Journal of the ICRU Vol 4 No 2 (2004) Report 72 Oxford University Press
5
DOI: 10.1093/jicru/ndh029
DOSIMETRY COMPARISONS
5.1 COMPARISON OF BETA-RAY SOURCE CALIBRATIONS As described earlier (Section 2.2), beta-ray ophthalmic applicators have been produced since 1950. Between 1950 and 1983, eight manufacturers of 90Sr----90Y applicators have been identified (Soares, 1995). 106Ru----106Rh applicators have been produced since the early 1960s, and the sources are still in production. To calibrate the sources, the manufacturers have used methods that have been locally developed, in some cases being based on the calibration of a few reference sources at a standards laboratory. Except in a few cases, the methods of calibration have not been well documented. Until 1989, no comparison of the methods of calibration had been reported. In 1989, a comparison of the calibration at the NIST with that from the laboratory of the only remaining manufacturer of the 90 Sr----90Y sources was reported, revealing a large
discrepancy in the calibration results (Goetsch, 1989; Soares, 1991). This result gave rise to a critical analysis of the methods of calibration, which led to better knowledge of the physical basis of calibration and to considerably improved measurement techniques (described in Section 3.2.2). After the major revision of measurement techniques at the NIST (Soares, 1991), comparison of the calibration results with the improved technique were carried out for a total of 58 90Sr----90Y sources representing seven former manufacturers and the only remaining manufacturer. A summary of the manufacturer’s source and calibration information is shown in Table 5.1 (Soares, 1995). If the manufacturer quoted a dose rate in the archaic unit of reb (roentgen-equivalent-beta), a conversion of 0.00982 Gy reb1 was used (Section 3.1.1). For all the sources examined from a given manufacturer, the average difference in the surface-absorbed-dose rates obtained by the NIST from those quoted by the manufacturer varied from 25 to þ36 % among the manufacturers, while the relative standard deviation for a given manufacturer could be as low as 4 % or as high as 17 % (see Table 5.1). The individual differences for a single source were up to 61 %. These differences are significant, if the uncertainty of the calibrations at the other manufacturers were of the same level as that reported by Nycomed Amersham. Significant differences in the uniformity of the dose rate over the effective source area were found among the source types of different manufacturers, and also among individual sources of a given type. Of particular interest is the comparison between the NIST and the then remaining manufacturer of 90Sr----90Y sources, Nycomed Amersham. Taking into account the known differences of the calibration methods between the standards laboratory and the manufacturer, the average difference of calibration results could be reduced to 13 % with a relative standard deviation of 4 %. Because the quoted uncertainty of the calibration by the NIST is 3.5 % (one standard deviation; cf. Table 3.3) and that of the calibration by Nycomed Amersham is 20 %, the difference might not be significant. In view of the individual differences among sources of the same
ª International Commission on Radiation Units and Measurements 2004
Downloaded from http://jicru.oxfordjournals.org/ at University of California, San Diego on June 8, 2015
Comparisons of the calibrations of sources among standard dosimetry laboratories are important to create confidence in the quoted uncertainties of the calibrations. The analysis of the results of such comparisons should lead to reductions in measurement uncertainties for all the laboratories involved. Comparisons of actual dose distributions are not always straightforward to carry out. Dose distributions measured or calculated by different methods are usually compared at a few relevant points, and the consistency expressed as the maximum deviations of relative doses in percent or, in case of very steep dose gradients, as the maximum deviations of the positions of isodose lines in millimeters. In the following, only comparisons dealing with beta-ray planar and concave sources are discussed. For beta-ray sources used in intravascular brachytherapy, only a few laboratories can provide calibrations, and results of comparisons among them are not yet available. For low-energy photon sources, comparisons of calibrations at the primary-standard level have not been possible because only one laboratory (NIST) provides such calibrations.
DOSIMETRY OF BETA RAYS AND LOW-ENERGY PHOTONS FOR BRACHYTHERAPY WITH SEALED Table 5.1. Summary of sources and their calibration by the manufacturers, and the results of calibration comparison with a standards laboratory (NIST), for 90Sr----90Y ophthalmic applicators (from Soares, 1995, with permission). Dates of manufacture
Model
Calibration method
Units
Number compared
Average difference (%)
SD (%)
Tracelab ICN/Tracelab
1950----1969 1969----late 1970s
RA-1
Extrapolation chamber: 5-mm diameter collecting electrode; 0.2----0.5 mm air gap
reb
18
18
15
Atlantic Research Corporation Atomchem New England Nuclear
early 1960s----1969
B1
rad
8
þ2.5
12
1969----late 1970s 1976----1983
NB-1
rad
8
25
4
Nycomed Amersham (earlier Amersham International)
1950s----present
SIA.20
rad
10
23
4
3M Isotope Products Lab. Nuclear Associates Technical Operations Manning Research
1967----1976 1970s----1978
6DIA
rad rad
5 2
þ2.5 þ14
13 ------
1950s and 1960s
reb
3
4.1
------
1960s
reb
4
þ36
17
Intercomparison with old NIST calibrations Extrapolation chamber: 3-mm diam. collecting electrode; 0.1----0.25-mm air gap; at the depth of 7 mg/cm2 TLD Intercomparison with old NIST calibrations
type (large standard deviation in the above comparisons), the results should not be used to make generalizations about other sources of a given type. In case there is doubt about the accuracy of a particular applicator calibration, it should be re-calibrated using the modern techniques recommended in Section 3.2. In addition to this comprehensive comparison for old 90Sr----90Y sources, systematic comparisons of calibration methods for planar 90Sr----90Y, and planar and concave 106Ru----106Rh sources (under current production) have been carried out for this Report (Soares et al., 2001). In this comparison, different techniques presented in Sections 3.2 and 4.4 have been applied at a few laboratories. A summary of the results is presented in Table 5.2. If one averages across all results by different techniques for the three sources, the results agree to within 30 % for the planar 90Sr----90Y source (eight results), within 35 % for the planar 106Ru----106Rh source (nine results), and within 40 % for the concave 106 Ru----106Rh source (six results). However, for the planar sources, the two extrapolation-chamber measurements agree to within 12 %, and the results for the thinnest detectors (GAFChromic film, 0.3 mm TLD and 0.4 mm scintillator) agree to within 12 %
with the mean of extrapolation-chamber results. The results of the two most reliable measurements for the concave source (GAFChromic film and 0.3 mm TLD in WT1) are consistent to within 3 %. The other detectors, yielding higher disagreement are far from ideal point-like detectors. The larger deviations are probably attributable to the finite thickness and cross-sectional area of these detectors, particularly when the source to be measured is not highly uniform (e.g., 106Ru----106Rh sources). The estimated relative uncertainties (one standard deviation) and their dominant component for each type of calibration are given in Table 5.3. The component relative uncertainty for source----detector positioning varies considerably from case to case, because the dose gradients near the reference depth of 1 mm are very large depending on the source: 6.5 %, 3 % and 2 % per 0.1 mm for the 90Sr----90Y flat source, the 106Ru----106Rh flat source and the 106 Ru----106Rh curved source, respectively, and positioning in water is generally less accurate than in plastic. In view of the stated relative uncertainties, the consistency of the results for the extrapolation chambers and the thinnest detectors is good. The results support the conclusion that the extrapolation chamber should be the primary method for planar 68
Downloaded from http://jicru.oxfordjournals.org/ at University of California, San Diego on June 8, 2015
Manufacturer
DOSIMETRY COMPARISIONS Table 5.2. Results of comparisons for the calibration of ophthalmic applicators (Soares et al., 2001): Absorbed-dose rate to water on the central axis at the reference depth of 1 mm (reference date January 1 1997). Source
Extrapolation Extrapolation GAFchromic chamber chamber film in WT1
GAFchromic film in polycarbonate
TLD, 0.3 mm, in WT1
Scintillator, 0.4 mm, in water
TLD, 1 mm, in WT1
Alanine, 1.2 mm, in WT1
Diamond in water
Sr----90Y 0.237 Gy s1 0.260b Gy s1 0.229 Gy s1 0.258 Gy s1 0.248 Gy s1 0.278 Gy s1 0.308 Gy s1 0.272 Gy s1 -----NEN 0258 106 Ru----106Rh 1.61c mGy s1 1.82c mGy s1 1.62 mGy s1 1.63c mGy s1 1.40 mGy s1 1.69 mGy s1 1.75 mGy s1 1.49 mGy s1 1.99 mGy s1 CCB 570 Planar 106 Ru----106Rh ----------2.48 mGy s1 2.10d mGy s1 2.55 mGy s1 -----2.79 mGy s1 2.89 mGy s1 3.17 mGy s1 CCB 511 concave 90
No distinction has been made between measurements in plastic and water phantom materials. Estimated from a contact measurement with a factor of 0.573 applied to correct to a depth of 1 mm. (c) Estimated from a contact measurement with a factor of 0.742 applied to correct to a depth of 1 mm. (d) Estimated from a measurement at a depth of 1.2 mm with a factor of 1.082 applied to correct to a depth of 1 mm. (b)
Table 5.3. Results of comparisons for the calibration of ophthalmic applicators (Soares et al., 2001): estimated relative uncertainties (one standard deviation, %) for the calibrations by different methods (cf. Table 5.2). Source
Extrapolation chamber
GAFchromic film in WT1
TLD, 0.3 mm, in WT1
Scintillator, 0.4 mm, in water
TLD, 1 mm, in WT1
Alanine, 1.2 mm, in WT1
Diamond in water
90 Sr----90Y NEN 0258 106 Ru----106Rh CCB 570 planar 106 Ru----106Rh CCB 511 concave Major component uncertainty
6
7.5
8----10a
7
8----10a
5----7b
----
6
7.5
8----10a
7
8----10a
5----7b
10
------
7.5
8----10a
------
8----10a
5----7b
10
Area of collecting electrode
Variation of thickness of sensitive emulsion coating
Effective measuring point of a finite detector
Accuracy of positioning in water
Effective measuring point of a finite detector
Effective measuring point of a finite detector
Accuracy of positioning in water
(a)
Depends on the depth in the phantom, applicator type, and the TLD thickness. When irradiated to absorbed doses of 10----20 Gy.
(b)
mainly ophthalmic applicators with planar 90Sr----90Y sources and concave 106Ru----106Rh sources. The various measurement and calculation techniques described in Section 4 have been applied. A number of selected publications are collected in Table 5.4, including those specially conducted for this Report (Cross et al., 2001; Soares et al., 2001).
sources, as recommended in Section 3.2, with a relative uncertainty of 4 mm. This translates into large spreads in the dose measured at this radius, as indicated in the radial-dose plot of this film. This non-uniformity makes comparisons of off-axis measurements problematic, particularly if only a single trace across the source surface is made.
of the detectors. These data are also the most difficult to compare. In the special study (Soares et al., 2001) for this Report, measurements at off-axis points were performed only with two of the detection systems: radiochromic film and plastic scintillators. As an example of the amount of data available from the radiochromic film measurements, isodose-contour plots and radial-dose distributions obtained from a 90Sr----90Y planar source at 1 mm are shown in Figure 5.3, and 74
DOSIMETRY COMPARISIONS
Figure 5.5 (Soares et al., 2001) shows off-axis measurement results for the two planar sources, 90 Sr----90Y and 106Ru----106Rh, at several depths between contact and 5 mm. For comparison, the results of a Monte Carlo calculation with the EGS4 code (Cross et al., 2001) are also shown in this figure. To simplify the presentation, the film data shown are averages for each radius, rather than the wide spread shown in Figure 5.4. For treatment planning in clinical practice, the central-axis and off-axis dose distributions are converted to isodose contours. Figure 5.6 gives an example of typical isodose contours in the eye, for planar and concave 106Ru----106Rh sources, calculated for this Report by Cross using the ACCEPT 3.0 Monte Carlo code. The above results support the general recommendation that the uniformity of each individual source should be checked (Section 7). For this purpose, radiochromic film or small scintillation detectors
offer a convenient technique with acceptable accuracy (Section 4.4.2). 5.2.2
Low-energy photon sources
Recent measurements and calculations of dose distributions for low-energy photon sources generally agree within experimental uncertainties. The values recommended in this Report for the general formalism of dose distributions (Section 4. 1) are the current (Rivard et al., 2004) consensus values obtained from available measured and calculated data. The relative uncertainties in dose-rate constants are 5 %, and for the radial-dose functions they are estimated to be about 3----5 % in the range 0.5----5 cm. The relative uncertainties of the anisotropy functions are difficult to estimate with the available data, but a reasonable conjecture might be perhaps from 5 % increasing to 20 % at some distances and angles.
75
Downloaded from http://jicru.oxfordjournals.org/ at University of California, San Diego on June 8, 2015
Figure 5.6. Isodose contours in the eye (12.2 mm radius), calculated by the ACCEPT 3.0 Monte Carlo code, for a circular, (a) planar and (b) concave 106Ru----106Rh applicator (BEBIG CCB570 and BEBIG CCB346, respectively) directly against the eye (Cross, calculations for this report). The sources are covered by a 0.1 mm thick silver window, have a 0.9 mm thick silver backing, and an active diameter of (a) 20 mm and (b) 21 mm. The numbers on the curves give the dose relative to that on the axis at a depth of 1 mm. The ‘‘central plane’’ is one through the center of the eye and normal to the source axis.
5
DOI: 10.1093/jicru/ndh029
DOSIMETRY COMPARISONS
5.1 COMPARISON OF BETA-RAY SOURCE CALIBRATIONS As described earlier (Section 2.2), beta-ray ophthalmic applicators have been produced since 1950. Between 1950 and 1983, eight manufacturers of 90Sr----90Y applicators have been identified (Soares, 1995). 106Ru----106Rh applicators have been produced since the early 1960s, and the sources are still in production. To calibrate the sources, the manufacturers have used methods that have been locally developed, in some cases being based on the calibration of a few reference sources at a standards laboratory. Except in a few cases, the methods of calibration have not been well documented. Until 1989, no comparison of the methods of calibration had been reported. In 1989, a comparison of the calibration at the NIST with that from the laboratory of the only remaining manufacturer of the 90 Sr----90Y sources was reported, revealing a large
discrepancy in the calibration results (Goetsch, 1989; Soares, 1991). This result gave rise to a critical analysis of the methods of calibration, which led to better knowledge of the physical basis of calibration and to considerably improved measurement techniques (described in Section 3.2.2). After the major revision of measurement techniques at the NIST (Soares, 1991), comparison of the calibration results with the improved technique were carried out for a total of 58 90Sr----90Y sources representing seven former manufacturers and the only remaining manufacturer. A summary of the manufacturer’s source and calibration information is shown in Table 5.1 (Soares, 1995). If the manufacturer quoted a dose rate in the archaic unit of reb (roentgen-equivalent-beta), a conversion of 0.00982 Gy reb1 was used (Section 3.1.1). For all the sources examined from a given manufacturer, the average difference in the surface-absorbed-dose rates obtained by the NIST from those quoted by the manufacturer varied from 25 to þ36 % among the manufacturers, while the relative standard deviation for a given manufacturer could be as low as 4 % or as high as 17 % (see Table 5.1). The individual differences for a single source were up to 61 %. These differences are significant, if the uncertainty of the calibrations at the other manufacturers were of the same level as that reported by Nycomed Amersham. Significant differences in the uniformity of the dose rate over the effective source area were found among the source types of different manufacturers, and also among individual sources of a given type. Of particular interest is the comparison between the NIST and the then remaining manufacturer of 90Sr----90Y sources, Nycomed Amersham. Taking into account the known differences of the calibration methods between the standards laboratory and the manufacturer, the average difference of calibration results could be reduced to 13 % with a relative standard deviation of 4 %. Because the quoted uncertainty of the calibration by the NIST is 3.5 % (one standard deviation; cf. Table 3.3) and that of the calibration by Nycomed Amersham is 20 %, the difference might not be significant. In view of the individual differences among sources of the same
ª International Commission on Radiation Units and Measurements 2004
Downloaded from http://jicru.oxfordjournals.org/ at University of California, San Diego on June 8, 2015
Comparisons of the calibrations of sources among standard dosimetry laboratories are important to create confidence in the quoted uncertainties of the calibrations. The analysis of the results of such comparisons should lead to reductions in measurement uncertainties for all the laboratories involved. Comparisons of actual dose distributions are not always straightforward to carry out. Dose distributions measured or calculated by different methods are usually compared at a few relevant points, and the consistency expressed as the maximum deviations of relative doses in percent or, in case of very steep dose gradients, as the maximum deviations of the positions of isodose lines in millimeters. In the following, only comparisons dealing with beta-ray planar and concave sources are discussed. For beta-ray sources used in intravascular brachytherapy, only a few laboratories can provide calibrations, and results of comparisons among them are not yet available. For low-energy photon sources, comparisons of calibrations at the primary-standard level have not been possible because only one laboratory (NIST) provides such calibrations.
DOSIMETRY OF BETA RAYS AND LOW-ENERGY PHOTONS FOR BRACHYTHERAPY WITH SEALED Table 5.1. Summary of sources and their calibration by the manufacturers, and the results of calibration comparison with a standards laboratory (NIST), for 90Sr----90Y ophthalmic applicators (from Soares, 1995, with permission). Dates of manufacture
Model
Calibration method
Units
Number compared
Average difference (%)
SD (%)
Tracelab ICN/Tracelab
1950----1969 1969----late 1970s
RA-1
Extrapolation chamber: 5-mm diameter collecting electrode; 0.2----0.5 mm air gap
reb
18
18
15
Atlantic Research Corporation Atomchem New England Nuclear
early 1960s----1969
B1
rad
8
þ2.5
12
1969----late 1970s 1976----1983
NB-1
rad
8
25
4
Nycomed Amersham (earlier Amersham International)
1950s----present
SIA.20
rad
10
23
4
3M Isotope Products Lab. Nuclear Associates Technical Operations Manning Research
1967----1976 1970s----1978
6DIA
rad rad
5 2
þ2.5 þ14
13 ------
1950s and 1960s
reb
3
4.1
------
1960s
reb
4
þ36
17
Intercomparison with old NIST calibrations Extrapolation chamber: 3-mm diam. collecting electrode; 0.1----0.25-mm air gap; at the depth of 7 mg/cm2 TLD Intercomparison with old NIST calibrations
type (large standard deviation in the above comparisons), the results should not be used to make generalizations about other sources of a given type. In case there is doubt about the accuracy of a particular applicator calibration, it should be re-calibrated using the modern techniques recommended in Section 3.2. In addition to this comprehensive comparison for old 90Sr----90Y sources, systematic comparisons of calibration methods for planar 90Sr----90Y, and planar and concave 106Ru----106Rh sources (under current production) have been carried out for this Report (Soares et al., 2001). In this comparison, different techniques presented in Sections 3.2 and 4.4 have been applied at a few laboratories. A summary of the results is presented in Table 5.2. If one averages across all results by different techniques for the three sources, the results agree to within 30 % for the planar 90Sr----90Y source (eight results), within 35 % for the planar 106Ru----106Rh source (nine results), and within 40 % for the concave 106 Ru----106Rh source (six results). However, for the planar sources, the two extrapolation-chamber measurements agree to within 12 %, and the results for the thinnest detectors (GAFChromic film, 0.3 mm TLD and 0.4 mm scintillator) agree to within 12 %
with the mean of extrapolation-chamber results. The results of the two most reliable measurements for the concave source (GAFChromic film and 0.3 mm TLD in WT1) are consistent to within 3 %. The other detectors, yielding higher disagreement are far from ideal point-like detectors. The larger deviations are probably attributable to the finite thickness and cross-sectional area of these detectors, particularly when the source to be measured is not highly uniform (e.g., 106Ru----106Rh sources). The estimated relative uncertainties (one standard deviation) and their dominant component for each type of calibration are given in Table 5.3. The component relative uncertainty for source----detector positioning varies considerably from case to case, because the dose gradients near the reference depth of 1 mm are very large depending on the source: 6.5 %, 3 % and 2 % per 0.1 mm for the 90Sr----90Y flat source, the 106Ru----106Rh flat source and the 106 Ru----106Rh curved source, respectively, and positioning in water is generally less accurate than in plastic. In view of the stated relative uncertainties, the consistency of the results for the extrapolation chambers and the thinnest detectors is good. The results support the conclusion that the extrapolation chamber should be the primary method for planar 68
Downloaded from http://jicru.oxfordjournals.org/ at University of California, San Diego on June 8, 2015
Manufacturer
DOSIMETRY COMPARISIONS Table 5.2. Results of comparisons for the calibration of ophthalmic applicators (Soares et al., 2001): Absorbed-dose rate to water on the central axis at the reference depth of 1 mm (reference date January 1 1997). Source
Extrapolation Extrapolation GAFchromic chamber chamber film in WT1
GAFchromic film in polycarbonate
TLD, 0.3 mm, in WT1
Scintillator, 0.4 mm, in water
TLD, 1 mm, in WT1
Alanine, 1.2 mm, in WT1
Diamond in water
Sr----90Y 0.237 Gy s1 0.260b Gy s1 0.229 Gy s1 0.258 Gy s1 0.248 Gy s1 0.278 Gy s1 0.308 Gy s1 0.272 Gy s1 -----NEN 0258 106 Ru----106Rh 1.61c mGy s1 1.82c mGy s1 1.62 mGy s1 1.63c mGy s1 1.40 mGy s1 1.69 mGy s1 1.75 mGy s1 1.49 mGy s1 1.99 mGy s1 CCB 570 Planar 106 Ru----106Rh ----------2.48 mGy s1 2.10d mGy s1 2.55 mGy s1 -----2.79 mGy s1 2.89 mGy s1 3.17 mGy s1 CCB 511 concave 90
No distinction has been made between measurements in plastic and water phantom materials. Estimated from a contact measurement with a factor of 0.573 applied to correct to a depth of 1 mm. (c) Estimated from a contact measurement with a factor of 0.742 applied to correct to a depth of 1 mm. (d) Estimated from a measurement at a depth of 1.2 mm with a factor of 1.082 applied to correct to a depth of 1 mm. (b)
Table 5.3. Results of comparisons for the calibration of ophthalmic applicators (Soares et al., 2001): estimated relative uncertainties (one standard deviation, %) for the calibrations by different methods (cf. Table 5.2). Source
Extrapolation chamber
GAFchromic film in WT1
TLD, 0.3 mm, in WT1
Scintillator, 0.4 mm, in water
TLD, 1 mm, in WT1
Alanine, 1.2 mm, in WT1
Diamond in water
90 Sr----90Y NEN 0258 106 Ru----106Rh CCB 570 planar 106 Ru----106Rh CCB 511 concave Major component uncertainty
6
7.5
8----10a
7
8----10a
5----7b
----
6
7.5
8----10a
7
8----10a
5----7b
10
------
7.5
8----10a
------
8----10a
5----7b
10
Area of collecting electrode
Variation of thickness of sensitive emulsion coating
Effective measuring point of a finite detector
Accuracy of positioning in water
Effective measuring point of a finite detector
Effective measuring point of a finite detector
Accuracy of positioning in water
(a)
Depends on the depth in the phantom, applicator type, and the TLD thickness. When irradiated to absorbed doses of 10----20 Gy.
(b)
mainly ophthalmic applicators with planar 90Sr----90Y sources and concave 106Ru----106Rh sources. The various measurement and calculation techniques described in Section 4 have been applied. A number of selected publications are collected in Table 5.4, including those specially conducted for this Report (Cross et al., 2001; Soares et al., 2001).
sources, as recommended in Section 3.2, with a relative uncertainty of 4 mm. This translates into large spreads in the dose measured at this radius, as indicated in the radial-dose plot of this film. This non-uniformity makes comparisons of off-axis measurements problematic, particularly if only a single trace across the source surface is made.
of the detectors. These data are also the most difficult to compare. In the special study (Soares et al., 2001) for this Report, measurements at off-axis points were performed only with two of the detection systems: radiochromic film and plastic scintillators. As an example of the amount of data available from the radiochromic film measurements, isodose-contour plots and radial-dose distributions obtained from a 90Sr----90Y planar source at 1 mm are shown in Figure 5.3, and 74
DOSIMETRY COMPARISIONS
Figure 5.5 (Soares et al., 2001) shows off-axis measurement results for the two planar sources, 90 Sr----90Y and 106Ru----106Rh, at several depths between contact and 5 mm. For comparison, the results of a Monte Carlo calculation with the EGS4 code (Cross et al., 2001) are also shown in this figure. To simplify the presentation, the film data shown are averages for each radius, rather than the wide spread shown in Figure 5.4. For treatment planning in clinical practice, the central-axis and off-axis dose distributions are converted to isodose contours. Figure 5.6 gives an example of typical isodose contours in the eye, for planar and concave 106Ru----106Rh sources, calculated for this Report by Cross using the ACCEPT 3.0 Monte Carlo code. The above results support the general recommendation that the uniformity of each individual source should be checked (Section 7). For this purpose, radiochromic film or small scintillation detectors
offer a convenient technique with acceptable accuracy (Section 4.4.2). 5.2.2
Low-energy photon sources
Recent measurements and calculations of dose distributions for low-energy photon sources generally agree within experimental uncertainties. The values recommended in this Report for the general formalism of dose distributions (Section 4. 1) are the current (Rivard et al., 2004) consensus values obtained from available measured and calculated data. The relative uncertainties in dose-rate constants are 5 %, and for the radial-dose functions they are estimated to be about 3----5 % in the range 0.5----5 cm. The relative uncertainties of the anisotropy functions are difficult to estimate with the available data, but a reasonable conjecture might be perhaps from 5 % increasing to 20 % at some distances and angles.
75
Downloaded from http://jicru.oxfordjournals.org/ at University of California, San Diego on June 8, 2015
Figure 5.6. Isodose contours in the eye (12.2 mm radius), calculated by the ACCEPT 3.0 Monte Carlo code, for a circular, (a) planar and (b) concave 106Ru----106Rh applicator (BEBIG CCB570 and BEBIG CCB346, respectively) directly against the eye (Cross, calculations for this report). The sources are covered by a 0.1 mm thick silver window, have a 0.9 mm thick silver backing, and an active diameter of (a) 20 mm and (b) 21 mm. The numbers on the curves give the dose relative to that on the axis at a depth of 1 mm. The ‘‘central plane’’ is one through the center of the eye and normal to the source axis.
