This thesis investigates methods to safely reduce the treatment margin applied during external beam radiotherapy for prostate cancer. This margin is applied to allow the prostate to move within the pelvis, but still receive an adequate radiation dose. The treatment margin also includes part of the rectum and bladder, which receive some radiation dose and consequently cause treatment toxicity. To reduce treatment margin and, therefore, treatment toxicity, we investigate a diet intervention which

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... aims to reduce the movement of the prostate. We also investigate an adaptive margin, where a smaller margin is applied when reduced prostate movement is predicted.