Anticipating prostatic volume changes due to prostate brachytherapy.

The purpose of the study was to determine which clinical parameters might predict individual prostate volume changes from prostate brachytherapy. Fifty consecutive, unselected patients treated at the University of Washington by I-125 or Pd-103 implantation for prostatic carcinoma in 1998 were analyzed. The prostate contours on preimplant transrectal ultrasound (TRUS) images were digitized and the prostate volumes calculated. Postimplant axial CT images of the prostate was obtained at 0.5 cm intervals with patients in the supine position the morning after the implant. The postimplant prostate volume increased by an average factor of 1.7 (+/-0.34) compared with the preimplant volume, the size increase being primarily in the anterior-posterior dimension. The absolute volume change was similar in patients with small vs. large preimplant prostate volume (r = -0.39), but the proportional change was less in patients with a larger prostate volume (r = -0.71). Because patients with a small preimplant prostate had proportionately greater volume increase, their postimplant target coverage was generally less. No single parameter, including preimplant prostate volume, preimplant hormonal deprivation, or supplemental external beam radiation therapy (EBRT) can accurately predict the degree of swelling. The precise significance of and practical solution to implant-related prostate volume changes remains to be determined.