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ICRU report 38 reference isodose dimensions and MG-HRS as estimators of tissue volume irradiated by intracavitary implants
Proceedings of the 33rd Annual ASTRO Meeting
16
88 A NOMOGRAPH
FOR PERMANENT
Janaki M. Srinivasan, Memorial
IMPLANTS
Lowell L. Anderson
Sloan-Kettering
OF 103Pd SEEDS
and Louis B. Harrison
Cancer Center, New York, NY 10021
Purpose: Because its half-life (17.0 days) is shorter than the half-life (59.6 days) of 1251 and because it emits photons of even lower energy, 103Pd is being substituted for 1251 in permanent implants for which it is desired to deliver a higher initial dose rate with readily achieved radiation protection. We have constructed a nomograph to assist in determining both the total seed strength required and the appropriate needle spacing for 103Pd implants. Materials & Methods: We have calculated the “matched peripheral dose” (MPD), i. e., the dose for which the isodose contocz volume is equal to the target volume, for sixty-four l251 and fifteen 103Pd actual implants as if 103Pd had been used for all of them, employing a computer lookup table based on single-seed dose distribution measurements in solid water. The calculated data were used to obtain a least-squares fil: to a linear relationship between the logarithm of the apparent activity for a given MPD and the logarithm of the average dimension, da (cm). Results: We found that, for lo3Pd seeds, an apparent activity (in mCi) equal to 3.2 d&56 produces a nominal MPD of 11,500 cGy for this group of implants. A 103Pd nomograph similar to the 125I nomograph has been constructed on the basis of this power function relationship. The corresponding (16,000 cGy) function for 1251 is 1.34 d&2. Conclusion: Our nomographic guide for planning 103Pd implants calls for total seed strength to increase significantly faster as a function of target volume average dimension than is the case for 1251. It will greatly facilitate the intraoperative application of to3Pd seeds.
a9 ICRU REPORT 38 REFE:RENCE ISODOSE DIMENSIONS VOLUME IRRADIATED BY INTRACAVITARY IMPLANTS
AND
MG-HRS
AS ESTIMATORS
OF TISSUE
Avraham Eisbruch, M.D., Jeffrey F. Williamson, Ph.D., Duncan Ross Dickson, B.S. and Perry W. Grigsby, M.D. Radiation Oncology Center, Medlinckrodt Institute of Radiology, Washington University, St. Louis, MO 63110 m: Several investigators have proposed using the product of ICRU Report 38 reference iscdose dimensions to estimate tissue volumes encompassed by iscdose surfaces, to better predict outcome and sequela of irradiation of uterine cervix cancer. We examined the accuracy of this model in estimating isodose volume, and propose a new model which predicts volume using mg-hr alone. and Metho&: Orthogonal films from 128 patients were used to reconstruct the 137~ssource coordinates of 203 intracavitary implants. The source location, strength and duration data were used to calculate dose-volume histograms (DVH) yielding the volume enclosed by each dose level as well as the orthogonal dimension (H,W,and T) of each isodose as defined by ICRU Report 38. Using bony landmarks to align simulation films for different insertions in the same patient, similar calculations were repeated for combined implant source coordinates. The ratio T x W x H to actual volume was computed for 160 individual and 37 combined implant plans consisting of tandem and ovoids and for 43 individual and 14 combined implant plans consisting only of tandems and/or cylinders. Curve-fitting techniques were used to see whether tissue volume, as evaluated by DVH’s, could be predicted by simple functions of mg-hrs and dose. Results: Curve fitting revealed that volume could be predicted by a modified power-law function of the mghr/dose ratio, x, using four best-fit parameters: predicted volume = (104.8 - 8.103 x + 0.437 x2) . x 1.635, where x = mgRaEq-hr/cGy. This was used to calculate the ratio predicted volume/actual volume for the implants. Results are given for the individual implants, in terms of mean f % standard deviation: hrs mpdell
llIm!im + -Jicls 20 40 60 80
1.903 + 7.2% 2.0!)1* 18% 2.346 * 27% 2.5!>0If:32%
1.722 * 7.5% 1.632 + 13% 1.567 + 22% 1.570 f 28%
1.016 f 6.5% 1.094f 11% 1.187 f 13%
1.035 * 2.8% 1.040 + 4.4% 1.066 + 6.5% 1.103 & 8.3%
The results for combined implants show similar means but smaller variability; the variability of the mg-hrs model remains smaller than that of the ICRU model. In 95% of the implants, the volume predicted by our power-law model is within * 10% of the actual volume when mghr/cCiy = 0.8. This variability decreases with increasing mghr/cGy. Conclusions: The product of ICRU report 38 orthogonal dimensions is unsatisfactory in predicting the actual volume irradiated due to the large variations in WHT/vcdume between implants, and the large systematic differences between different implant types. Isodose surface volumes can be accurately predicted knowing only mg-hrs, resulting in relatively small patient-to-patient variations. Specification of volume enclosed by an isodose curve does not add substantially to the information gained by specifying mg-hr.
16
88 A NOMOGRAPH
FOR PERMANENT
Janaki M. Srinivasan, Memorial
IMPLANTS
Lowell L. Anderson
Sloan-Kettering
OF 103Pd SEEDS
and Louis B. Harrison
Cancer Center, New York, NY 10021
Purpose: Because its half-life (17.0 days) is shorter than the half-life (59.6 days) of 1251 and because it emits photons of even lower energy, 103Pd is being substituted for 1251 in permanent implants for which it is desired to deliver a higher initial dose rate with readily achieved radiation protection. We have constructed a nomograph to assist in determining both the total seed strength required and the appropriate needle spacing for 103Pd implants. Materials & Methods: We have calculated the “matched peripheral dose” (MPD), i. e., the dose for which the isodose contocz volume is equal to the target volume, for sixty-four l251 and fifteen 103Pd actual implants as if 103Pd had been used for all of them, employing a computer lookup table based on single-seed dose distribution measurements in solid water. The calculated data were used to obtain a least-squares fil: to a linear relationship between the logarithm of the apparent activity for a given MPD and the logarithm of the average dimension, da (cm). Results: We found that, for lo3Pd seeds, an apparent activity (in mCi) equal to 3.2 d&56 produces a nominal MPD of 11,500 cGy for this group of implants. A 103Pd nomograph similar to the 125I nomograph has been constructed on the basis of this power function relationship. The corresponding (16,000 cGy) function for 1251 is 1.34 d&2. Conclusion: Our nomographic guide for planning 103Pd implants calls for total seed strength to increase significantly faster as a function of target volume average dimension than is the case for 1251. It will greatly facilitate the intraoperative application of to3Pd seeds.
a9 ICRU REPORT 38 REFE:RENCE ISODOSE DIMENSIONS VOLUME IRRADIATED BY INTRACAVITARY IMPLANTS
AND
MG-HRS
AS ESTIMATORS
OF TISSUE
Avraham Eisbruch, M.D., Jeffrey F. Williamson, Ph.D., Duncan Ross Dickson, B.S. and Perry W. Grigsby, M.D. Radiation Oncology Center, Medlinckrodt Institute of Radiology, Washington University, St. Louis, MO 63110 m: Several investigators have proposed using the product of ICRU Report 38 reference iscdose dimensions to estimate tissue volumes encompassed by iscdose surfaces, to better predict outcome and sequela of irradiation of uterine cervix cancer. We examined the accuracy of this model in estimating isodose volume, and propose a new model which predicts volume using mg-hr alone. and Metho&: Orthogonal films from 128 patients were used to reconstruct the 137~ssource coordinates of 203 intracavitary implants. The source location, strength and duration data were used to calculate dose-volume histograms (DVH) yielding the volume enclosed by each dose level as well as the orthogonal dimension (H,W,and T) of each isodose as defined by ICRU Report 38. Using bony landmarks to align simulation films for different insertions in the same patient, similar calculations were repeated for combined implant source coordinates. The ratio T x W x H to actual volume was computed for 160 individual and 37 combined implant plans consisting of tandem and ovoids and for 43 individual and 14 combined implant plans consisting only of tandems and/or cylinders. Curve-fitting techniques were used to see whether tissue volume, as evaluated by DVH’s, could be predicted by simple functions of mg-hrs and dose. Results: Curve fitting revealed that volume could be predicted by a modified power-law function of the mghr/dose ratio, x, using four best-fit parameters: predicted volume = (104.8 - 8.103 x + 0.437 x2) . x 1.635, where x = mgRaEq-hr/cGy. This was used to calculate the ratio predicted volume/actual volume for the implants. Results are given for the individual implants, in terms of mean f % standard deviation: hrs mpdell
llIm!im + -Jicls 20 40 60 80
1.903 + 7.2% 2.0!)1* 18% 2.346 * 27% 2.5!>0If:32%
1.722 * 7.5% 1.632 + 13% 1.567 + 22% 1.570 f 28%
1.016 f 6.5% 1.094f 11% 1.187 f 13%
1.035 * 2.8% 1.040 + 4.4% 1.066 + 6.5% 1.103 & 8.3%
The results for combined implants show similar means but smaller variability; the variability of the mg-hrs model remains smaller than that of the ICRU model. In 95% of the implants, the volume predicted by our power-law model is within * 10% of the actual volume when mghr/cCiy = 0.8. This variability decreases with increasing mghr/cGy. Conclusions: The product of ICRU report 38 orthogonal dimensions is unsatisfactory in predicting the actual volume irradiated due to the large variations in WHT/vcdume between implants, and the large systematic differences between different implant types. Isodose surface volumes can be accurately predicted knowing only mg-hrs, resulting in relatively small patient-to-patient variations. Specification of volume enclosed by an isodose curve does not add substantially to the information gained by specifying mg-hr.