Computed tomography in definitive radiotherapy of prostatic carcinoma, part 2: definition of target volume.

Pretreatment computed tomography (CT) scanning of the pelvis was performed in 100 consecutive patients with carcinoma of the prostate treated with external irradiation (82 patients) or interstitial I125 (18 patients). Treatment plan modifications prompted by CT scan findings were most frequent in patients with (clinical) involvement of the seminal vesicles in whom the conventional treatment planning often resulted in an underestimate of tumor volume. Seventeen of 32 (53%) such patients required an enlargement of treatment fields to adequately encompass the target volume. Using skeletal landmarks as reference, the dimensions of the prostate, seminal vesicles and the detectable tumor and their topographic relationships were systematically tabulated. These measurements provide a basis for the definition of the target volume in patients with carcinoma of the prostate in whom CT scans might not be available.