A simple algorithm to predict non‐compliance with organ at risk dose‐volume constraints when planning intensity modulated post‐prostatectomy radiation treatment: ‘Why we should put the CART before the horse’

INTRODUCTION: It is not always apparent when the optimal IMRT/VMAT plan for post-prostatectomy radiotherapy (PPRT) has been achieved. Individual variation in patient anatomy is a key contributor. This study aimed to create a model to determine the probability of rectum and/or bladder doses exceeding planning goals based on individual patient anatomy prior to planning. METHODS: The IMRT/VMAT PPRT plans from 200 men were randomly and evenly allocated into the Training Cohort and the Validation Cohort. Univariate and multivariate analysis of the Training Cohort identified variables which impacted bladder and rectal doses. Significant variables were included in a Classification and Regression Tree (CART) analysis. The resultant algorithm was then applied to the Validation Cohort. RESULTS: On multivariate analysis, prescription dose; bladder and rectal volume; lymph node treatment; and percentage of bladder and rectal overlap with the PTV were significant variables. Following CART analysis, the overlap volume (OV) for both rectum (rectum overlap > 20%) and bladder (bladder overlap > 20%) were the key drivers of meeting planning goals. Treatment of pelvic lymph nodes was included as the secondary driving factor for bladder (but not rectal) dose. On application to the Validation Cohort, CART analysis predicted 95% and 87% of patients who would meet bladder and rectal planning goals respectively. CONCLUSIONS: A simple algorithm was developed to predict plan quality by using the OV of the bladder and rectum with the PTV. This algorithm may be used a priori to assess the planning process in the context of variable anatomy, and to optimise planning quality and efficiency.