On the Need to Compensate for Edema-Induced Dose Reductions in Preplanned 131Cs Prostate Brachytherapy

Abstract

Surgical trauma-induced edema and its protracted resolution can lead to significant dose reductions in preplanned (131)Cs prostate brachytherapy. The purpose of this work was to examine whether these dose reductions should be actively compensated for and to estimate the magnitude of the additional irradiation needed for dose compensation. The quantitative edema resolution characteristics observed by Waterman et al. were used to examine the physical and radiobiologic effects of prostate edema in preplanned (131)Cs implants. The need for dose compensation was assessed using the dose responses observed in (125)I and (103)Pd prostate implants. The biologically effective dose, calculated with full consideration of edema evolution, was used to estimate the additional irradiation needed for dose compensation. We found that the edema-induced dose reduction in preplanned (131)Cs implants could easily exceed 10% of the prescription dose for implants with moderate or large edema. These dose reductions could lead to a >10% reduction in the biochemical recurrence-free survival for individual patients if the effect of edema was ignored. For a prescribed dose of 120 Gy, the number of 2-Gy external beam fractions needed to compensate for a 5%, 10%, 15%, 20%, and 25% edema-induced dose reduction would be one, four, six, seven, and nine, respectively, for prostate cancer with a median potential doubling time of 42 days. The required additional irradiation increased for fast-growing tumors and/or those less efficient in sublethal damage repair. Compensation of edema-induced dose reductions in preplanned (131)Cs prostate brachytherapy should be actively considered for those implants with moderate or large edema.