Purpose: To compare the urethral and prostate absolute and biologic effective doses (BEDs) for 131 Cs and 125 I prostate permanent implant brachytherapy (PPI).
Methods and materials: Eight previously implanted manually planned 125 I PPI patients were replanned manually with 131 Cs, and re-planned using Inverse Planning Simulated Annealing. 131 Cs 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 alpha/beta ratio of 2 and was also calculated for the prostate using an alpha/beta ratio of 6 and a urethral alpha/beta 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 (alpha/beta = 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 125 I and 131 Cs, respectively). For the inverse planning simulated annealing plan comparisons (alpha/beta = 2), the prostate D90 BED was significantly lower with 131 Cs than with 125 I (median, 177 Gy2 vs. 187 Gy2, respectively; p = 0.01). However, the nuD30BED was significantly greater with 131 Cs than with 125 I (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 131 Cs (p = 0.01), compared with 125 I, when the prostate alpha/beta ratio was 6 and the urethral alpha/beta 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.