Simultaneous cone beam CT scanning during prostate radiotherapy to produce clinically useful images.

220 Background: Cone beam CT (CBCT) images can be acquired during delivery of rotational radiotherapy. We assessed the quality of simultaneous CBCTs captured during prostate radiotherapy and studied the effects of arc delivery time on image quality. Methods: Fifty patients with localised prostate cancer were treated with radical rotational radiotherapy. Treatment was delivered using a single eight or 10 MV arc on an Elekta Synergy linear accelerator. Standard (stCBCT) and simultaneous CBCTs (siCBCT) were captured on fractions 1, 6, 11, and 16. The siCBCT image was reconstructed and optimised using in-house software. The image quality was assessed by four observers using a validated scoring tool. The scoring clinician was blinded to whether the scan was stCBCT or siCBCT. Results: Seventy-four siCBCT scans were performed with a mean arc delivery time of 120 seconds during which 688 CBCT frames were acquired. Following a software upgrade the subsequent 122 siCBCTs were obtained with a mean arc delivery time ...