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 of 83 seconds during which 502 CBCT frames were acquired. There was moderate agreement between the four observers regarding the quality of the CBCTs (Fleiss’ kappa statistic 0.51). StCBCTs were of good quality and the four observers agreed that 98 to 100% of these scans were of sufficient quality to be used to make a clinical decision. The four observers assessed that 35 to 79% of siCBCTs were of sufficient quality to be used to make clinical decisions. Before the software upgrade 65 to 93% of siCBCTs were judged to be of sufficient quality to make a clinical decision ("slow" simultaneous CBCT). Following the software upgrade this proportion fell to 16 to 69% ("fast" simultaneous CBCT). Conclusions: SiCBCT can produce images which are clinically useful. Image quality is affected by bowel gas, patient habitus and scatter from the MV beam. Reducing arc delivery time and the number of CBCT frames acquired by approximately one-third reduces the quality of siCBCTs. Further development and refinement of siCBCT scanning is warranted. Future reductions in arc delivery times may have to be limited if the use of siCBCT scanning becomes standard practice.