A feature alignment score for online cone‐beam CT‐based image‐guided radiotherapy for prostate cancer

Catriona Hargrave, Timothy Deegan, Michael Poulsen, Tomasz Bednarz, Fiona Harden, Kerrie Mengersen

To develop a method for scoring online cone-beam CT (CBCT)-to-planning CT image 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. Radiation therapists and radiation oncologists participated in workshops where they reviewed prostate CBCT-IGRT case examples and completed a paper-based survey of image feature matching practices. For thirty-six prostate cancer patients, one daily CBCT was retrospectively contoured then registered with their plan to simulate delivered dose if 1) no online set-up corrections and 2) online image alignment and set-up corrections, were performed. Survey results were used to select variables for inclusion in classification and regression tree (CART) and boosted regression trees (BRT) modelling of volume variation metric thresholds predictive of delivering planned dose to the prostate, proximal seminal vesicles (PSV), bladder and rectum. Weighted functions incorporating the CART and BRT results were used to calculate a score of individual tumor and organ at risk image feature alignment (FASTV_OAR ). Scaled and weighted FASTV_OAR were then used to calculate a score of overall treatment compliance (FASglobal ) for a given CBCT-planning CT registration. The FASTV_OAR were assessed for sensitivity, specificity and predictive power. FASglobal thresholds indicative of high, medium or low overall treatment plan compliance were determined using coefficients from multiple linear regression analysis. Thirty-two participants completed the prostate CBCT-IGRT survey. While responses demonstrated consensus of practice for preferential ranking of planning CT and CBCT match features in the presence of deformation and rotation, variation existed in the specified thresholds for observed volume differences requiring patient re-positioning or repeat bladder and bowel preparation. The CART and BRT modelling indicated that for a given registration, a Dice similarity coefficient >0.80 and >0.66 for the prostate and PSV 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 a CBCT anterior rectum wall >1.0 mm anterior to the planning CT anterior rectum wall were predictive of delivered dose >5% of planned rectum dose. A normalized volume difference <0.88, and a CBCT bladder wall >13.5 mm inferior and >5.0 mm posterior to the planning CT bladder were predictive of delivered dose >5% of planned bladder dose. A FASTV_OAR >0 is indicative of delivery of planned dose. For calculated FASTV_OAR for 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 calculated FASglobal of were low <60, medium 60-80 and high >80, with a 27% misclassification rate for the test data. A FASglobal incorporating nested FASTV_OAR and volume variation metric thresholds predictive of treatment plan compliance was developed, offering an alternative to pre-treatment dose calculations to assess treatment delivery accuracy. This article is protected by copyright. All rights reserved.