Dosimetric Comparison of Image Guidance by Megavoltage Computed Tomography versus Bone Alignment for Prostate Cancer Radiotherapy

Daily image guidance in irradiation of prostate cancer can be based on simple portal images or on soft-tissue imaging. This study compares daily bone alignment with daily pretreatment megavoltage computed tomography (MVCT). Ten patients with a total of 356 fractions were analyzed. Before each fraction, the patient was positioned to match the prostate on pretreatment MVCT and planning CT. In seven fractions, rectum distension prevented a satisfactory match and the fraction was restarted after the patient went to the restroom. After treatment, organs were manually contoured on each daily MVCT and doses recalculated. Bone alignment was simulated by a software that matches the bones on MVCT and planning CT. In the seven interrupted fractions, median improvement of rectum volume receiving full fraction dose was 14 cm(3) between simulated treatment before and actual treatment after the patient went to the restroom. In the 349 noninterrupted fractions, the average difference of the isodose that covers 95% of the prostate between actual treatment position and simulated bone match position was < 1% and there was no significant change in the rectum volume with a fraction dose a parts per thousand yen 2 Gy. Full fraction dose rectum irradiation can be avoided with daily MVCT by interruption of single fractions. There was no relevant benefit of daily MVCT in the noninterrupted fractions with the margins used in this study.