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Dosimetry for irregular shaped fields of beta rays
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1976 Phys. Med. Biol. 21 504 (http://iopscience.iop.org/0031-9155/21/4/002) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 137.132.123.69 This content was downloaded on 03/10/2015 at 12:18
Please note that terms and conditions apply.
PHYS. MED. BIOL.,
1976, VOL. 21, NO. 4,504-508.
@
1976
Dosimetry for Irregular Shaped Fields of Beta Rays S. J. SUPE, PH.Dand SHIV DATTA,
M.SC.
Radiological Monitoring and Research Section, Division of Radiological Protection, Bhabha Atomic Research Centre, Modular Laboratories, Trombay, Bombay 400 085, India
Received 23 February 1976 ABSTRACT. The feasibilityof using various shapesand sizes of field limiting devices and collimators with p-rayeye applicators has necessitated the studyof dosimetry for these fields. A method of calculating surface and depth doses for any shaped field from the data for circular fields is presented. The depth dose evaluation is based on a measured dose function which is defined as the dose rate at a particular depth for a particular circular field. The evaluated values for the surface and depth dose were compared with experimentally obtained values for three non-circular fields. The good agreement in these data indicates the practicability of the method suggested.
1. Introduction Strontium-90 applicators have been used for the treatmentof eye lesions for a number of years. These applicatorswere usually used without anyfield limiting or collimating devices, regardless of the size of the lesion, and resulted in unduly large doses to the lens of the eye. Recently, therefore, the use of field limiting devices (Supe and Cunningham 1963) as well as collimators (Supe 1972, Supe and Rao 1974) has been investigated and shown to be possible for almost all cases. The feasibility of matching the field shape to that of lesion size has raised new problems of dosimetry. As various lesion shapes could be encountered the present dataaboutdosimetry(Krohmer 1951, Kastner and Greenberg 1952, Tochilin and Golden 1953, Freitag, Gore, Greenfield and Hartman 1956, Sinclair andTrott 1956, McTaggart,West, Claypool and Collins 1961, Chhabra 1962, Jones and Dermentzoglou 1971) which pertain only to circular fields would be insufficient for the treatment of the widely varying shapes of the lesions. Furthermore, it would be atedious job to carry out dosimetric studiesfor the varietyof fieldshaping devices which might beused in practice. It was, therefore, felt that a method of estimation of surface and depth doses from the available data for circular fields would be worth investigating. The results of these investigations are presented in this work. 2.
Materials and methods
The data requiredfor this purpose were obtained for a 50 mCi 90Sr-90YRA-1 (Tracerlab) typeeye applicator withfield limiting diaphragms andfor a 450 mCi goSr-goY source with collimator of 1.05 cm length. The uniformity of these sources was tested byscanning the autoradiographs of the sources with a 0.5 mm aperture densitometer. It was found that the RA-1 applicator was uniform
Dosimetry for Irregular Shaped Fields
of Beta Rays
505
within 5 3% and the 450 mCi source was uniform within f 5%. This was also confirmed by using the pin-hole camera method (Supe 1965). The surface dose rate at thesurface of the field limiting diaphragm and the end of the collimator respectively forthe two sources were measured for varying diameters of the fields. An extrapolation chamber witha collecting electrode of 0.9 mm diameter was used for the field diameters larger than 1.5 mm. For field diameters less than 1.5 mm the surface dose was obtained by exposing N 550 films and scanning them with a 0.5 mm aperture densitometer. These values were not expected to be very accurate andwere smoothed by extrapolating the curves obtained for the variation of dose rate with the diameter of the field. The data so obtained are presented in figs 1 and 2 as curves for zero depth in tissue. Tssue depth [mm) 0.0
0.25 0.50
:I
121
01 0
P
//
l
0.75
7.o 1
9
1.5 2.0 2.5
30 3.5 L. 0 Radius of field (mm) Fig. 1. Variation of dose functions for circular fields with radius of field size for an R A - l type 50 mCi soSr-guYeye applicator with field shaping devices.
(mm1
Tissue
0.75 1.0
2.0 2 5
3 0 3.5 1.0
Rodlus of fteld Irnrn)
Fig. 2. Variation of dose functions for circular fields with radius of field size for a 430 mCi 90Sr-90Y source with collimators.
S. J. Xupe and Shiv Datta
506
The central axis depth dose data for a range of field diameters were also obtained for the two sources using the extrapolation chamber. For fields less than 1.5 mm diameter a film method, as described by Supe, Rao and Sawant (1975), was used. The accuracy of the data so obtained, particularly for the was notexpectedto be good. field sizes smaller than 1 mmindiameter, Smoothed data obtained by cross-plotting the variation of depth dose a t a particular depth with the diameter of the field size were used in calculations. The surfacedose rate for a particularfield sizewas multiplied by thecorresponding percentage depth dose data at different depths for the same field size. This gives the dose rates a t different depths on the central axis for circular fields. These dose rates were defined as the 'Dose Functions' for @-rays. These dose functions for the two sources are presented in figs 1 and 2. Field limiting devices for use with the RA-1 type applicator were constructed with field sizes of 6 x 6, 6 x 4 and 6 x 3 mm2. Collimators for the 450 mCi source with fieldsizes of 6.6 x 8.2, 6 x 8 and 6 x 6 mm2 were also fabricated. An extrapolation chamber was used to measure the surface dose rates and the central axis depth doses for these field sizes with the corresponding sources. These data are presented in tables 1, 2 and 2a. A computational method of predicting the surface and depth dose data from the basic measured dose functions (figs 1 and 2)has been developed.The various field sizes were split into 10' sectors and the surface and depth doses D in each case were evaluated using an expression
where Diis the dose function a t a particular depth for a circular field of radius equal to thenominal radius of the ith sector;n is the number of sectors ( n = 36 when 10" sectors are used). The evaluated dose data arepresented in tables 1, 2 and 2a. 3. Discussion It can be seen from the tables that thecomputed and experimental values of surface anddepth dose arein fairly good agreement, andthe procedure
Table 1. Surface dose rates for various field sizes ~~
~
Collimator/field limiting davice size (mm) 6x6 6x4 6x3 6.6 X 8.2 6x8 6x6
t
Dose rate. Extrapolation evaluated chamber Source to surface measurements value s")t distance(rad (cm) 0.1 115.2 0.1 81.2 0.1 62.1 19.5 1.05 1.0518.7 1.0513.8
117.0 77.5 65.0 20.1 19.2 14.5
Dose rates for the 450 mCi goSr-gOY source.
Dose rate, (rad S-l)
Dosimetry for Irregular Shaped Fields of Beta Rays
507
Table 2. Experimental and evaluated depth dose data (RA-l applicator with field shaping devices) Centre of field 1mm off-centre 2 mm off-centre Depth (mm) Experimental Evaluated Experimental Evaluated Experimental Evaluated ~~~
Collimator : 6 x 8 mm 0 1 2 3
100.0 49.0 26.5 13.3
100.0 45.3 22.7 11.3
0 1 2 3
100.0 46.5 24.5 12.2
100.0 44.9 21.7 10.2
0 1 2 3
100.0 46.0 23.5 11.0
100.0 43.7 20.6 9.0
97.9 44.0 22.6 10.8
86.0 41.0 21.5 10.5
92.2 41.2 20.2 10.0
6 x 6 mm 96.9 43.5 20.7 9.9
86.0 39.0 20.0 9.8
91.1 41.0 20.3 9.7
6 x 3 mm 96.8 41.9 20.5 8.4
86.5 37.0 19.5 9.2
90.7 38.7 18.7 8.4
96.2 48.0 24.0 12.0
Collimator: 93.0 45.0 23.0 11.0
Collimator: 93.0 44.0 22.5 10.2
Table 2a. Experimentalandevaluated dose data (450 mCi source with collimators) Collimator size 6.6 X 8.2 mm 86 Xx 86 m m Depth (mm) Experimental Evaluated Experimental Evaluated Experimental Evaluated ~~
~~
0 1 2 3
100 100 64.9 40.9 26.6
100 100 71.7 41.8 25.1
100
100 66.6 46.7 29.0
73.0 43.0 38.7 26.0
64.9 38.7 24.4
69.0 22.3
described abovetherefore seems satisfactoryforpracticalradiotherapeutic purposes. With practice,amanualcomputation can easily be carried out within a period of two hours. We are indebted to Dr. K. G. Vohra and Shri G. Subrahmanian for theirkeen interest in these investigations. Grateful thanks are also due t o Shri S. M. Rao for his help inexperimentalmeasurements.We are indebted to Dr. J. R . Cunningham of Princess Margarette Hospital, Toronto, for his suggestions for improving the text. RESUMI~ La dosimhtrie pour les champs de rayonnements beta de formes irrbgulikres
La praticabilith de l’emploi de collimateurs et de dispositifs limiteurs de champs de formats et de formes diffbrentes, avec des applicateursoculaires ri rayons p, a nhcessitb l’htude de la dosimbtrie pour ces champs. Une mhthode de calcul der?doses en surface et en profondeur pour un champ de
508
Dosimetry for Irregular Shaped Fields
of Beta Rays
n'importe quelle forme, iL partir des donnires disponibles pour des champs circulaires, est prirsentire dens cet exposir. L'Bvaluation dela dose en profondeur est fondire sur unefonctionde dose mesurBe, dirhie comme irtant le taux de dosage b une profondeur particulibre, pour un champ circulaire particulier. Les valeurs calculires pour les doses en profondeur et en surface furent comparbes aux valeurs obtenues expirrimentalement pour trois champs non-circulaires. La bonne correspondance de ces donnBes indique la praticabilitir de la mirthode suggirrire.
ZUSAMMENFASSUNG Dosimetrie fur unregelmassig geformte Felder von Betastrahlen Die Moglichkeit der Verwendung verschiedener Formenund Grossen von Feldbegrenzungsgerlten undKollimatoren bei P-Strahlen-Augenapplikatorenhat die Untersuchung der Dosimetrie fur diese Felder erforderlich gemacht.ZurBerechnung von Oberflachen- und Tiefendosen fur Felder jedweder Form aufgrund der Werte fur Kreisfelder wird eine Methode vorgestellt. Die Abschatzung der Tiefendosis basiert auf einer Massdosisfunktion, die d e h i e r t ist als Dosisrate in bestimmter Tiefe fur ein bestimmtes Kreisfeld. Die abgeschatzten Wertefur Oberflachenund Tiefendosis wurden mit experimentellerzielten Werten fur drei nichtkreisformige Felder verglichen. Die dabei erwiesene guteUbereinstimmung deutet auf die Durchfuhrbarkeit der vorgeschlagenen JIethode hin.
REFERENCES CHHABRA,A. S., 1962, Radiology, 79, 1001. FREITAG,A. B.,GORE, W. A., GREEKFIELD,M. A., and HARTMAN,J. A., 1956, Am. J . Roentg., 75, 1169. JONES, C. H., a n d DERMENTZOGLOU, F., 1971, Br. J . Radiol., 44, 203. KASTNER,J., and GREENBERG,L., 1952, Radiology, 58, 731. J. S., 1951, Am. J . Roentg., 66, 791. KROHMER, MCTAGGART, W. C., WEST,W. D., CLAYPOOL, H.A., and COLLINS,V. P., 1961, Radiology, 76, 278. SIXCLAIR,W. K., a n d TROTT,K.G., 1956, Br. J . Radiol., 29, 15. SUPE, S. J., and CUNNINGHAM, J. R . , 1963, Am. J . Roentg., 89, 570. SUPE, S. J . , 1965, Am. J . Roentg., 94, 989. SUPE, S.J . , 1972, Am. J . Roentg., 114, 24. SUPE, S. J., a n d RAO,S.M., 1974, Br. J . Radiol., 47, 337. SCJPE,S. J., RAO, S."l, and SAWAKT,S. G., 1975, Am. J . Roentg., 123, 36. E., a n d GOLDEN,R . , 1953, Nucleonics, 11, 26. TOCHICIN,
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My IOPscience
Dosimetry for irregular shaped fields of beta rays
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1976 Phys. Med. Biol. 21 504 (http://iopscience.iop.org/0031-9155/21/4/002) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 137.132.123.69 This content was downloaded on 03/10/2015 at 12:18
Please note that terms and conditions apply.
PHYS. MED. BIOL.,
1976, VOL. 21, NO. 4,504-508.
@
1976
Dosimetry for Irregular Shaped Fields of Beta Rays S. J. SUPE, PH.Dand SHIV DATTA,
M.SC.
Radiological Monitoring and Research Section, Division of Radiological Protection, Bhabha Atomic Research Centre, Modular Laboratories, Trombay, Bombay 400 085, India
Received 23 February 1976 ABSTRACT. The feasibilityof using various shapesand sizes of field limiting devices and collimators with p-rayeye applicators has necessitated the studyof dosimetry for these fields. A method of calculating surface and depth doses for any shaped field from the data for circular fields is presented. The depth dose evaluation is based on a measured dose function which is defined as the dose rate at a particular depth for a particular circular field. The evaluated values for the surface and depth dose were compared with experimentally obtained values for three non-circular fields. The good agreement in these data indicates the practicability of the method suggested.
1. Introduction Strontium-90 applicators have been used for the treatmentof eye lesions for a number of years. These applicatorswere usually used without anyfield limiting or collimating devices, regardless of the size of the lesion, and resulted in unduly large doses to the lens of the eye. Recently, therefore, the use of field limiting devices (Supe and Cunningham 1963) as well as collimators (Supe 1972, Supe and Rao 1974) has been investigated and shown to be possible for almost all cases. The feasibility of matching the field shape to that of lesion size has raised new problems of dosimetry. As various lesion shapes could be encountered the present dataaboutdosimetry(Krohmer 1951, Kastner and Greenberg 1952, Tochilin and Golden 1953, Freitag, Gore, Greenfield and Hartman 1956, Sinclair andTrott 1956, McTaggart,West, Claypool and Collins 1961, Chhabra 1962, Jones and Dermentzoglou 1971) which pertain only to circular fields would be insufficient for the treatment of the widely varying shapes of the lesions. Furthermore, it would be atedious job to carry out dosimetric studiesfor the varietyof fieldshaping devices which might beused in practice. It was, therefore, felt that a method of estimation of surface and depth doses from the available data for circular fields would be worth investigating. The results of these investigations are presented in this work. 2.
Materials and methods
The data requiredfor this purpose were obtained for a 50 mCi 90Sr-90YRA-1 (Tracerlab) typeeye applicator withfield limiting diaphragms andfor a 450 mCi goSr-goY source with collimator of 1.05 cm length. The uniformity of these sources was tested byscanning the autoradiographs of the sources with a 0.5 mm aperture densitometer. It was found that the RA-1 applicator was uniform
Dosimetry for Irregular Shaped Fields
of Beta Rays
505
within 5 3% and the 450 mCi source was uniform within f 5%. This was also confirmed by using the pin-hole camera method (Supe 1965). The surface dose rate at thesurface of the field limiting diaphragm and the end of the collimator respectively forthe two sources were measured for varying diameters of the fields. An extrapolation chamber witha collecting electrode of 0.9 mm diameter was used for the field diameters larger than 1.5 mm. For field diameters less than 1.5 mm the surface dose was obtained by exposing N 550 films and scanning them with a 0.5 mm aperture densitometer. These values were not expected to be very accurate andwere smoothed by extrapolating the curves obtained for the variation of dose rate with the diameter of the field. The data so obtained are presented in figs 1 and 2 as curves for zero depth in tissue. Tssue depth [mm) 0.0
0.25 0.50
:I
121
01 0
P
//
l
0.75
7.o 1
9
1.5 2.0 2.5
30 3.5 L. 0 Radius of field (mm) Fig. 1. Variation of dose functions for circular fields with radius of field size for an R A - l type 50 mCi soSr-guYeye applicator with field shaping devices.
(mm1
Tissue
0.75 1.0
2.0 2 5
3 0 3.5 1.0
Rodlus of fteld Irnrn)
Fig. 2. Variation of dose functions for circular fields with radius of field size for a 430 mCi 90Sr-90Y source with collimators.
S. J. Xupe and Shiv Datta
506
The central axis depth dose data for a range of field diameters were also obtained for the two sources using the extrapolation chamber. For fields less than 1.5 mm diameter a film method, as described by Supe, Rao and Sawant (1975), was used. The accuracy of the data so obtained, particularly for the was notexpectedto be good. field sizes smaller than 1 mmindiameter, Smoothed data obtained by cross-plotting the variation of depth dose a t a particular depth with the diameter of the field size were used in calculations. The surfacedose rate for a particularfield sizewas multiplied by thecorresponding percentage depth dose data at different depths for the same field size. This gives the dose rates a t different depths on the central axis for circular fields. These dose rates were defined as the 'Dose Functions' for @-rays. These dose functions for the two sources are presented in figs 1 and 2. Field limiting devices for use with the RA-1 type applicator were constructed with field sizes of 6 x 6, 6 x 4 and 6 x 3 mm2. Collimators for the 450 mCi source with fieldsizes of 6.6 x 8.2, 6 x 8 and 6 x 6 mm2 were also fabricated. An extrapolation chamber was used to measure the surface dose rates and the central axis depth doses for these field sizes with the corresponding sources. These data are presented in tables 1, 2 and 2a. A computational method of predicting the surface and depth dose data from the basic measured dose functions (figs 1 and 2)has been developed.The various field sizes were split into 10' sectors and the surface and depth doses D in each case were evaluated using an expression
where Diis the dose function a t a particular depth for a circular field of radius equal to thenominal radius of the ith sector;n is the number of sectors ( n = 36 when 10" sectors are used). The evaluated dose data arepresented in tables 1, 2 and 2a. 3. Discussion It can be seen from the tables that thecomputed and experimental values of surface anddepth dose arein fairly good agreement, andthe procedure
Table 1. Surface dose rates for various field sizes ~~
~
Collimator/field limiting davice size (mm) 6x6 6x4 6x3 6.6 X 8.2 6x8 6x6
t
Dose rate. Extrapolation evaluated chamber Source to surface measurements value s")t distance(rad (cm) 0.1 115.2 0.1 81.2 0.1 62.1 19.5 1.05 1.0518.7 1.0513.8
117.0 77.5 65.0 20.1 19.2 14.5
Dose rates for the 450 mCi goSr-gOY source.
Dose rate, (rad S-l)
Dosimetry for Irregular Shaped Fields of Beta Rays
507
Table 2. Experimental and evaluated depth dose data (RA-l applicator with field shaping devices) Centre of field 1mm off-centre 2 mm off-centre Depth (mm) Experimental Evaluated Experimental Evaluated Experimental Evaluated ~~~
Collimator : 6 x 8 mm 0 1 2 3
100.0 49.0 26.5 13.3
100.0 45.3 22.7 11.3
0 1 2 3
100.0 46.5 24.5 12.2
100.0 44.9 21.7 10.2
0 1 2 3
100.0 46.0 23.5 11.0
100.0 43.7 20.6 9.0
97.9 44.0 22.6 10.8
86.0 41.0 21.5 10.5
92.2 41.2 20.2 10.0
6 x 6 mm 96.9 43.5 20.7 9.9
86.0 39.0 20.0 9.8
91.1 41.0 20.3 9.7
6 x 3 mm 96.8 41.9 20.5 8.4
86.5 37.0 19.5 9.2
90.7 38.7 18.7 8.4
96.2 48.0 24.0 12.0
Collimator: 93.0 45.0 23.0 11.0
Collimator: 93.0 44.0 22.5 10.2
Table 2a. Experimentalandevaluated dose data (450 mCi source with collimators) Collimator size 6.6 X 8.2 mm 86 Xx 86 m m Depth (mm) Experimental Evaluated Experimental Evaluated Experimental Evaluated ~~
~~
0 1 2 3
100 100 64.9 40.9 26.6
100 100 71.7 41.8 25.1
100
100 66.6 46.7 29.0
73.0 43.0 38.7 26.0
64.9 38.7 24.4
69.0 22.3
described abovetherefore seems satisfactoryforpracticalradiotherapeutic purposes. With practice,amanualcomputation can easily be carried out within a period of two hours. We are indebted to Dr. K. G. Vohra and Shri G. Subrahmanian for theirkeen interest in these investigations. Grateful thanks are also due t o Shri S. M. Rao for his help inexperimentalmeasurements.We are indebted to Dr. J. R . Cunningham of Princess Margarette Hospital, Toronto, for his suggestions for improving the text. RESUMI~ La dosimhtrie pour les champs de rayonnements beta de formes irrbgulikres
La praticabilith de l’emploi de collimateurs et de dispositifs limiteurs de champs de formats et de formes diffbrentes, avec des applicateursoculaires ri rayons p, a nhcessitb l’htude de la dosimbtrie pour ces champs. Une mhthode de calcul der?doses en surface et en profondeur pour un champ de
508
Dosimetry for Irregular Shaped Fields
of Beta Rays
n'importe quelle forme, iL partir des donnires disponibles pour des champs circulaires, est prirsentire dens cet exposir. L'Bvaluation dela dose en profondeur est fondire sur unefonctionde dose mesurBe, dirhie comme irtant le taux de dosage b une profondeur particulibre, pour un champ circulaire particulier. Les valeurs calculires pour les doses en profondeur et en surface furent comparbes aux valeurs obtenues expirrimentalement pour trois champs non-circulaires. La bonne correspondance de ces donnBes indique la praticabilitir de la mirthode suggirrire.
ZUSAMMENFASSUNG Dosimetrie fur unregelmassig geformte Felder von Betastrahlen Die Moglichkeit der Verwendung verschiedener Formenund Grossen von Feldbegrenzungsgerlten undKollimatoren bei P-Strahlen-Augenapplikatorenhat die Untersuchung der Dosimetrie fur diese Felder erforderlich gemacht.ZurBerechnung von Oberflachen- und Tiefendosen fur Felder jedweder Form aufgrund der Werte fur Kreisfelder wird eine Methode vorgestellt. Die Abschatzung der Tiefendosis basiert auf einer Massdosisfunktion, die d e h i e r t ist als Dosisrate in bestimmter Tiefe fur ein bestimmtes Kreisfeld. Die abgeschatzten Wertefur Oberflachenund Tiefendosis wurden mit experimentellerzielten Werten fur drei nichtkreisformige Felder verglichen. Die dabei erwiesene guteUbereinstimmung deutet auf die Durchfuhrbarkeit der vorgeschlagenen JIethode hin.
REFERENCES CHHABRA,A. S., 1962, Radiology, 79, 1001. FREITAG,A. B.,GORE, W. A., GREEKFIELD,M. A., and HARTMAN,J. A., 1956, Am. J . Roentg., 75, 1169. JONES, C. H., a n d DERMENTZOGLOU, F., 1971, Br. J . Radiol., 44, 203. KASTNER,J., and GREENBERG,L., 1952, Radiology, 58, 731. J. S., 1951, Am. J . Roentg., 66, 791. KROHMER, MCTAGGART, W. C., WEST,W. D., CLAYPOOL, H.A., and COLLINS,V. P., 1961, Radiology, 76, 278. SIXCLAIR,W. K., a n d TROTT,K.G., 1956, Br. J . Radiol., 29, 15. SUPE, S. J., and CUNNINGHAM, J. R . , 1963, Am. J . Roentg., 89, 570. SUPE, S. J . , 1965, Am. J . Roentg., 94, 989. SUPE, S.J . , 1972, Am. J . Roentg., 114, 24. SUPE, S. J., a n d RAO,S.M., 1974, Br. J . Radiol., 47, 337. SCJPE,S. J., RAO, S."l, and SAWAKT,S. G., 1975, Am. J . Roentg., 123, 36. E., a n d GOLDEN,R . , 1953, Nucleonics, 11, 26. TOCHICIN,
