1991, The British Journal of Radiology, 64, 836-841
Memorandum from the Institute of Physical Sciences in Medicine Back-scatter and F-factors for low- and medium-energy X-ray beams in radiotherapy Summary of recommendations by a working party of the Institute of Physical Sciences in Medicine This memorandum has been authorized by the Scientific Committee of the IPSM. The members of the working party were: S. C. Klevenhagen (chairman), R. J. Aukett, J. E. Burns, R. M. Harrison, R. T. Knight, A. E. Nahum and K. E. Rosser. Thanks are due to J. E. Burns for preparing the several drafts on which this summary and the full report are based. The full report of the working party is published in Physics in Medicine and Biology (IPSM, 1991). (Received March 1991) Keywords: Dosimetry for radiotherapy, Back-scatter, Low-energy X-radiation
At present, the dosimetry of low- and medium-energy X-radiation for radiotherapy purposes in the UK is based on the method recommended in International Commission on Radiation Units and Measurements (ICRU) Report 23 (1973), and incorporated into the current Hospital Physicists' Association (HPA) code of practice (1983). The essential points are summarized here using the same symbols and units as in the code of practice. For an X-ray beam between 150 kV and 300 kV (0.5mmCu and 4mmCu half-value layer (HVL)), ICRU recommended that output measurements should be made using an ionization chamber with its centre 5 cm deep in a water phantom. Absorbed dose is given by: D= 1.145 RNkF (la) or by: D = 0.0\ RNXF (lb) where D = absorbed dose to water in grays at the position of the centre of the chamber, with the chamber replaced by water, R = instrument reading, corrected to the same standard ambient conditions as the calibration factor, Nk = calibration factor to convert the instrument reading to air kerma in grays for standard ambient conditions (at present 20°C, 1013 mbar, and 50% RH for calibrations at NPL), Nx = calibration factor to convert the instrument reading to exposure in rontgens for standard ambient conditions, F = a conversion factor, in rad/rontgen, which depends on the HVL of the X-ray beam being used. For an X-ray beam between 40 kV and 150 kV (0.5 mm Al and 8 mm Al HVL), ICRU recommended that output measurements should be made with the chamber under scatter-free conditions in air on the Reprints can be obtained from The British Institute of Radiology, 36 Portland Place, London WIN 4AT, at a cost of £2.00 plus postage. 836
beam axis, at a distance from the target equal to the source-surface distance (SSD) normally used. The absorbed dose to water in grays on the beam axis at the surface of a water phantom is given by: D= 1.145 RNkFB
(2a)
D = 0.0l
(2b)
or by: RKFB
where the symbols have the same meanings as in equations la and lb, with the addition that B is the back-scatter factor appropriate to the field size and radiation quality used. The HPA code of practice recommended that values of back-scatter factors should be derived from BJR Supplement 17 (1983), and F-factors from ICRU Report 23 (1973). However, in 1987 the International Atomic Energy Agency (IAEA) published a code of practice with different values of back-scatter factors and procedures for converting air kerma into absorbed dose to water that give different results from those obtained using the HPA code of practice. The Institute of Physical Sciences in Medicine (IPSM) therefore set up a working party to examine the two discrepancies and to recommend what action should be taken. This paper contains a summary of its recommendations; a fuller account is published elsewhere (IPSM, 1991). Back-scatter factors IAEA back-scatter data In 1984 Grosswendt used Monte Carlo methods to calculate back-scatter factors for X-radiation generated at voltages between 10 kV and 100 kV, and at 100 cm SSD. His data were subsequently used in the IAEA code of practice (IAEA, 1987) and recommended for use in radiotherapy. These new values differed considerably from those in BJR Supplement 17 (1983), particularly for HVLs between 0.1 mm Al and 1.0 mm Al. The British Journal of Radiology, September 1991
Back-scatter and F-factors for low-energy X-rays
1.20
HVL 0.7mmAl HVL O.AmmAI HVL0.7mmAI . HVLO.AmmAl
5
10 Field diameter (cm)
In a later paper, Grosswendt (1990) extended the X-ray energy range upwards to 280 kV generating potential, and included 661 keV Cs-137 gamma-radiation; back-scatter factors were calculated for a range of SSDs from 10 cm to 100 cm, and for field sizes from 1 cm to 20 cm diameter.
15
Figure 1. Circles and dashed lines are back-scatter factors in BJR Supplement 17. Crosses and continuous lines are new back-scatter factors in this report.
phantom thickness is decreased. Klevenhagen (1982) provided formulae for calculating the magnitude of this effect and discussed the effect of SSD on the build-up of back-scatter with phantom thickness.
Effect of X-ray spectrum The effect on the back-scatter factor of different Results from working party X-ray spectra for a given HVL appears to be rather In order to discover which set of values was correct, uncertain, with conflicting evidence given by Johns et al the IPSM working party re-determined back-scatter (1954) and Harrison (1982). factors both by calculation and experiment, paying Burns (1975) surveyed the radiological literature on particular attention to energies below 1 mm Al HVL, the range of kV-filter combinations used in radiotherapy where the discrepancies appeared to be greatest. and found that when kV was plotted against HVL on Knight and Nahum (1990) used Monte Carlo log-log graph paper, the results clustered around a methods to calculate back-scatter factors; their values straight line. The qualities used by Grosswendt (1990) agree to about 1% with Grosswendt (1990) and also for his Monte Carlo calculations of back-scatter also lie with the back-scatter factors in BJR Supplement 17 on or close to the same straight line. Therefore, it was (1983) for medium-energy X-radiation above assumed that the qualities he used are typical of those 0.5 mm Cu HVL. Back-scatter factors measured using a found in radiotherapy, and the working party decided flat ionization chamber (Klevenhagen, 1989) are in to use his values as the basis for new back-scatter excellent agreement with Grosswendt except for small factors to replace those in BJR Supplement 17. fields (5 cm diameter and smaller) at 4 mm Al HVL where they are about 3% lower. Measurements using Handling of the data for this report thermoluminescent dosemeters (Harrison et al, 1990) Grosswendt's back-scatter factors were plotted show the rather larger random scatter which is expected against HVL in order to interpolate values for the HVLs when using this type of dosemeter, but they agree much found in BJR Supplement 17. Some smoothing was better with Grosswendt (1990) than with BJR necessary and the data of Knight and Nahum (1990) Supplement 17. and Klevenhagen (1989) were used to confirm the appropriate shape of the curves. The data were also Effect of source-surface distance and phantom thicknessplotted against field size and SSD in order to interpolate Grosswendt's (1990) calculations in the range back-scatter factors for the field sizes and SSDs 10-100 cm SSD showed that back-scatter factors published in BJR Supplement 17, and were then increase with increasing SSD, particularly for large rounded to 1%. fields and for X-radiation above 1.0 mm Al HVL; this The final values are presented in Tables 1.3 and 1.4 in has been confirmed by Knight and Nahum (1990). The a form that matches the irradiation conditions and new values presented in this report take this effect into format used in BJR Supplement 17. Table numbers are account. the same as the corresponding tables in that publication. All the calculations and measurements discussed so far are for a thick phantom, giving nearly full back- Comparison of new and old back-scatter factors scatter. Quimby et al (1938) and Wachsmann et al The greatest change in back-scatter occurs between (1954) showed that back-scatter decreases as the 0.1 mm Al and 1.0 mm Al HVL for large field sizes. The Vol. 64, No. 765
837
S.C. Klevenhagen et al
Tables 1.3 Corrected values of back-scatter factors for Tables 1.3 in Central Axis Depth Dose Data for Use in Radiotherapy, BJR Supplement 17 (1983) Table 1.3.2. HVL 002mm Al
Table 1.3.1. HVL 001 mm Al SSD (cm) Diameter (cm) BSF
SSD (cm) Diameter (cm) BSF
10 1-6-50 100
Table 1.3.4. HVL 007mm Al
Table 1.3.3. HVL 004mm Al SSD (cm) Diameter (cm) BSF
10 1-6-5-0 101
20 1-6-10-0 101
30 3-0-16-0 101
Table 1.3.5. HVL 010 mm Al SSD (cm) Diameter (cm) BSF
10 1-6-5-0 100
SSD (cm) Diameter (cm) BSF
10 1-6-5-0 101
20 1-6-10-0 101
30 30-160 101
20 1-6-10-0 103-104
30 30-160 103-104
Table 1.3.6. HVL 0-20 mm Al
10 1-6-5-0 1-02
20 1-6-10-0 102
30 3-0-16-0 1-02
SSD (cm) Diameter (cm) BSF
10 1-6-5-0 1-03-1-04
Table 1.3.7. HVL 0-40 mm Al SSD (cm) Diameter (cm) BSF
1-6 104
10 30 106
5-0 1-07
1-6 104
15 30 106
5-0 107
3-0 106
50 107
30
10 30 108
50 110
1-6 106
15 30 109
50 110
30 109
50 110
100 107
160 107
100 1-12
160 1-12
Table 1.3.8. HVL 0-70 mm Al SSD (cm) Diameter (cm) BSF
838
1-6 106
30
The British Journal of Radiology, September 1991
Back-scatter and F-factors for low-energy X-rays
Tables 1.4 Corrected values of back-scatter factors for Tables 1.4 in Central Axis Depth Dose Data for Use in Radiotherapy, BJR Supplement 17 (1983) Table 1.4.1. HVL 10 mm Al Diameter (cm)
0
1
2
3
4
5
6
8
10
15
20
SSD 10 cm 15 cm 20 cm 25 cm 30 cm
10 10 10 10 10
105 105 105 105 105
109 109 109 109 109
110 110 110 110 110
112 1-12 1-12 112 1-12
1-12 1-13 113 113 1-13
113 114 114 1-14 1-14
114 115 1-15 1-15 115
115 116 116 116 116
116 117 1-17
117 117
Table 1.4.2. HVL 2 0 m m Al Diameter (cm)
0
1
2
3
4
5
6
8
10
15
20
SSD 10 cm 15 cm 20 cm 25 cm 30 cm
10 10 10 10 10
106 106 106 106 106
111 111 111 111 111
114 114 1-14 114 114
116 116 116 116 116
117 1-18 118 118 118
1-18 119 119 1-20 1-20
1-20 1-21 1-22 1-22 1-22
1-22 1.23 1-23 1-24 1-24
1-26 1-26 1-27
1-27 1-28
Table 1.4.3. HVL 3 0 m m Al Diameter (cm)
0
1
2
3
4
5
6
8
10
15
20
SSD 10 cm 15 cm 20 cm 25 cm 30 cm
10 10 10 10 10
106 106 106 106 106
1-12 1-12 112 112 112
116 116 116 116 116
118 118 119 119 119
1-20 1-21 1-21 1 21 1-22
1-22 1-23 1-23 1-24 1-24
1-25 1-25 1-26 1-26 1-27
1-26 1-27 1-28 1-29 1-29
1-32 1-32 1-33
1-34 1-35
Table 1.4.4. HVL 4 0 m m Al Diameter (cm)
0
1
2
3
4
5
6
8
10
15
20
SSD 10 cm 15 cm 20 cm 25 cm 30 cm
10 10 10 10 10
106 106 106 106 106
1-12 112 1-12 112 1-12
116 116 116 116 1-17
1-20 1-20 1-20 1-20 1-20
1-22 1-22 1-23 1-23 1-23
1-24 1-25 1-25 1-25 1-26
1-27 1-28 1-29 1-30 1-30
1-29 1-31 1-32 1-32 1-33
1-36 1-37 1-38
1-39 1-40
Table 1.4.5. HVL 8 0 m m Al Diameter (cm)
0
1
2
3
4
5
6
8
10
15
20
SSD 10 cm 15 cm 20 cm 25 cm 30 cm
10 10 10 10 10
105 105 105 105 105
111 111 111 111 111
115 115 115 115 116
119 119 1-20 1-20 1-20
1-22 1-23 1-23 1-24 1-24
1-25 1-26 1-26 1-27 1-27
1-29 1-30 1-31 1-32 1-32
1-32 1-34 1-35 1-36 1-36
1-41 1-42 1-43
1-45 1-46
Vol. 64, No. 765
839
S.C. Klevenhagen et al
new back-scatter factors reach their maximum values for much smaller field sizes than those in BJR Supplement 17, as shown in Figure 1. Changes for HVLs between 1.0 mm Al to 8 mm Al are less marked. No new data are provided for medium-energy X-radiation (0.5 mm Cu to 4 mm Cu HVL), because the values of Grosswendt (1990), supported by Knight and Nahum (1990), agree so well with those in BJR Supplement 17 that corrections are unnecessary. No data are available to derive back-scatter factors for the two short SSD X-ray units which are dealt with in Tables 1.2 of BJR Supplement 17. Earlier published back-scatter factors When comparing the results of radiotherapy over a number of years, it is desirable to correct for any changes to the back-scatter factors that have been used to calculate the exposure or dose received by patients. In the UK, back-scatter factors have generally been taken from data published in the British Journal of Radiology. The first review of back-scatter factors was published by Mayneord and Lamerton (1941) and covered the medium- and high-energy X-ray range from 0.5 mm Cu to 17 mm Cu HVL. The first supplement of the British Journal of Radiology to publish back-scatter data was Supplement 5 (1953); this covered the low-energy range (0.01-8 mm Al HVL) only. In the next relevant BJR supplement to be published (BJR Supplement 10, 1961), back-scatter factors for low-energy X-radiation were significantly different from those in BJR Supplement 5 (1953), and the back-scatter factors for medium-energy X-radiation differed significantly from those published by Mayneord and Lamerton (1941). However, back-scatter factors then remained unchanged through BJR Supplement 11 (1972) to the most recent BJR Supplement 17 (1983), except for a slight change in the values for 4mmCu HVL in Supplement 17. The new back-scatter factors in this report therefore represent the first change in recommended values for radiotherapy since BJR Supplement 10 in 1961. Recommendation on back-scatter factors by working party The IPSM working party recommends that, for radiotherapy dosimetry, the new back-scatter factors in Tables 1.3 and 1.4 should be used in place of those in the corresponding tables of BJR Supplement 17. It suggests that no change should be made in the backscatter factors for short SSDs given in Tables 1.2 of BJR Supplement 17 until more accurate data become available. Dose-conversion F-factors In 1987 the IAEA published a code of practice which recommended procedures for converting the response of an ionization chamber calibrated in terms of air kerma into absorbed dose to water for medium-energy X-radiation. These give rise to different results from those obtained using the current HPA code of practice (HPA, 1983). 840
Table 2. Values of the conversion factor F, taken from ICRU Report 23 HVL of primary beam
F (rad/R)
(mm Al) 0.5 1.0 2.0 4.0 6.0 8.0
0.89 0.88 0.87 0.87 0.88 0.89
(mm Cu) 0.5 1.0 1.5 2.0 3.0 4.0
0.89 0.91 0.93 0.94 0.95 0.96
If absorbed dose to water is calculated from a measurement with a chamber in air, together with the application of a back-scatter factor, the difference between the two codes of practice is not large, amounting to 2% at about O.lmmCu HVL and decreasing for larger HVLs until the results almost agree at about 4 mm Cu HVL. However, for X-ray beams with generating potentials of 100 kV and above (about 0.2mmCu HVL), the IAEA code of practice recommends that measurements should be carried out with the chamber inserted into a water phantom and claims that a correction must be applied for the effect of displacing water by the chamber. The use of the IAEA code gives values of absorbed dose that are 12% higher than those obtained using the HPA code of practice at 100 kV. This discrepancy decreases for higher HVLs, becoming only about 1% at 4 mm Cu HVL. The IPSM working party compared the HPA and IAEA codes of practice with results reported in four scientific papers. Two of these papers compared absorbed dose, determined using the method described in the HPA code of practice, with absorbed dose derived from calorimetric measurements. The other two papers reported determinations of the effect of displacing phantom material by an ionization chamber. The papers are not in exact agreement with each other, but they do tend to support results obtained using the HPA code of practice rather than the IAEA code. In view of this evidence the IPSM working party recommends that no change should be made to the procedure for converting air kerma (or exposure) into absorbed dose to water for medium-energy X-radiation for the time being, until more work on this subject is published. A more detailed justification for this recommendation is contained in the report of the working party (IPSM, 1991). The HPA and IAEA conversion procedures are also compared and discussed by Rosser (1991). The British Journal of Radiology, September 1991
Back-scatter and F-factors for low-energy X-rays
For convenience, the values of the dose-conversion F-factors in the current HPA code of practice are reproduced here as Table 2, to be used in Equations 1 or 2 as appropriate.
References BIR & HPA WORKING PARTY, 1953. Central Axis Depth Dose Data. British Journal of Radiology, Supplement 5 (British Institute of Radiology, London). 1961. Depth Dose Tables for Use in Radiotherapy. British Journal of Radiology, Supplement 10 (British Institute of Radiology, London). 1972. Central Axis Depth Dose Data for Use in Radiotherapy. British Journal of Radiology, Supplement 11 (British Institute of Radiology, London). 1983. Central Axis Depth Dose Data for Use in Radiotherapy. British Journal of Radiology, Supplement 17 (British Institute of Radiology, London). BURNS, J. E., 1975. X-ray Qualities for the Calibration of Reference Instruments. Proceedings of the 3rd meeting of Section I of the CCEMRI, Annexe R(I)6 (BIPM, Sevres). GROSSWENDT, B., 1984. Backscatter factors for X-rays generated at voltages between 10 and 100 kV. Physics in Medicine and Biology, 29, 579-591. 1990. Dependence of the photon back-scatter factor for water on source-to-phantom distance and irradiation field size. Physics in Medicine and Biology, 35, 1233-1245. HARRISON, R. M., 1982. Back-scatter factors for diagnostic radiology (1-4 mm Al HVL). Physics in Medicine and Biology, 27, 1465-1474. HARRISON,
R. M.,
WALKER,
C.
&
AUKETT,
R. J.,
1990.
Measurement of back-scatter factors for low-energy radiotherapy (0.1-2.0 mm Al HVL) using thermoluminescence dosimetry. Physics in Medicine and Biology, 35, 1247-1253, 1715-1716.
Vol. 64, No. 765
HPA, 1983. Revised code of practice for the dosimetry of 2 to 35 MV X-ray, and of caesium-137 and cobalt-60 gamma-ray beams. Physics in Medicine and Biology, 28, 1097-1104. IAEA, 1987. Absorbed Dose Determination in Photon and Electron Beams. Technical Reports Series 277 (International Atomic Energy Agency, Vienna). ICRU, 1973. Measurement of Absorbed Dose in a Phantom Irradiated by a Single Beam of X or Gamma Rays. ICRU Report 23. (International Commission on Radiation Units and Measurements, Bethesda). IPSM, 1991. Report of the Institute of Physical Sciences in Medicine working party on low- and medium-energy X-ray dosimetry. Physics in Medicine and Biology, 36, (in press). JOHNS, H. E., HUNT, J. W. & FEDORUK, S. O., 1954. Surface
back-scatter in the 100 kV to 400 kV range. British Journal of Radiology, 27, 443-448. KLEVENHAGEN, S. C , 1982. The build-up of backscatter in the energy range 1 mm Al to 8 mm Al HVT. Physics in Medicine and Biology, 27, 1035-1043. 1989. Experimentally determined backscatter factors for X-rays generated at voltages between 16 and 140 kV. Physics in Medicine and Biology, 34, 1871-1882. KNIGHT, R. T. & NAHUM, A. E., 1990. Back-scatter data
submitted to IPSM working party. MAYNEORD, W. V. & LAMERTON, L. F., 1941. A survey of depth
dose data. British Journal of Radiology, 14, 255-264, and (a) to (p). QUIMBY, E. H., MARINELLI, L. D. & FARROW, J. H., 1938. A
study of back scatter. American Journal of Roentgenology, 39, 799-815. ROSSER, K. E., 1991. Comparison of Factors Given in ICRU Report 23 and IAEA TRS 277 for Converting from Exposure or Air Kerma into Absorbed Dose to Water for Medium-energy X-radiation. NPL Report RSA(EXT)15 (National Physical Laboratory, Teddington). WACHSMANN, F., HECKEL, K. & SCHIRREN, C. G.,
1954.
Die
Grosse der Riickstreuung bei verschiedener Tiefe des Streukorpers. Strahlentherapie, 94, 161-168.
841
Memorandum from the Institute of Physical Sciences in Medicine Back-scatter and F-factors for low- and medium-energy X-ray beams in radiotherapy Summary of recommendations by a working party of the Institute of Physical Sciences in Medicine This memorandum has been authorized by the Scientific Committee of the IPSM. The members of the working party were: S. C. Klevenhagen (chairman), R. J. Aukett, J. E. Burns, R. M. Harrison, R. T. Knight, A. E. Nahum and K. E. Rosser. Thanks are due to J. E. Burns for preparing the several drafts on which this summary and the full report are based. The full report of the working party is published in Physics in Medicine and Biology (IPSM, 1991). (Received March 1991) Keywords: Dosimetry for radiotherapy, Back-scatter, Low-energy X-radiation
At present, the dosimetry of low- and medium-energy X-radiation for radiotherapy purposes in the UK is based on the method recommended in International Commission on Radiation Units and Measurements (ICRU) Report 23 (1973), and incorporated into the current Hospital Physicists' Association (HPA) code of practice (1983). The essential points are summarized here using the same symbols and units as in the code of practice. For an X-ray beam between 150 kV and 300 kV (0.5mmCu and 4mmCu half-value layer (HVL)), ICRU recommended that output measurements should be made using an ionization chamber with its centre 5 cm deep in a water phantom. Absorbed dose is given by: D= 1.145 RNkF (la) or by: D = 0.0\ RNXF (lb) where D = absorbed dose to water in grays at the position of the centre of the chamber, with the chamber replaced by water, R = instrument reading, corrected to the same standard ambient conditions as the calibration factor, Nk = calibration factor to convert the instrument reading to air kerma in grays for standard ambient conditions (at present 20°C, 1013 mbar, and 50% RH for calibrations at NPL), Nx = calibration factor to convert the instrument reading to exposure in rontgens for standard ambient conditions, F = a conversion factor, in rad/rontgen, which depends on the HVL of the X-ray beam being used. For an X-ray beam between 40 kV and 150 kV (0.5 mm Al and 8 mm Al HVL), ICRU recommended that output measurements should be made with the chamber under scatter-free conditions in air on the Reprints can be obtained from The British Institute of Radiology, 36 Portland Place, London WIN 4AT, at a cost of £2.00 plus postage. 836
beam axis, at a distance from the target equal to the source-surface distance (SSD) normally used. The absorbed dose to water in grays on the beam axis at the surface of a water phantom is given by: D= 1.145 RNkFB
(2a)
D = 0.0l
(2b)
or by: RKFB
where the symbols have the same meanings as in equations la and lb, with the addition that B is the back-scatter factor appropriate to the field size and radiation quality used. The HPA code of practice recommended that values of back-scatter factors should be derived from BJR Supplement 17 (1983), and F-factors from ICRU Report 23 (1973). However, in 1987 the International Atomic Energy Agency (IAEA) published a code of practice with different values of back-scatter factors and procedures for converting air kerma into absorbed dose to water that give different results from those obtained using the HPA code of practice. The Institute of Physical Sciences in Medicine (IPSM) therefore set up a working party to examine the two discrepancies and to recommend what action should be taken. This paper contains a summary of its recommendations; a fuller account is published elsewhere (IPSM, 1991). Back-scatter factors IAEA back-scatter data In 1984 Grosswendt used Monte Carlo methods to calculate back-scatter factors for X-radiation generated at voltages between 10 kV and 100 kV, and at 100 cm SSD. His data were subsequently used in the IAEA code of practice (IAEA, 1987) and recommended for use in radiotherapy. These new values differed considerably from those in BJR Supplement 17 (1983), particularly for HVLs between 0.1 mm Al and 1.0 mm Al. The British Journal of Radiology, September 1991
Back-scatter and F-factors for low-energy X-rays
1.20
HVL 0.7mmAl HVL O.AmmAI HVL0.7mmAI . HVLO.AmmAl
5
10 Field diameter (cm)
In a later paper, Grosswendt (1990) extended the X-ray energy range upwards to 280 kV generating potential, and included 661 keV Cs-137 gamma-radiation; back-scatter factors were calculated for a range of SSDs from 10 cm to 100 cm, and for field sizes from 1 cm to 20 cm diameter.
15
Figure 1. Circles and dashed lines are back-scatter factors in BJR Supplement 17. Crosses and continuous lines are new back-scatter factors in this report.
phantom thickness is decreased. Klevenhagen (1982) provided formulae for calculating the magnitude of this effect and discussed the effect of SSD on the build-up of back-scatter with phantom thickness.
Effect of X-ray spectrum The effect on the back-scatter factor of different Results from working party X-ray spectra for a given HVL appears to be rather In order to discover which set of values was correct, uncertain, with conflicting evidence given by Johns et al the IPSM working party re-determined back-scatter (1954) and Harrison (1982). factors both by calculation and experiment, paying Burns (1975) surveyed the radiological literature on particular attention to energies below 1 mm Al HVL, the range of kV-filter combinations used in radiotherapy where the discrepancies appeared to be greatest. and found that when kV was plotted against HVL on Knight and Nahum (1990) used Monte Carlo log-log graph paper, the results clustered around a methods to calculate back-scatter factors; their values straight line. The qualities used by Grosswendt (1990) agree to about 1% with Grosswendt (1990) and also for his Monte Carlo calculations of back-scatter also lie with the back-scatter factors in BJR Supplement 17 on or close to the same straight line. Therefore, it was (1983) for medium-energy X-radiation above assumed that the qualities he used are typical of those 0.5 mm Cu HVL. Back-scatter factors measured using a found in radiotherapy, and the working party decided flat ionization chamber (Klevenhagen, 1989) are in to use his values as the basis for new back-scatter excellent agreement with Grosswendt except for small factors to replace those in BJR Supplement 17. fields (5 cm diameter and smaller) at 4 mm Al HVL where they are about 3% lower. Measurements using Handling of the data for this report thermoluminescent dosemeters (Harrison et al, 1990) Grosswendt's back-scatter factors were plotted show the rather larger random scatter which is expected against HVL in order to interpolate values for the HVLs when using this type of dosemeter, but they agree much found in BJR Supplement 17. Some smoothing was better with Grosswendt (1990) than with BJR necessary and the data of Knight and Nahum (1990) Supplement 17. and Klevenhagen (1989) were used to confirm the appropriate shape of the curves. The data were also Effect of source-surface distance and phantom thicknessplotted against field size and SSD in order to interpolate Grosswendt's (1990) calculations in the range back-scatter factors for the field sizes and SSDs 10-100 cm SSD showed that back-scatter factors published in BJR Supplement 17, and were then increase with increasing SSD, particularly for large rounded to 1%. fields and for X-radiation above 1.0 mm Al HVL; this The final values are presented in Tables 1.3 and 1.4 in has been confirmed by Knight and Nahum (1990). The a form that matches the irradiation conditions and new values presented in this report take this effect into format used in BJR Supplement 17. Table numbers are account. the same as the corresponding tables in that publication. All the calculations and measurements discussed so far are for a thick phantom, giving nearly full back- Comparison of new and old back-scatter factors scatter. Quimby et al (1938) and Wachsmann et al The greatest change in back-scatter occurs between (1954) showed that back-scatter decreases as the 0.1 mm Al and 1.0 mm Al HVL for large field sizes. The Vol. 64, No. 765
837
S.C. Klevenhagen et al
Tables 1.3 Corrected values of back-scatter factors for Tables 1.3 in Central Axis Depth Dose Data for Use in Radiotherapy, BJR Supplement 17 (1983) Table 1.3.2. HVL 002mm Al
Table 1.3.1. HVL 001 mm Al SSD (cm) Diameter (cm) BSF
SSD (cm) Diameter (cm) BSF
10 1-6-50 100
Table 1.3.4. HVL 007mm Al
Table 1.3.3. HVL 004mm Al SSD (cm) Diameter (cm) BSF
10 1-6-5-0 101
20 1-6-10-0 101
30 3-0-16-0 101
Table 1.3.5. HVL 010 mm Al SSD (cm) Diameter (cm) BSF
10 1-6-5-0 100
SSD (cm) Diameter (cm) BSF
10 1-6-5-0 101
20 1-6-10-0 101
30 30-160 101
20 1-6-10-0 103-104
30 30-160 103-104
Table 1.3.6. HVL 0-20 mm Al
10 1-6-5-0 1-02
20 1-6-10-0 102
30 3-0-16-0 1-02
SSD (cm) Diameter (cm) BSF
10 1-6-5-0 1-03-1-04
Table 1.3.7. HVL 0-40 mm Al SSD (cm) Diameter (cm) BSF
1-6 104
10 30 106
5-0 1-07
1-6 104
15 30 106
5-0 107
3-0 106
50 107
30
10 30 108
50 110
1-6 106
15 30 109
50 110
30 109
50 110
100 107
160 107
100 1-12
160 1-12
Table 1.3.8. HVL 0-70 mm Al SSD (cm) Diameter (cm) BSF
838
1-6 106
30
The British Journal of Radiology, September 1991
Back-scatter and F-factors for low-energy X-rays
Tables 1.4 Corrected values of back-scatter factors for Tables 1.4 in Central Axis Depth Dose Data for Use in Radiotherapy, BJR Supplement 17 (1983) Table 1.4.1. HVL 10 mm Al Diameter (cm)
0
1
2
3
4
5
6
8
10
15
20
SSD 10 cm 15 cm 20 cm 25 cm 30 cm
10 10 10 10 10
105 105 105 105 105
109 109 109 109 109
110 110 110 110 110
112 1-12 1-12 112 1-12
1-12 1-13 113 113 1-13
113 114 114 1-14 1-14
114 115 1-15 1-15 115
115 116 116 116 116
116 117 1-17
117 117
Table 1.4.2. HVL 2 0 m m Al Diameter (cm)
0
1
2
3
4
5
6
8
10
15
20
SSD 10 cm 15 cm 20 cm 25 cm 30 cm
10 10 10 10 10
106 106 106 106 106
111 111 111 111 111
114 114 1-14 114 114
116 116 116 116 116
117 1-18 118 118 118
1-18 119 119 1-20 1-20
1-20 1-21 1-22 1-22 1-22
1-22 1.23 1-23 1-24 1-24
1-26 1-26 1-27
1-27 1-28
Table 1.4.3. HVL 3 0 m m Al Diameter (cm)
0
1
2
3
4
5
6
8
10
15
20
SSD 10 cm 15 cm 20 cm 25 cm 30 cm
10 10 10 10 10
106 106 106 106 106
1-12 1-12 112 112 112
116 116 116 116 116
118 118 119 119 119
1-20 1-21 1-21 1 21 1-22
1-22 1-23 1-23 1-24 1-24
1-25 1-25 1-26 1-26 1-27
1-26 1-27 1-28 1-29 1-29
1-32 1-32 1-33
1-34 1-35
Table 1.4.4. HVL 4 0 m m Al Diameter (cm)
0
1
2
3
4
5
6
8
10
15
20
SSD 10 cm 15 cm 20 cm 25 cm 30 cm
10 10 10 10 10
106 106 106 106 106
1-12 112 1-12 112 1-12
116 116 116 116 1-17
1-20 1-20 1-20 1-20 1-20
1-22 1-22 1-23 1-23 1-23
1-24 1-25 1-25 1-25 1-26
1-27 1-28 1-29 1-30 1-30
1-29 1-31 1-32 1-32 1-33
1-36 1-37 1-38
1-39 1-40
Table 1.4.5. HVL 8 0 m m Al Diameter (cm)
0
1
2
3
4
5
6
8
10
15
20
SSD 10 cm 15 cm 20 cm 25 cm 30 cm
10 10 10 10 10
105 105 105 105 105
111 111 111 111 111
115 115 115 115 116
119 119 1-20 1-20 1-20
1-22 1-23 1-23 1-24 1-24
1-25 1-26 1-26 1-27 1-27
1-29 1-30 1-31 1-32 1-32
1-32 1-34 1-35 1-36 1-36
1-41 1-42 1-43
1-45 1-46
Vol. 64, No. 765
839
S.C. Klevenhagen et al
new back-scatter factors reach their maximum values for much smaller field sizes than those in BJR Supplement 17, as shown in Figure 1. Changes for HVLs between 1.0 mm Al to 8 mm Al are less marked. No new data are provided for medium-energy X-radiation (0.5 mm Cu to 4 mm Cu HVL), because the values of Grosswendt (1990), supported by Knight and Nahum (1990), agree so well with those in BJR Supplement 17 that corrections are unnecessary. No data are available to derive back-scatter factors for the two short SSD X-ray units which are dealt with in Tables 1.2 of BJR Supplement 17. Earlier published back-scatter factors When comparing the results of radiotherapy over a number of years, it is desirable to correct for any changes to the back-scatter factors that have been used to calculate the exposure or dose received by patients. In the UK, back-scatter factors have generally been taken from data published in the British Journal of Radiology. The first review of back-scatter factors was published by Mayneord and Lamerton (1941) and covered the medium- and high-energy X-ray range from 0.5 mm Cu to 17 mm Cu HVL. The first supplement of the British Journal of Radiology to publish back-scatter data was Supplement 5 (1953); this covered the low-energy range (0.01-8 mm Al HVL) only. In the next relevant BJR supplement to be published (BJR Supplement 10, 1961), back-scatter factors for low-energy X-radiation were significantly different from those in BJR Supplement 5 (1953), and the back-scatter factors for medium-energy X-radiation differed significantly from those published by Mayneord and Lamerton (1941). However, back-scatter factors then remained unchanged through BJR Supplement 11 (1972) to the most recent BJR Supplement 17 (1983), except for a slight change in the values for 4mmCu HVL in Supplement 17. The new back-scatter factors in this report therefore represent the first change in recommended values for radiotherapy since BJR Supplement 10 in 1961. Recommendation on back-scatter factors by working party The IPSM working party recommends that, for radiotherapy dosimetry, the new back-scatter factors in Tables 1.3 and 1.4 should be used in place of those in the corresponding tables of BJR Supplement 17. It suggests that no change should be made in the backscatter factors for short SSDs given in Tables 1.2 of BJR Supplement 17 until more accurate data become available. Dose-conversion F-factors In 1987 the IAEA published a code of practice which recommended procedures for converting the response of an ionization chamber calibrated in terms of air kerma into absorbed dose to water for medium-energy X-radiation. These give rise to different results from those obtained using the current HPA code of practice (HPA, 1983). 840
Table 2. Values of the conversion factor F, taken from ICRU Report 23 HVL of primary beam
F (rad/R)
(mm Al) 0.5 1.0 2.0 4.0 6.0 8.0
0.89 0.88 0.87 0.87 0.88 0.89
(mm Cu) 0.5 1.0 1.5 2.0 3.0 4.0
0.89 0.91 0.93 0.94 0.95 0.96
If absorbed dose to water is calculated from a measurement with a chamber in air, together with the application of a back-scatter factor, the difference between the two codes of practice is not large, amounting to 2% at about O.lmmCu HVL and decreasing for larger HVLs until the results almost agree at about 4 mm Cu HVL. However, for X-ray beams with generating potentials of 100 kV and above (about 0.2mmCu HVL), the IAEA code of practice recommends that measurements should be carried out with the chamber inserted into a water phantom and claims that a correction must be applied for the effect of displacing water by the chamber. The use of the IAEA code gives values of absorbed dose that are 12% higher than those obtained using the HPA code of practice at 100 kV. This discrepancy decreases for higher HVLs, becoming only about 1% at 4 mm Cu HVL. The IPSM working party compared the HPA and IAEA codes of practice with results reported in four scientific papers. Two of these papers compared absorbed dose, determined using the method described in the HPA code of practice, with absorbed dose derived from calorimetric measurements. The other two papers reported determinations of the effect of displacing phantom material by an ionization chamber. The papers are not in exact agreement with each other, but they do tend to support results obtained using the HPA code of practice rather than the IAEA code. In view of this evidence the IPSM working party recommends that no change should be made to the procedure for converting air kerma (or exposure) into absorbed dose to water for medium-energy X-radiation for the time being, until more work on this subject is published. A more detailed justification for this recommendation is contained in the report of the working party (IPSM, 1991). The HPA and IAEA conversion procedures are also compared and discussed by Rosser (1991). The British Journal of Radiology, September 1991
Back-scatter and F-factors for low-energy X-rays
For convenience, the values of the dose-conversion F-factors in the current HPA code of practice are reproduced here as Table 2, to be used in Equations 1 or 2 as appropriate.
References BIR & HPA WORKING PARTY, 1953. Central Axis Depth Dose Data. British Journal of Radiology, Supplement 5 (British Institute of Radiology, London). 1961. Depth Dose Tables for Use in Radiotherapy. British Journal of Radiology, Supplement 10 (British Institute of Radiology, London). 1972. Central Axis Depth Dose Data for Use in Radiotherapy. British Journal of Radiology, Supplement 11 (British Institute of Radiology, London). 1983. Central Axis Depth Dose Data for Use in Radiotherapy. British Journal of Radiology, Supplement 17 (British Institute of Radiology, London). BURNS, J. E., 1975. X-ray Qualities for the Calibration of Reference Instruments. Proceedings of the 3rd meeting of Section I of the CCEMRI, Annexe R(I)6 (BIPM, Sevres). GROSSWENDT, B., 1984. Backscatter factors for X-rays generated at voltages between 10 and 100 kV. Physics in Medicine and Biology, 29, 579-591. 1990. Dependence of the photon back-scatter factor for water on source-to-phantom distance and irradiation field size. Physics in Medicine and Biology, 35, 1233-1245. HARRISON, R. M., 1982. Back-scatter factors for diagnostic radiology (1-4 mm Al HVL). Physics in Medicine and Biology, 27, 1465-1474. HARRISON,
R. M.,
WALKER,
C.
&
AUKETT,
R. J.,
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Measurement of back-scatter factors for low-energy radiotherapy (0.1-2.0 mm Al HVL) using thermoluminescence dosimetry. Physics in Medicine and Biology, 35, 1247-1253, 1715-1716.
Vol. 64, No. 765
HPA, 1983. Revised code of practice for the dosimetry of 2 to 35 MV X-ray, and of caesium-137 and cobalt-60 gamma-ray beams. Physics in Medicine and Biology, 28, 1097-1104. IAEA, 1987. Absorbed Dose Determination in Photon and Electron Beams. Technical Reports Series 277 (International Atomic Energy Agency, Vienna). ICRU, 1973. Measurement of Absorbed Dose in a Phantom Irradiated by a Single Beam of X or Gamma Rays. ICRU Report 23. (International Commission on Radiation Units and Measurements, Bethesda). IPSM, 1991. Report of the Institute of Physical Sciences in Medicine working party on low- and medium-energy X-ray dosimetry. Physics in Medicine and Biology, 36, (in press). JOHNS, H. E., HUNT, J. W. & FEDORUK, S. O., 1954. Surface
back-scatter in the 100 kV to 400 kV range. British Journal of Radiology, 27, 443-448. KLEVENHAGEN, S. C , 1982. The build-up of backscatter in the energy range 1 mm Al to 8 mm Al HVT. Physics in Medicine and Biology, 27, 1035-1043. 1989. Experimentally determined backscatter factors for X-rays generated at voltages between 16 and 140 kV. Physics in Medicine and Biology, 34, 1871-1882. KNIGHT, R. T. & NAHUM, A. E., 1990. Back-scatter data
submitted to IPSM working party. MAYNEORD, W. V. & LAMERTON, L. F., 1941. A survey of depth
dose data. British Journal of Radiology, 14, 255-264, and (a) to (p). QUIMBY, E. H., MARINELLI, L. D. & FARROW, J. H., 1938. A
study of back scatter. American Journal of Roentgenology, 39, 799-815. ROSSER, K. E., 1991. Comparison of Factors Given in ICRU Report 23 and IAEA TRS 277 for Converting from Exposure or Air Kerma into Absorbed Dose to Water for Medium-energy X-radiation. NPL Report RSA(EXT)15 (National Physical Laboratory, Teddington). WACHSMANN, F., HECKEL, K. & SCHIRREN, C. G.,
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