Dosimetric effects of edema in permanent prostate seed implants: A rigorous solution

Purpose:To derive a rigorous analytic solution to the dosimetric effects of prostate edema so that its impact on the conventional pre-implant and post -implant dosimetry can be studied for any given radioactive isotope and ede ma characteristics. Methods and Materials: The edema characteristics observed by Waterman et al (Int. J. Rad. Onc. Biol. Phys, 41:1069-1077; 1998) was used to model the t ime evolution of the prostate and the seed locations. The total dose to any part of prostate tissue from a seed implant was calculated analytically by parameterizing the dose fall-off from a radioactive seed as a single inver se power function of distance, with proper account of the edema-induced tim e evolution. The dosimetric impact of prostate edema was determined by comp aring the dose calculated with full consideration of prostate edema to that calculated with the conventional dosimetry approach where the seed locatio ns and the target volume are assumed to be stationary. Results: A rigorous analytic solution on the relative dosimetric effects of prostate edema was obtained. This solution proved explicitly that the rela tive dosimetric effects of edema, as found in the previous numerical studie s by Yue et. al, tint, J, Radiat, Oncol, Biol, Phys. 43, 447-454, 1999), ar e independent of the size and the shape of the implant target volume and ar e independent of the number and the locations of the seeds implanted. It al so showed that the magnitude of relative dosimetric effects is independent of the location of dose evaluation point within the edematous target volume , It implies that the relative dosimetric effects of prostate edema are uni versal with respect to a given isotope and edema characteristic. A set of m aster tables for the relative dosimetric effects of edema were obtained for a wide range of edema characteristics for both I-125 and Pd-103 prostate s eed implants. Conclusions: A rigorous analytic solution of the relative dosimetric effect s of prostate edema has been derived for a class of edema characterized by Waterman et al, The solution proved that the dosimetric effects caused by t he edema are universal functions of edema characteristics for a given isoto pe. It provides an efficient tool to examine the relative dosimetric effect s of edema for any given edema characteristics and for any isotopes that ma y be considered for prostate implants. (C) 2000 Elsevier Science Inc.