A Castration-Resistant Prostate Tumor With Unusual Anatomy Treated Using Daily Online Adaptive Radiotherapy.

Daily online adaptive radiotherapy (OART) is useful in radiotherapy of prostate cancer to reduce doses to the rectum and bladder which pose a challenge because of daily variation in shape and size. It also helps to reduce target margins while still maintaining target coverage. We present a case of prostate cancer resistant to androgen deprivation therapy and systemic therapy which was difficult to treat with definitive radiotherapy because of the unusual anatomical shape of the tumor impinging into the rectum. Cone beam computed tomography-based daily OART helped in adapting the dose to changes in the prostate volume during the course of radiation and helped reduce the dose to the rectum while maintaining coverage of the target volume. We could achieve excellent biochemical and radiological responses without significant toxicity.