CBCT-Based Adaptive Assessment Workflow for Intensity Modulated Proton Therapy for Head and Neck Cancer

Abstract

Purpose Anatomical changes and patient setup uncertainties during intensity modulated proton therapy (IMPT) of head and neck (HN) cancers demand frequent evaluation of delivered dose. This work investigated a cone-beam computed tomography (CBCT) and deformable image registration based therapy workflow to demonstrate the feasibility of proton dose calculation on synthetic computed tomography (sCT) for adaptive IMPT treatment of HN cancer. Materials and Methods Twenty-one patients with HN cancer were enrolled in this study, a retrospective institutional review board protocol. They had previously been treated with volumetric modulated arc therapy and had daily iterative CBCT. For each patient, robust optimization (RO) IMPT plans were generated using ±3 mm patient setup and ±3% proton range uncertainties. The sCTs were created and the weekly delivered dose was recalculated using an adaptive dose accumulation workflow in which the planning computed tomography (CT) was deformably registered to CBCTs and Hounsfield units transferred from the planning CT. Accumulated doses from ±3 mm/±3% RO-IMPT plans were evaluated using clinical dose-volume constraints for targets (clinical target volume, or CTV) and organs at risk. Results Evaluation of weekly recalculated dose on sCTs showed that most of the patient plans maintained target dose coverage. The primary CTV remained covered by the V95 > 95% (95% of the volume receiving more than 95% of the prescription dose) worst-case scenario for 84.5% of the weekly fractions. The oral cavity accumulated mean dose remained lower than the worst-case scenario for all patients. Parotid accumulated mean dose remained within the uncertainty bands for 18 of the 21 patients, and all were kept lower than RO-IMPT worst-case scenario for 88.7% and 84.5% for left and right parotids, respectively. Conclusion This study demonstrated that RO-IMPT plans account for most setup and anatomical uncertainties, except for large weight-loss changes that need to be tracked throughout the treatment course. We showed that sCTs could be a powerful decision tool for adaptation of these cases in order to reduce workload when using repeat CTs.

Evaluating Proton Dose and Associated Range Uncertainty Using Daily Cone-Beam CT

“…One notable difference between the current and previous studies is that measurement was used as the baseline for our study. Much of the literature investigating CBCT-based proton dose calculation converted CBCT HU to CT HU and then used CT HU for proton dose calculation ( 3 , 23 – 27 ). However, as mentioned above and demonstrated in this study, using CT as the baseline would introduce the inherent range uncertainties associated with the CT HU-D-stopping power conversion, which could potentially be removed by direct CBCT HU-D-stopping power conversion.…”

A narrative review of intensity-modulated proton therapy for head and neck cancer

“…Factors that may impact Bragg peak localization are artifacts (e.g., dental or surgical hardware); air space in cavities (e.g., air-filled sinuses or oral cavity); and patient's anatomical fluctuations (e.g., tumor shrinkage, weight change, or daily variations in patient positioning). Therefore, (10).…”

Proton beam radiation therapy treatment for head and neck cancer

“…Implementation of automated adaptive replanning is highly recommended to account for the anatomical variations and patient's weight change. [7][8][9] The two primary modes of PBT delivery are passive-scatter protons therapy or intensity-modulated proton therapy (IMPT). Similar…”

“…To circumnavigate the issue of artifacts and other heterogeneous structures interfering with Bragg peak localization, a short and reliable beam path needs to be selected that avoids areas such as the mouth, spinal cord, and other hollow structures. Implementation of automated adaptive replanning is highly recommended to account for the anatomical variations and patient's weight change 7–9 …”