CT-based dosimetry for transperineal I-125 prostate brachytherapy

Abstract

Purpose: To demonstrate CT-based target coverage achieved by an experienced brachytherapy team, with a documented history of clinical efficacy. These results may serve as a point of reference by which other implant teams could evaluate their technical results.

Methods and materials: Two to 4 h after implantation, a CT scan of the pelvis was obtained, taking 3-mm thick images at 3 mm scan intervals. The outer margins of the rectum and prostate were outlined on each CT section by a single investigator (KW). CT scans from 20 consecutive, unselected patients with stage T1 or T2 prostatic carcinoma who had transperineal I-125 implants at MSKCC in 1995 were analyzed. The prescribed minimum peripheral prostate dose was 140 Gy, based on a preimplant CT scan of the prostate. Target volumes were determined based on planar reconstruction of the prostate.

Results: An average of 84% of the target (range: 76-92%) was covered by the 140 Gy isodose line. An average of 90% of the target (range: 81-96%) was covered by the 120 Gy isodose line. The average minimum target dose was 57 Gy (range: 40-70 Gy). There was a loose correlation between the minimum dose and the degree of target coverage (r = 0.33).

Conclusions: Based on CT scanning on the day of the implant, coverage of 80% or more of the target volume by the prescription dose is probably adequate. Accurate evaluation is a complex problem that is made imprecise by difficulty in target identification and by volume changes after the implant.