Comparison of Dosimetric and Biologic Effective Dose Parameters for Prostate and Urethra Using 131Cs and 125I for Prostate Permanent Implant Brachytherapy
Received 18 February 2008; received in revised form 27 April 2008
Purpose
To compare the urethral and prostate absolute and biologic effective doses (BEDs) for 131Cs and 125I prostate permanent implant brachytherapy (PPI).
Methods and Materials
Eight previously implanted manually planned 125I PPI patients were replanned manually with 131Cs, and re-planned using Inverse Planning Simulated Annealing. 131Cs activity and the prescribed dose (115 Gy) were determined from that recommended by IsoRay. The BED was calculated for the prostate and urethra using an α/β ratio of 2 and was also calculated for the prostate using an α/β ratio of 6 and a urethral α/β ratio of 2. The primary endpoints of this study were the prostate D90 BED (pD90BED) and urethral D30 BED normalized to the maximal potential prostate D90 BED (nuD30BED).
Results
The manual plan comparison (α/β = 2) yielded no significant difference in the prostate D90 BED (median, 192 Gy2 for both isotopes). No significant difference was observed for the nuD30BED (median, 199 Gy2 and 202 Gy2 for 125I and 131Cs, respectively). For the inverse planning simulated annealing plan comparisons (α/β = 2), the prostate D90 BED was significantly lower with 131Cs than with 125I (median, 177 Gy2 vs. 187 Gy2, respectively; p = 0.01). However, the nuD30BED was significantly greater with 131Cs than with 125I (median, 192 Gy2 vs. 189 Gy2, respectively; p = 0.01). Both the manual and the inverse planning simulated annealing plans resulted in a significantly lower prostate D90 BED (p = 0.01) and significantly greater nuD30BED for 131Cs (p = 0.01), compared with 125I, when the prostate α/β ratio was 6 and the urethral α/β ratio was 2.
Conclusion
This report highlights the controversy in comparing the dose to both the prostate and the organs at risk with different radionuclides.
∗Department of Radiation Oncology, University of California, San Francisco, School of Medicine, San Francisco, CA
†Department of Radiation Oncology, Odette Cancer Centre at Sunnybrook Hospital, University of Toronto, Toronto, ON, Canada
‡University of California, San Francisco, Helen Diller Family Comprehensive Cancer Center Biostatistics Core, San Francisco, CA
Reprint requests to: Jean Pouliot, Ph.D., Department of Radiation Oncology, University of California, San Francisco, School of Medicine, 1600 Divisadero St., Suite H1031, San Francisco, CA 94143. Tel: (415) 353-8900; Fax: (415) 353-8679
ABSTRACT