Abstract
PurposeAnatomical 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 MethodsTwenty-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.ResultsEvaluation 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.ConclusionThis 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.
Highlights
Radiotherapy for head and neck (HN) cancer is used as a primary treatment or as an adjuvant to surgery to achieve tumor control and limit toxicity to nearby organs at risk (OARs)
Of the 21 patients, 9 had weekly doses of CTV1 below V95, from which a total of 17 weekly doses corresponded to undercoverage, and 12 of these were lower than the robust optimization (RO)-intensity modulated proton therapy (IMPT) worst-case scenario
Patient 12 had CTV2 weekly doses below V95, and the accumulated dose was lower than the RO worst-case scenario
Summary
Radiotherapy for head and neck (HN) cancer is used as a primary treatment or as an adjuvant to surgery to achieve tumor control and limit toxicity to nearby organs at risk (OARs). The complex anatomy in the HN region has made intensity modulated radiation therapy (IMRT) the treatment of choice because of its ability to create highly conformal treatment plans in treatment sites with complex target geometry where many OARs are proximal to the targets. IMRT in patients with substantial target volume changes or significant weight loss can be challenging and may result in underdosage of targets and overdosage of OARs [1,2,3]. It has been well documented that significant anatomical changes are common during HN treatment courses, which may require plan adaptation to maintain optimal target coverage and continued sparing of OARs. Adaptive radiotherapy (ART) in the HN setting typically requires the patient to be re-scanned and replanned during the treatment course in order to accommodate for these changes. ART is time consuming and resource intensive; the clinician’s judgment plays an important role in determining the need for adaptation [3, 4]
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