Using CBCT to Monitor Treatment Dose Delivery Accuracy in Head and Neck Proton Radiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> In comparison with photon radiotherapy, intensity modulated proton therapy (IMPT) has higher sensitivities to anatomical and geometrical changes. In IMPT, variations in tumor size, patient geometry and setup can compromise target coverage or augment dose to nearby organs at risk (OAR). As a result, weekly quality assurance CT scans (QACTs) are commonly used to monitor treatment accuracy and provide indicators for adaptive radiotherapy (ART). This study presents the validation of a tool that applies deformable image registration and image processing techniques to generate accurate noise-free CBCT-based virtual CTs used for proton dose calculation to monitor treatment accuracy in head and neck (HN) patients. <h3>Materials/Methods</h3> A single-institution retrospective IRB-approved analysis was conducted on 25 randomly selected HN patients (15 [58%] males, 10 [42%] females) treated with IMPT at our institution between January 2019 and August 2020. Patients had same-day QACT and CBCTs (total of 194 scans) which were imported into a research version of the treatment planning system. A vendor-developed prototype (RaySearch Laboratories, Sweden) tool was used to correct CBCT artifacts and subsequently create a virtual-CT (vCTs) by deforming the planning CT (pCT). The treatment plan was subsequently recomputed on both the QACT and vCT scans. Differences for dose parameters of clinical target volumes (CTVs) and primary OARs such as oral cavity, larynx, spinal cord, mandible, and parotids were compared between vCT and the reference QACT image sets. <h3>Results</h3> Patient averaged differences between CTV coverage parameters estimated from vCT and QACTs were within a 3% standard deviation; CTV mean doses were 0.4% ± 0.7%, and D_2% and D_95% were 0.8% ± 1.6%, and 0.9% ± 1.7%, respectively. Similarly, good correspondence was observed for CTV V_95% (difference = 0.3% ± 1.0%). High agreement was also observed for OARs mean dose (parotid 0.6% ± 1.0%) and max doses (D_0.01cc): oral cavity (0.9% ± 1.5%), larynx (0.8% ± 1.2%), spinal cord (0.7% ± 1.4%), and mandible (1.0% ± 1.2%). Except for 5 cases where the vCT suffered from uncorrected CBCT artifacts, vCT and QACT estimated changes in target coverage metrics (in percent) with respect to the planning CT were within 3% agreement. Virtual-CTs also accurately indicated scenarios where adaptive radiotherapy (e.g., replan) was initiated based on the QACTs. <h3>Conclusion</h3> For all patients, the dose distributions computed on the vCTs closely matched the reference QACT scans. Our results suggest that CBCT-based dosimetry can be used to evaluate treatment plans, and guide the decision for ART over the treatment course.