Changes in Organs at Risk Dose Estimates Due to Anatomic Changes and Registration Method During Head and Neck Radiotherapy and its Relationship to Percent Weight Loss

Abstract

<h3>Purpose/Objective(s)</h3> Weight loss and anatomic changes are common during radiation therapy for head and neck cancers. The best method for assessing the impact of anatomic changes on dose distribution is unknown. We compared planned doses to organs at risk (OARs) to delivered dose estimates using a cone beam computed tomography (CBCT) from the final week of treatment. <h3>Materials/Methods</h3> Ten patients who underwent bilateral neck radiation and had weekly extended CBCT's available to capture critical OARs were identified. CBCT from the final week was reconstructed to 2mm slices to match the resolution of the CT simulation (CTsim). The CBCT's were registered to the sim using both rigid and deformable registration techniques. The parotid glands, spinal cord, and skin (defined as 3mm from body contour) were contoured on the CBCT's. Mean dose to the parotids, and maximum dose (D0.035cc) to the spinal cord and skin were extracted from the treatment plan based on CTsim, rigid CBCT and deformed CBCT respectively. Independent t-tests were used to determine and compare mean percent (%) dose changes in OAR's from initially planned to delivered dose estimates using the rigid and deformed CBCT's. Linear regression was then performed to identify which set of OAR doses were the best predictors of the variation in % mean weight change during the treatment course. Statistical software was used for the statistical analyses. <h3>Results</h3> Among the 10 patients analyzed, mean % change in mean right parotid dose was -6.99% between planned and rigid CBCT estimated dose and +12.02% between planned and deformed CBCT estimated dose, for a mean total difference of 19.01% (SE 5.61%, p=0.004). These differences were statistically significant. A similar trend was observed for the left parotid gland, with mean % changes of -1.67%, +6% and 7.67% respectively (SE 5.62%, p=0.19). No differences were observed for cord (mean difference 0.19%, SE 1.27%, p = 0.89) or skin (mean difference 0.26%, SE 2.83%, p = 0.92). On regression analysis the delivered dose estimates from the rigid CBCT model were a better predictor of % weight change (R²=0.543) compared to the deformed CBCT (R²=0.463) or CT sim planned doses (R²=0.460) <h3>Conclusion</h3> The data shows a significant difference in estimating dose delivered to the right parotid gland using final week CBCT between using rigid and deformable registration techniques. Change in dose to left parotid gland also trended towards significance and may prove to be meaningful with a larger sample size. There were no significant differences in cord or skin % dose changes. We found that doses collected from the Rigid CBCT were the best predictors of % weight change during the whole course of treatment on regression analysis. Future work will focus on mid treatment dose estimates using a larger sample size, and correlation with clinical outcomes.