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Evaluation of Prostate Edema and Seed Migration Via Seed Position Change From Consequent Post-Implant CT Scan in Prostate Brachytherapy

Y Peng

Y Peng1*, (1) Department of Radiation Oncology, Indiana University, Indianapolis, IN 46232

SU-E-T-330 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To evaluate prostate edema and seed migration based on seed position change determined from post-implant CT scans.
Methods: Early stage prostate cancer patients with permanent seed implant brachytherapy were acquired post-implant CT scans on Day-0 and Day-30, respectively. Seeds coordinates(x,y,z) were determined directly from those CTs and were sorted to different slices according to their z coordinates. The prostate center (xc,yc,zc) was the mathematic average of the coordinates for all seeds for one CT scan. Distance from each seed to the prostate center and weighted distance, defined as fraction of seeds in a certain distance to the prostate center, were calculated and compared between at day-0 and at day-30. Slice center was the average coordinates of all seeds in that slice. We assume the boundary of the edema is determined by the largest convex polygon constructed by those seeds having largest distance to the slice center at each direction. The area (A) of the convex polygon was summation of (xiyi+1-xi+1yi), where xi and yi were the ith seed coordinate on the apex. We further assume the edema is the summation of volume [Vi=(Ai+Ai+1+(Ai*Ai+1)^0.5)*hi/3] between each slice, where hi was distance between adjacent slices.
Results: Distance from prostate center to each seed, volumes between adjacent slices and the total volumes show the shrinkage of the prostate from day-0 to day-30. Population weighted distance was 2.1 and 1.9 cm on subsequent CTs. The volume ratio of the edema ranged from 1.3 to 1.7 from day-0 and day-30 CTs.
Conclusion: Post-implant edema increased the prostate volume by factors ranging from 1.3 to 1.7. The results showed that the distance index method was consistent with that of volume calculation in the evaluation of the edema change. The method proposed in this study can be used to evaluate seed migration and edema change.

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