven, Belgium.
0167-8140/90/$03.50
0 1990 Elsevier Science Publishers
be less than & 3.5%, expressed as one relative standard deviation [ 121. From calculations of Goitein [ 51 the 5% accuracy requirement as proposed by the ICRU [ 81 should be considered as a 1.5 S.D. [12]. Each step involved in the planning or accomplishment of the treatment can contribute to the total uncertainty in the absorbed dose delivered to the patient [ 1,4,9,20]. An ultimate check of absorbed dose is only possible by means of in vivo dosimetry. This technique is useful as an overall
B.V. (Biomedical
Division)
142 check of basic dosimetry, treatment unit parameters, planning and calculation methods and the daily set-up of the patient. In vivo dosimetry has already a long history. In 1932, Sievert performed routinely patient dose measurements with small ionisation chambers. Thermoluminescence dosemeters for in vivo dosimetry were introduced in routine therapy in the 1960s [2,3,11,18]. Today, there is a growing interest for semiconductor in vivo dosimetry, mainly under the impulse of the work of Rikner [6,16,17]. The main advantage of using semiconductor detectors is that there is no delay between irradiation and results, allowing a direct check of all treatment parameters when an error in dose delivery is detected. This makes it possible to identify the sources of the error, and to correct them immediately. In this study, the calibration method and the physical characteristics of the semiconductor detectors are described and the results of entrance dose measurements performed with semiconductor detectors on patients, treated for head and neck and brain malignancies, are discussed.
Material and methods The use of semiconductor detector in vivo dosimetry for quality assurance in external beam radiotherapy has been evaluated for several months. Entrance dose measurements have been performed weekly on patients treated for head and neck and brain malignancies on a 6 MV linear accelerator. The 6 MV X-ray beam was produced by a Mevatron 6700 Siemens linear accelerator. The treatment unit is supplied with an automatic verification system (Mevamatic-Siemens) to check treatment set-up parameters such as field size, wedges, gantry and collimator rotation. The treatment couch parameters are not automatically checked.
I. Material 1.a. Semiconductor detectors P-type EDP-10 semiconductor detectors were used in connection with a DPD-6 electrometer (Therados). P-type silicon diodes are more resistant to radiation damage than n-type silicon diodes; the sensitivity drop of the diode response with accumulated dose is less pronounced and the signal of p-type silicon diodes shows a linear dependence on dose rate [ 161. The build-up cap material of the EDP-10 diodes is made of 0.75 mm steel and 4.05 mm PVC, the water-equivalent thickness of the cap is about 10 mm. When used for entrance dose measurements, the build-up cap of the diodes has to be relevant for the depth of dose maximum for the beam energy used in order to achieve conditions of electronic equilibrium. EDP-10 silicon diodes can only be used for 6 MV X-ray beams with depth of dose maximum of 15 mm of water, when relevant corrections are applied for the lack of electronic equilibrium. 1.b. Ionisation chamber The ionisation chamber used for calibration of the semiconductor detectors was a 0.6 cm3 NE 2505/3B thimble chamber connected to a 2500/3 Ionex dosemeter (Nuclear Enterprises). II. Determination of absorbed dose with semiconductor detectors 1I.a. Calibrationprocedure A calibration is necessary in order to determine the calibration factor for each individual diode. The calibration was performed with the diodes positioned on the surface of a polystyrene phantom at the center of a 10 x 10 cm2 field at S.S.D. 100 cm. The signal of the diode was compared to the absorbed dose determined with the ionisation chamber, positioned at the field center, at the depth of dose maximum: 15 mm, defined as the entrance dose. The calibration factor (F,,) was then determined as the ratio of the absorbed dose deter-
143 mined with the ionisation chamber (IC) and the semiconductor (SC) signal in reference conditions. absorbed Fcal =
dose IC
CF lOI--
1
oo--
signal SC
Therefore, the measured dose in reference conditions is equal to: signal x F_,.
0 99 --
0 98 --
1I.b. Correctionfactors The influence of collimator opening, S.S.D., the presence of wedges and trays on the semiconductor signal was investigated and the respective correction factors (CFcO1, CFsSD, CFwedge, CFtray) were determined. They were normalised to a 10 x 10cm2 field at S.S.D. 100cm. The results are shown in Figs. 1 and 2 and Tables I and II. After determination of the correction fac-
80
85
90 95 100 SSD IN cm
105
110
115
120
Fig. 2. Correction factors as a function of S.S.D. The correction factors are determined as the ratio of the reading of the ionisation chamber and the reading of the semiconductor detector for a 10 x 10 cm* collimator opening at the given S.S.D. and normalised to the same ratio ofthe reference field. Curves 1 to 4 correspond to four different diodes.
tors (CF), the semiconductor signal can be converted in measured dose for the different treatment conditions met in clinical practice.
CF 101 -.
Measured
dose = signal x Fca, x
(Cl;coi x CF,s,
1.00 --
0.99 --
TABLE Correction Wedge
0 98 -.
5
10
15
20
25
30
“C” IN CM
Fig. 1. Correction factors as a function of field size. The correction factors are determined as the ratio of the reading of the ionisation chamber and the reading of the semiconductor detector for the given field size at S.S.D. 100 cm normalised to the same ratio for the reference field (10 x 10 cm2 at S.S.D. 100 cm). “C-is the side of the square field at 100 cm. Curves 1 to 4 correspond to four different diodes.
15” 30” 45”
x C&edge x CFtraJ
I factors for wedges. Diode identification 1
2
3
4
1.015 1.017 1.025
1.009 1.011 1.017
1.007 1.010 1.015
1.007 1.010 1.015
The correction factors (CF) are determined as the ratio of the reading of the ionisation chamber and the reading of the semiconductor detector for the wedged field (15”, 30” and 45” wedge) divided by the same ratio for the open field R R 25 wedge/ ;;” no wedge. The correction R SC SC mined for four different diodes.
factors are deter-
144 TABLE
II
Correction “C” (in cm)
5 10 15 20 25 30
factors for trays. Diode identification 1
2
3
4
1 1 1 0.995 0.995 0.995
1 1 0.995 0.995 0.995 0.990
1 1 1 0.995 0.995 0.990
1 1 1 0.995 0.995 0.990
The correction factors are determined as the ratio of the reading of the ionisation chamber and the reading of the semiconductor detector for a given field size with the tray in the beam divided by the same ratio for the open field (see Table I). “C” is the side of the square field at 100 cm. The correction factors are determined for four different diodes.
S.S.D. decreases, the number of contaminating electrons and low energy photons able to reach the semiconductor is larger and the ratio of the ionisation chamber reading and the semiconductor reading decreases. When beam modifying devices such as wedges and trays are used, not only the beam contamination but also the dose rate is modified and a correction factor has to be applied. The correction factors for wedges or trays placed in the beam, are listed in Tables I and II, respectively. The variation of the wedge correction factor with field size was less than 0.5% and has been neglected. The correction factor for the tray varied between 0 and 1% depending on field size and it has been applied. III. Determination of the expected dose
The signal variation in the range of temperatures and dose rates used in clinical practice was not significant for open fields and therefore the relative correction factors have been neglected. As the silicon diodes were always used on a surface perpendicular to the beam axis, we have checked that the correction factors for the directional dependence of the diode signal could be neglected. 11.~. Discussion As the signal of the detector is proportional to the electron fluence at the depth of the sensitive part of the semiconductor, the ratio of the ionisation chamber reading and the semiconductor reading depends on the collimator size (Fig. 1). For large collimator sizes, the contamination of the primary beam due to electrons and low energy photons born in the machine head, increases the surface dose and decreases the depth of the maximum dose. As a result, the thickness of the build-up cap of the diodes, which was sufficient to assure electronic equilibrium for large field sizes, was not sufficient for the smaller ones and a different correction factor has been applied depending on the collimator size. The variation of the correction factor as a function of S.S.D. is shown in Fig. 2. When the
The expected dose (defined as the dose at the depth of dose maximum) was calculated from the prescribed tumor dose with the algorithms of the treatment planning system. IV. Measurement procedure on patients The measurements were performed on a fixed day once a week. One of the authors (G. Leunens) was responsible for the positioning of the semiconductor detectors and the reading of the semiconductor signal. The treatment set-ups were performed as usual by the technicians and after completion, the diodes were positioned on the skin of the patients in the center of the irradiation field. After irradiation, the measured dose was calculated immediately as a percentage value of the expected dose. When discrepancies of k 5 y0 were encountered, a check of the treatment set-up was performed with the patient still on the treatment couch in treatment position. About 50 fields were measured each week. The supplementary set-up time due to the achievement of the measurement procedure was about 20 min on a whole treatment day (about 0.5 min per treatment set-up) and was acceptable as compared to the total set-up time.
145 Results The results were plotted in histograms as the frequency distribution of the ratio of the measured dose and the expected dose (MD/ED) in percentage. N was the number of treatment set-ups measured, the mean value (X) and one standard deviation (S .D.) were calculated. The frequency distribution of all the measures has been studied first. In sections II and III separate groups of patients have been investigated to detect the groups for which the uncertainty was larger or for which a systematic error appeared. I. Overall results of the entrance dose measurements
Entrance dose measurements were performed on a total number of 554 treatment set-ups (23 1 treatment fields). The 68 patients included in the study were patients treated for head and neck and brain malignancies. The results of the entrance dose measurements, obtained with semiconductor detectors, showed a distribution with one relative standard deviation of 2.75 y0 and a mean value of 97.8% (Fig. 3). The discrepancy between the 100 i
80 t
m
0
SD= 2.75%
:
.-_
MD*ED
Fig. 3. Overall results. The overall results of all the patients are plotted in the histogram as the frequency distribution (total number N = 554) ofthe ratio ofmeasured and expected dose (MD/ED) as a percentage. The overall uncertainty, expressed as 1 SD. is 2.75% and the mean value (X) is 97.8”/,. The discrepancy between the measured and the expected mean value is 2.2%.
measured and the expected mean value was thus 2.2%. The comparison with a gaussian distribution showed a dissymmetry of the histogram. 1.a. The systematic error on the mean value The 2.2 y0 systematic error on the mean value was investigated. It was due to inaccuracies in the algorithm used for the dose calculation with the treatment planning system (1) and to an inaccuracy on the absorbed dose determination with the ionisation chamber in the region of the maximum build-up depth (2). (1) Scatter defects due to the use of shielding blocks or when part of the field is in air, were not taken into account by the computer software. Phantom measurements were performed to study the importance of the scatter defects in the 6 MV X-ray beam. The overestimation on the expected dose in the field center varied between 0.5 and l%, depending on the extent of the shielding blocks and the missing tissue. The expected dose was corrected and recalculated with new algorithms taking into account scatter defects (Bridier, pers. commun.). (2) The Dutch dosimetry protocol [ 131, includes the application of a displacement correction factor for the ionisation chamber as a function of the Quality Index of the beam (0.9875 in the 6 MV X-ray beam of quality index = 0.67). The displacement correction factor has to be applied when measurements are performed on the exponential part of the depth-dose curves in order to correct for displacement between the effective measurement point and the center of the ionisation chamber. This factor should not have been applied at depth of dose maximum due to the shape of the depth-dose curve. The application of the displacement correction factor at depth of dose maximum has led to an underestimation of the measured dose of 1.25%. This error should have been avoided by calibrating the diodes with the ionisation chamber positioned at the reference depth of 5 cm. However, the estimation of the entrance dose is necessary for the determination of the patient transmission: the ratio of the exit
146
1;
A. / N = 554
z 60.3 P f l&l--
1
1
1
= 100.6%
SD=
2.95 %
B.lNz474
1
Fig. 4. Overall results corrected for the systematic error. (A) This histogram shows the overall results after correction of the systematic error due to inaccuracies in the calculation algorithm and in the absorbed dose determination with the ionisation chamber in the region of depth of dose maximum. The mean value (X) is 100.6% and the S.D. is 2.95%. (B) The black histogram (N = 474, X = 99.69x, S.D. = 2.3%) shows the frequency distribution without the patient groups, treated with techniques in which larger deviations are detected. Discrepancies between measured and expected dose of more than 2 SD. are considered as large deviations.
and entrance doses (part 2 of present paper, in preparation). The results of the measurements were corrected for both these errors (Fig. 4, X = 100.6%, S.D. = 2.95 %). All the following histograms have been calculated from corrected values. 1.b. Large deviations of the measured dose from the expected dose
Although the standard deviation was reasonably small, large errors have been detected in 3 y0 of the measured treatment set-ups. A large error was defined as a discrepancy between measured and expected dose larger than 5 %, in agreement with the ICRU recommendations (report 24). In 17 of the 554 (3%) measured treatment set-ups such large errors were detected (sections II.c., 111.~. and 1II.d.). I.c. Comparison between the measurements
per-
formed on open and wedged&Ids
The distribution observed for wedged fields was not larger than for open fields. A total number of
290 treatment set-ups without wedges and 264 treatment set-ups with wedges have been measured. The standard deviations were 3.34 and 2.3 1 %, respectively. This was in contradiction with the results of the entrance dose measurements obtained with silicon diodes by Nilsson et al. [ 141, who had found a larger spread for wedged fields. They studied the influence of the displacement of the diodes in wedged fields: a displacement of + 1.5 cm in a 45 ’ wedged field led to a variation of the measured dose of k 10% compared to the dose measured on the central axis. This demonstrated clearly that the positioning of the diodes in wedged fields is very critical and that interpretation of the results is impossible when the diodes are misaligned. This was the reason why much care has been especially taken for the correct positioning of the diodes, which was reflected in the small S.D. observed for the entrance dose measurements performed on wedged fields.
1.d. Reliability of the entrance dose measurements The reliability of the results was checked by phantom measurements simulating the treatment conditions including wedges, shielding blocks etc. Measurements have been performed with both silicon diodes and ionisation chamber. A total number of 87 different treatment set-ups have been simulated on a phantom for measurement. The agreement between the dose determined with the semiconductor and the dose measured with the ionisation chamber was always better dan 1% and was within 0.5% in 78% of the simulated set-ups. II. Results of the entrance dose measurements
on patients treatedfor head and neck malignancies
1I.a. Treatment technique The primary tumor and adjacent nodal regions were irradiated with two lateral fields with an isocentric technique. The lower cervical and the supraclavicular nodes were irradiated with a cervical anterior field at S.S.D. 100 cm.
147 Exception was made for patients with an extremely short neck or when the primary tumor was situated in the junction zone of the lateral fields and the cervical anterior field. Two extended lateral fields were then used (isocentric technique) and the treatment couch was rotated to avoid the homolateral shoulder and to include the lower cervical and supraclavicular nodes in the margins of the lateral fields. All patients were immobilized with individual plastic masks. 1I.b. Overall results A total number of 413 entrance dose measurements has been performed on 47 patients (179 treatment fields) treated for head and neck malignancies (Fig. 5, black and white). The mean value was 100.6% and the S.D. was 2.44%. Large errors were detected in 2.2% (9/413) of the measurements. 11.~. Results of lateral fields
without treatment couch rotation and of cervical anterior fields
364 measurements have been performed on lateral fields without treatment couch rotation and on cervical anterior fields. The mean value was
t/.; 90
95
1001 .
165
-
%
MD/ED
Fig. 5. Patients treated for head and neck malignancies alone. (A) The histogram of this patient group (N = 413) shows a frequency distribution of MD/ED with a mean value (X) of 100.6% and a S.D. of 2.44%. The distribution is asymmetric with a positive top. (B)The black histogram shows the frequency distribution without the patients, treated with large lateral fields and a treatment couch rotation, responsible for the asymmetry of the total histogram (N = 364, X = 99.76% and S.D. = 2.3%).
99.76%
and the S.D. was 2.3% (Fig. 5, black). Large deviations on lateral treatment fields have been detected in five measured treatment set-ups. The overdosage of 9 % was due to a positioning mistake of the patient on the treatment couch. The underdosages of 6% and 7 % were due to inaccuracies in the external contour of the patient, for instance, the contour diameter used for the treatment planning was 8.5 cm, the measured patient diameter being 6 cm. An underdosage of 6% was measured during a treatment session where repeated automatic treatment interruption of the linear accelerator occurred due to unsteady dose rate. The positioning and contour mistakes were immediately rectified after detection and the next entrance dose measurements showed good agreement between delivered and expected dose. Although the results of the entrance dose measurements performed on cervical anterior fields were good (N = 64, X = 99.93x, S.D. = 2.5%) it has to be stressed that the positioning of the silicon diodes was often difficult because several of these patients had a tracheostomy which was situated near the field center. One large deviation was detected leading to an overdosage of 12 % . This error was due to a mistake in the set-up of the treatment parameters: an isocentric technique was used while the prescribed technique was a beam at fixed S.S.D. (100 cm). The error in technique was rectified. 1I.d. Lateralfields with treatment couch rotation A total number of 49 entrance dose measurements has been performed on patients treated with lateral fields and a treatment couch rotation. The frequency distribution of the ratio MD/ED was plotted in white on Fig. 5 and separately on Fig. 6. The mean value was 103 %, the S.D. was 2.3 %. The systematic overdosage was due to nonstandard geometric conditions including the rotation of the treatment couch, and the irregular irradiated surface. This patient group was responsible for the asymmetric distribution of the overall
148 15 --
N = 49 x
90
95
8,’
100
105 *
: 102.92 %
r SD=
-_
1,:::
2.3
%
110
%
MD/ED
Fig. 6. Lateral fields with treatment couch rotation. The histogram shows the frequency distribution (N = 49) of MD/ED for lateral fields with treatment couch rotation. The mean value (X) is 102.92 y0 and the SD. is 2.3 %. The deviation on the mean value is due to non-standard geometric treatment conditions, enhancing the entrance dose.
results of the entrance dose measurements performed on patients treated for head and neck malignancies (Fig. 5, black and white). The overall frequency distribution was the sum of two gaussian distributions (Fig. 5, black, and Fig. 6). III. Results for brain tumors andpancranial
irradi-
ation
1II.a. Treatment technique Patients with brain tumors were treated with two or more beams using an isocentric technique. Large shielding blocks were often placed in the beam to protect healthy brain tissue as much as possible. Pancranial irradiation was performed with two lateral beams. Both isocentric and fixed S. S.D. beam techniques were used simultaneously in the department. A large proportion of the field was often outside the skull, leading to scatter defects due to missing tissue. All patients were immobilized with individual plastic masks.
5 f
90
95
1Ool .
r165
MD/ED
110
135
Fig. 7. Overall results for brain tumors and pancranial irradiation. (A) The histogram shows a frequency distribution (N = 141) with a mean value (X) of 100.6% and a S.D. of 4.09%. The overall uncertainty is larger than the uncertainty in the head and neck patient group. (B) The black histogram refers to the brain tumors without pancranial irradiations (N = 111, X = 99.98x, S.D. = 2.32%).
irradiation were plotted in Fig. 7. The total number of the measurements performed was 141, the mean value was 100.6% and the SD. was 4.09%. The frequency spread of the results was broader than for head and neck malignancies. The systematic error due to inaccuracies in the algorithm used for dose calculation was more important for this patient group because important shielding blocks were often used to protect the healthy brain tissue and because of scatter defects. Phantom measurements simulating the treatment conditions have been performed and showed an overestimation of the entrance dose at the field center of l-1.5% compared to 0.5% in head and neck treatments. Large errors have been detected in 5.7% (8/ 14 1) of the measured treatment set-ups, instead of 2.2% in the head and neck set-ups.
1II.b. Overall results of the entrance dose measurements
The ratios MD/ED for patients treated for brain tumors and for patients treated with pancranial
%
111.~. Results for brain tumors alone A total number of 111 entrance dose measurements has been performed with semiconductor
149 detectors on patients treated for brain tumors. The frequency distribution of the ratio MD/ED was plotted in Fig. 7 (black). The mean value was 99.98x, the SD. was 2.32% instead of 4% for the overall estimation of the uncertainty for all patients irradiated on the brain, including patients treated with pancranial irradiation. Large errors have been detected in 3 of the 111 treatment set-ups measured. The 8 % over&age was due to an inaccurate positioning of the patient on the treatment couch, decreasing the S.S.D. with 2 cm. The 6% underdosages were due to the use of very large shielding blocks, covering about 45% of the total field area. The margins of the shielding blocks were rather close to the field center, decreasing the entrance dose measured on the field center due to the penumbra of the shielding blocks. This was confirmed with phantom measurements simulating the treatment conditions. 1II.d. Results on patients treated with pancranial irradiation
Figure 8 showed the results of entrance dose measurements performed on patients treated with pancranial irradiation. A total number of 30 treatment set-ups has been measured. No gaussian distribution was found, the mean value was 102.9%.
Fig. 8. Results for pancranial irradiation alone. (A) The histogram shows the frequency distribution of MD/ED for both isocentric and fixed S.S.D. techniques (N = 30). Large deviations are detected: 13/30 results show deviations larger than f 3%. (B) The black histogram shows the distribution for the pancranial irradiations performed with isocentric techniques. In this patient group only 4/16 deviations are larger than k 3 % instead of 9/14 in the patient group treated with a fixed S.S.D.
Large deviations have been detected in the treatment set-ups using fixed S.S.D. Inaccurate settings of the S.S.D. were leading to an overdosage of 6 % and underdosages of 5 and 11% . The overdosage of 35% was due to a mistake in treatment set-up: the right lateral field was irradiated at a S.S.D. of 100 cm, the gantry was then rotated as for an isocentric irradiation to treat the left lateral field (the irradiation was performed at a S.S.D. of 86 cm instead of the prescribed 100 cm). The results for the patients treated with pancranial irradiations, showed a smaller deviation of the ratio MD/ED with an isocentric technique (Fig. 8, black) than with a fixed S. S.D. technique (Fig. 8, white). For fixed S.S.D., 9/14 measures showed errors larger than 3% and 3/14 larger than 5 y0 as compared with 3/ 16 and l/16, respectively, for isocentric techniques. This patient group was responsible for the larger uncertainty on the overall results of the entrance dose measurements on brain patients as compared to the results of the head and neck patient group.
Conclusions It has been shown that in vivo dosimetry performed with semiconductor detectors is a reliable method for patient dose control (section 1.d.). A high precision can be obtained when the correction factors for each parameter of influence on the diode response are carefully determined and applied to convert the semiconductor signal in absorbed dose (see section 1.a.). Much care has to be taken for the correct positioning of the semiconductor detectors at the field center, especially when measurements are performed in wedged fields (section I.c.). The main advantages of using semiconductor detectors for in vivo dosimetry is that results are available in real time allowing the detection of the sources of error due to the treatment set-up. Semiconductor in vivo dosimetry is also less time consuming as compared to TLD in vivo dosimetry. The standard deviation on the results of the
150 entrance dose measurements was reasonably small (2.75%) as compared with the results of Nilsson et al. [ 141: 4% at 6 MV for open fields and 6% for wedged fields. This was due to several reasons: the excellent stability of the treatment machine, the automatic verification system, the use of individual plastic masks for immobilisation, the good accuracy in daily treatment set-up and the achievement of the measurements by one selected person. The check of the overall treatment system led to the identification of two small, systematic errors: an inaccuracy in the algorithm used for dose calculation with the treatment planning system and an error in the determination of the absorbed dose with the ionisation chamber in the region of depth of maximum dose (section 1.a.). These inaccuracies have been corrected, improving the quality of the treatment chain. Two treatment techniques with large uncertainties have been identified: (1) The patients treated with large lateral fields and a treatment couch rotation for whom a systematic overdosage occurred due to non-standard geometric conditions affecting the dose calculation (section 1I.d.). (2) The patients treated with pancranial irradiation for whom a large frequency distribution was found due to the confusion between isocentric and fixed S.S.D. techniques in daily treatment set-up. This could probably be prevented with a uniform technique within a department for similar treatment fields. The results of the measurements performed on treatment set-ups using an isocentric technique were better than the results of the fixed S.S.D. beam measurements, proving the superiority in the precision in dose delivery that could be achieved with isocentric treatment setups (section 1II.d.). Large errors have been detected in 3% of the measured treatment set-ups. The sources of the errors could in all cases be identified and corrected (see Results), avoiding large under- or overdosages. The detection of such errors is espe-
cially critical for individual patients. These errors are directly related to treatment failures and previously “unexpected” complications. The present work has shown that periodic in vivodosimetryis useful as a check of the quality of the whole treatment chain. The quality of several treatment techniques can be evaluated and improved when needed, sytematic errors can be detected and corrected. However, the check of the dose delivered to a given patient can be performed only by systematic in vivodosimetry.Large errors can be detected and the sources of error can be identified and corrected decreasing the uncertainty on the total dose delivered to each individual patient. In vivo dosimetry has been proved to be very useful as part of a departmental quality assurance program.
Acknowledgments We wish to thank A. Rijnders for his support and J. Verstraete for his enthusiastic cooperation in this study. The authors greatly appreciate the secretarial work by Mrs. L. Minnen and the assistance in computer programming by Mr. J. Van Kelecom. This work was supported by grants from the Belgian Work Against Cancer.
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