A feature alignment score for online cone‐beamCT‐based image‐guided radiotherapy for prostate cancer
Authors: Hargrave Catriona , Deegan Timothy , Poulsen Michael , Bednarz Tomasz , Harden Fiona , Mengersen Kerrie 
Published: May 2018

PurposeTo develop a method for scoring online cone‐beamCT(CBCT)‐to‐planningCTimage feature alignment to inform prostate image‐guided radiotherapy (IGRT) decision‐making. The feasibility of incorporating volume variation metric thresholds predictive of delivering planned dose into weighted functions, was investigated.MethodsRadiation therapists and radiation oncologists participated in workshops where they reviewed prostateCBCT‐IGRTcase examples and completed a paper‐based survey of image feature matching practices. For 36 prostate cancer patients, one dailyCBCTwas retrospectively contoured then registered with their plan to simulate delivered dose if (a) no online setup corrections and (b) online image alignment and setup corrections, were performed. Survey results were used to select variables for inclusion in classification and regression tree (CART) and boosted regression trees (BRT) modeling of volume variation metric thresholds predictive of delivering planned dose to the prostate, proximal seminal vesicles (PSV), bladder, and rectum. Weighted functions incorporating theCARTandBRTresults were used to calculate a score of individual tumor and organ at risk image feature alignment (FASTV_OAR). Scaled and weightedFASTV_OARwere then used to calculate a score of overall treatment compliance (FASglobal) for a givenCBCT‐planningCTregistration. TheFASTV_OARwere assessed for sensitivity, specificity, and predictive power.FASglobalthresholds indicative of high, medium, or low overall treatment plan compliance were determined using coefficients from multiple linear regression analysis.ResultsThirty‐two participants completed the prostateCBCT‐IGRTsurvey. While responses demonstrated consensus of practice for preferential ranking of planningCTandCBCTmatch features in the presence of deformation and rotation, variation existed in the specified thresholds for observed volume differences requiring patient repositioning or repeat bladder and bowel preparation. TheCARTandBRTmodeling indicated that for a given registration, a Dice similarity coefficient >0.80 and >0.60 for the prostate andPSV, respectively, and a maximum Hausdorff distance <8.0 mm for both structures were predictive of delivered dose ± 5% of planned dose. A normalized volume difference <1.0 and aCBCTanterior rectum wall >1.0 mm anterior to the planningCTanterior rectum wall were predictive of delivered dose >5% of planned rectum dose. A normalized volume difference <0.88, and aCBCTbladder wall >13.5 mm inferior and >5.0 mm posterior to the planningCTbladder were predictive of delivered dose >5% of planned bladder dose. AFASTV_OAR>0 is indicative of delivery of planned dose. For calculatedFASTV_OARfor the prostate,PSV, bladder, and rectum using test data, sensitivity was 0.56, 0.75, 0.89, and 1.00, respectively; specificity 0.90, 0.94, 0.59, and 1.00, respectively; positive predictive power 0.90, 0.86, 0.53, and 1.00, respectively; and negative predictive power 0.56, 0.89, 0.91, and 1.00, respectively. Thresholds for the calculatedFASglobalof were low <60, medium 60–80, and high >80, with a 27% misclassification rate for the test data.ConclusionsAFASglobalincorporating nestedFASTV_OARand volume variation metric thresholds predictive of treatment plan compliance was developed, offering an alternative to pretreatment dose calculations to assess treatment delivery accuracy.