Author: Rachel B. Ger, Heng Li, Anh Tran 👨🔬
Affiliation: Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Johns Hopkins University 🌍
Purpose: Proton therapy patients undergo quality assurance CT scans (QACTs) during treatment to verify dosimetric accuracy and utilize robustness scenarios for setup and range uncertainties. For intra-cranial targets that are close to critical structures, we aimed to determine the optimal QACT frequency to minimize radiation exposure and the ideal setup uncertainty to ensure target coverage while reducing normal tissue dose.
Methods: Data from all patients with brain targets or targets within the head (e.g., optic nerves) treated between 2019-2023 were analyzed (122 patients) to assess replan rates and timing of adaptive plans to determine QACT frequency requirements based on beam paths through the neck or sinus cavity. Twenty patients clinically treated with 3mm setup uncertainty were replanned with 2mm uncertainty to determine if the lower setup uncertainty would be dosimetrically robust to encompass the daily setup variations while maintaining expected dose. These patients did not have beams traversing through the neck or sinus cavity. Daily CBCTs were converted to synthetic CTs, and clinical goals were evaluated against robustness expectations. Deviations from robustness were quantified.
Results: Only 2 patients required adaptive plans due to anatomical changes, both involving beams passing through the nasal cavity. Based on this, weekly QACTs were recommended for patients with beams passing through the nasal cavity or neck; others required only a week-1 QACT. Volumetric clinical goals for CTV were met in 96% and 99% of 2mm and 3mm plans, respectively. Maximum deviation for 2mm plans was -0.3%. OAR goals exceeded robustness in only 0.7% of synthetic CTs with 2mm uncertainty, with dose differences below 50cGy, except for one case with observed surface changes.
Conclusion: Optimized QACT frequency reduces unnecessary radiation exposure, with weekly scans only for specific beam paths. A 2mm setup uncertainty demonstrated robust target coverage and reduced OAR dose, minimizing potential adverse effects.