37 Background: CHHiP (CRUK/06/016) is a multicentre randomised controlled trial investigating the use of hypofractionated radiotherapy dose schedules for treatment of localised prostate cancer. RT treatment employs a complex target volume treated with either a multi-segment “forward” plan or inverse-planned intensity-modulated radiotherapy (IMRT). This study compares dose-volume histogram (DVH) and toxicity data for rectum and bladder for the two planning techniques. Methods: Three hundred thirty seven patients (230 forward-planned [F]; 107 inverse-planned [I]) with prospectively collected 2 year toxicity and DVH data for rectum and bladder dose constraints were included. Patients were paired for comparison by matching on (1) rectum and prostate + seminal vesicle PTV volumes for the rectal dose comparison (53 matched pairs available); and (2) bladder and prostate-alone PTV volumes for bladder dose comparison (61 matched pairs). For the toxicity comparison patients were additionally matched on randomised dose schedule. Results: Forward-planned patients had significantly larger volumes of rectum irradiated to 50 to 70Gy and bladder at 50 to 60Gy. In contrast, inverse-planned patients had significantly larger volumes of bladder irradiated to 74 Gy. Acute bowel toxicity (G2+ toxicity within 18 weeks of start of RT) was significantly worse in forward-planned patients (27/53 [52%] F vs 11/53 [21%] I; Mann-Whitney p=0.0002). There were no significant differences in acute urinary symptoms. Late toxicity was rare in both planning groups with only two patients (0 F; 2 I) reporting RTOG G2+ bowel/bladder effects at 24 months. Considering LENTSOM data, there was a suggestion of less late bowel toxicity in the inverse group (G1+ at 18 months: 16/53 (30%) F vs. 6/53 (11%) I; p=0.008). Patient reported outcomes of bowel habits showed no consistent differences between planning methods, although there was a tendency for worse urinary function scores in the inverse group at 12 months. Conclusions: Both planning techniques were associated with low late toxicity. Significant differences were found between DVHs from forward and inverse plans. There were some associations between DVH differences and normal tissue effects; these resolved by 2 years. Clinical trial information: CRUK/06/016.