Evaluating synthetic CT from deformable registration in directed adaptive radiotherapy for head and neck cancer

Abstract

Abstract Background and purposel Anatomical changes over the many weeks of radiotherapy for Head and Neck cancer patients can compromise the original treatment plan, potentially increase the risk of side effects and impact outcomes. It is difficult to decide, based on daily imaging alone, which patients will most benefit from adapted treatment plans. This work presents an automated approach to evidence‐based assessment of treatment plan robustness to anatomy changes. Methods Directed Adaptive Radiotherapy (DART) generates a synthetic CT for calculating dose changes in the treatment planning system (TPS). The process first enhances the CBCT, then uses a double fusion to insert the smaller field‐of‐view CBCT into the larger CT. The double fusion captures both neck tilt and weight loss changes, to accurately capture the position of the head and related critical organs. The deformed CT and contours are sent to TPS for recalculation of the original plan and quantify dose changes. A retrospective study was undertaken using 12 H&N patients who had a replanning CT during their treatment. The quality of the dose distributions calculated on synthetic CT and deformed contours were evaluated, along with the sensitivity and specificity of the synthetic CT as a predictor for adaptation. Results The contour and dose comparison between synthetic CT and clinical replan CT showed good agreement. The average Dice Similarity Coefficient (DSC) was > 80% for all contours, except for the sub‐mandibular (> 75%) and excluding cochlea. The average mean distance‐to‐agreement (MDA) was < 2 mm, and all MDAs were < 3 mm. The average DVH differences were within 1%, and all dose differences were within 3%. The total gamma passing rates were > 95%. Both the sensitivity and specificity of the synthetic CT for predicting the need for adaptation were 92%, based on a tolerance of 2 Gy change for both target coverage and OAR dose constraint limits. Conclusions By quantifying the impact of anatomical changes on dose distributions, the DART process helps determine whether a patient would benefit from adaptive replanning. The DART method improves efficiency by reducing unnecessary imaging and replanning when the treatment remains robust against anatomy changes.