Dose Reproducibility for PBS Proton Treatment of Breast Cancer Patients with and without Mask Immobilization

Abstract

<h3>Purpose/Objective(s)</h3> To compare radiation therapy dose reproducibility for proton pencil beam scanning (PBS) breast and chest wall (CW) patients with and without mask immobilization. <h3>Materials/Methods</h3> Patients treated between Jan 2019 and Feb 2022 in our department with at least one dose verification CT scan (V-CT) in treatment position were selected for this study. All patients were set up arms up using a patient positioning system, with (68 patients) or without (17 patients) mask immobilization in chin, neck, shoulder, and chest areas. Two to three single field uniform dose enface or near enface PBS proton beams were optimized using a commercial treatment planning system. Doses ranged from 25 to 60 Gy(RBE) in 5 to 45 fractions. Plans were re-calculated on V-CTs by registering V-CT to planning CT (P-CT) using the same kV X-ray alignment method used during treatment. CTV D95%, lung V40%, esophagus D0.01cc, and heart mean dose were compared for P-CTs and V-CTs. All dose values were percentages of prescription doses. Differences were considered to be large dose deteriorations (LDD) if: 1. CTV (V-CT D95%) – (P-CT D95%) < -5%, or 2. lung V-CT V40% > 20% and (V-CT V40%) – (P-CT V40%) > 5%, or 3. esophagus V-CT D0.01cc > 72% and (V-CT D0.01cc) – (P-CT D0.01cc) > 10%. Causes for the LDDs were investigated. <h3>Results</h3> The dose differences were shown in table 1. The percentages of patients with at least one LDD structure were 11.8% and 47.1% for patients with mask and without mask, respectively. Five CW patients without mask had lung V40% higher on all weekly V-CTs. Plans with LDDs large enough to raise clinical concerns were replanned if adjustment of setup would not solve the problem.