Typical case depicting the differences in shape and coverage between CTV RTOG and CTV MRI. Key - Purple: pre-op prostate, Blue: pre-op visible tumour, Red: CTV RTOG, Green: CTV MRI, Yellow: bladder. (a) pre-operative T2 MRI (b) planning CT scan (post-operative); the bladder neck has been pulled down. (c) CTV RTOG does not completely cover the region of the pre-operative prostate/ tumour. Although the bladder will now largely fill the location of the original prostate, the lateral soft tissue lateral remains at risk of microscopic disease. (d) CTV MRI extends 5 mm around the original prostate to cover the soft tissue that was adjacent to the prostate (e) Inferiorly the CTV RTOG treats more of the anterior bladder wall than CTV MRI where coverage may not be required in view of the original prostate location. (f) Superiorly the CTV MRI increases dose to the lateral bladder walls because it covers the soft tissue beside the bladder that remains at risk of microscopic disease in view of the original prostate/tumour location. CTV RTOG misses these areas.