Effect of extended field-of-view approaches on the accuracy of stopping power ratio estimation for single-energy computed tomography simulators.

BACKGROUND: Extended field-of-view (eFOV) methods have been proposed to generate larger demonstration FOVs for computed tomography (CT) simulators with a limited scanning FOV (sFOV) size in order to ensure accurate dose calculation and patient collision avoidance. Although the efficacy of these strategies has been evaluated for photon applications, the effect of stopping power ratio (SPR) estimation on proton therapy has not been studied. This study investigated the effect of an eFOV approach on the accuracy of SPR to water estimation in homogeneous and heterogeneous phantoms. MATERIALS AND METHODS: To simulate patient geometries, tissue-equivalent material (TEM) and customized extension phantoms were used. The TEM phantom supported various rod arrangements through predefined holes. Images were reconstructed to three FOV sizes using a commercial eFOV technique. A single-energy CT stoichiometric method was used to generate Hounsfield unit (HU) to SPR (HU-to-SPR) conversion curves for each FOV. To investigate the effect of rod location in the sFOV and eFOV regions, eight TEM rods were placed at off-center distances in the homogeneous phantom and scanned individually. Similarly, 16 TEM rods were placed in the heterogeneous TEM phantom and scanned simultaneously. RESULTS: The conversion curves derived from the sFOV and eFOV data were identical. The average SPR differences of soft-tissue, bone, and lung materials for rods placed at various off-center locations were 3.3%, 4.8%, and 39.6%, respectively. In the heterogeneous phantom, the difference was within 1.0% in the absence of extension. However, in the presence of extension, the difference increased to 2.8% for all rods, except for lung materials, whose difference was 4.8%. CONCLUSIONS: When an eFOV method is used, the SPR variation in phantoms considerably increases for all TEM rods, especially for lung TEM rods. This phenomenon may substantially increase the uncertainty of HU-to-SPR conversion. Therefore, image reconstruction with a standard FOV size is recommended.

Efficient method for whole-breast irradiation therapy using Halcyon linear accelerators.

BACKGROUND: The Halcyon is a linear accelerator-based treatment machine designed for a high-throughput simplified workflow. The machine features a compact jawless design, dual-layer multileaf collimators, and a single 6-MV flattening filter-free (FFF) beam. However, the machine's 6-MV FFF beam may restrict its applicability to conventional techniques, such as field-in-field (FiF) radiotherapy, for breast cancer treatment. This study developed a practical and efficient hybrid method for imaging, planning, and irradiation procedures for whole-breast irradiation using Halcyon linear accelerators. MATERIALS AND METHODS: The proposed method involves five major steps: (1) field arrangement, (2) planning target volume (PTV) generation and evaluation, (3) basal plan generation, (4) inverse planning intensity-modulated radiation therapy plan generation, and (5) plan evaluation and irradiation. The PTV is generated using isodose curves plotted on the basis of tangential fields, which are applied to create a basal plan. Subsequently, a basal-dose-compensation approach is applied to further optimize the treatment plan. This efficient workflow necessitates executing only one onboard cone-beam computed tomography procedure. This study included 10 patients with early-stage breast cancer who were treated at our center. The performance of the proposed method was evaluated by comparing its corresponding irradiation time and dose statistics with those derived for a dynamically flattened beam-based FiF (DFB-FiF) method. RESULTS: All plans were normalized to ensure that 98% of the prescribed dose covered 95% of the PTV. On average, the global maximum doses in the proposed and DFB-FiF methods were lower than 106%. The homogeneity index for right-sided (left-sided) breast cancer was 0.053 (0.056) in the proposed method and 0.073 (0.076) in the DFB-FiF method. The dose statistics of normal tissues, including the contralateral breast, heart, and lungs, were comparable between the methods. However, the irradiation time per monitor unit in the proposed method was approximately five times faster than that in the DFB-FiF method, but the planning time and complexity were similar between the methods. CONCLUSIONS: This study developed and evaluated an efficient and practical hybrid method for whole-breast irradiation using the Halcyon. This method can significantly reduce the irradiation time, while providing comparable dose statistics to the DFB-FiF method.