CTV to PTV margins for prostate irradiation. Three-dimensional quantitative assessment of interfraction uncertainties using portal imaging and serial CT scans.

Purpose: To determine the magnitude of setup and organ motion errors from a subset of prostate cancer patients treated with conventional conformal radiotherapy, and to estimate the CTV-PTV margin according to published margin recipes.

Methods And Materials: Twenty prostate cancer patients were treated with external radiotherapy using electronic portal images (EPIs). Weekly treatment EPIs and pelvic CT scans were obtained.