Evaluation of Deformable Dose Accumulation Accuracy for Head and Neck Cancer

Abstract

Accurate dose summation is crucial to adaptive radiotherapy, but moderate and large anatomic changes (e.g., tumor shrinkage, weight loss) present challenges to daily dose accumulation based on deformable image registration (DIR). This study sought to evaluate the accuracy of deformable dose accumulation commonly encountered during radiation therapy (RT) for head and neck cancer (HNC). Data for eight HNC patients were selected in this study. For each case, a planning kVCT and 15 daily MVCTs acquired during MVCT-guided RT on a helical tomotherapy system were analyzed. A commercial DIR algorithm was used to register the planning kVCT to each MVCT. The registration accuracy was evaluated using mean distance to agreement (MDA) and Dice coefficient (DC). The resultant displacement vector fields (DVFs) were used to map the doses calculated on daily MVCT to the planning image. The dose in the daily MVCTs and the deformed dose in the planning CT were converted to radiation energy by multiplying these doses by mass density in each voxel, where the mass density at each image voxel is converted from the Hounsfield units (HU) using HU-to-mass-density tables calibrated for the kVCT and MVCT scanners, respectively. Changes in dose, volume and energy before and after mapping were compared to each other for both planning target volume (PTV) and organ-at-risk (OAR), and the correlations of these changes were quantified to assess uncertainties in the dose mapping operations. For the eight cases, mean MDA and standard deviation were 0.45±0.09, 0.46±0.06, 0.44±0.01 mm and the DC were 0.958±0.02, 0.946±0.02, 0.949±0.01 for the PTV, left and right parotid respectively. Each of those result conforms to the AAPM task group 132 (TG-132) recommendations (DC > 0.8, MDA < CT voxel width: 2 mm). The average volume changes from fraction 1 to fraction 15 for PTV, left and right parotids are 6.3±3.0%, 5.3±1.5%, and 5.7±3.6%, respectively. The changes of V30 for dose calculated on daily MVCT and warped dose on planning CT were 10.0% and 7.8% for left parotid, and 28.4% and 27.1% for right parotid. The radiation dose (mean) mapped to the planning PTV is within 99% of the dose calculated in daily MVCT images, and radiation energy calculated in PTV and deformed PTVs differ 4.6±3.2% on average. In the left and right parotids, average energy losses were 7.6±2.5%, and 7.9±3.3%, and their mean dose differences are 1.2±0.3% (0.29 cGy) and 1.3±0.5% (0.58 cGy). For the 8 patients, the dose changes are positively correlated to the energy differences with their correlation coefficients equal to 0.56, 0.52 and 0.54 for the PTV, left and right parotids, respectively. The DIR algorithm performed led to acceptable results based on AAPM TG-132. The mean dose, DVH and total deposited energy showed sensitivity to volume changes in an increasing order. It is valuable to include these metrics in a comprehensive QA protocol for adaptive radiotherapy.