Assessing Clinical Feasibility of a LINAC-Based Electron FLASH Radiotherapy System Using an Anthropomorphic Phantom under Realistic Clinical Conditions

Abstract

<h3>Purpose/Objective(s)</h3> Ultra-high dose rate (UHDR) radiotherapy (RT) at ≥40 Gy/s has been shown preclinically to reduce toxicity with similar tumor control compared to conventional RT, a phenomenon known as the FLASH effect. We previously described a configuration of a clinical-use LINAC that can output ∼17 MeV electron UHDR RT at 45-80 Gy/s with clinically feasible beam profiles, field sizes (3-20 cm), and SSDs (70-90 cm). Safe and effective clinical trials are needed to advance translation of FLASH treatment in humans, such as one proposed at our institution for cutaneous lymphomas. This study characterizes the beam output of our electron UHDR RT set-up under realistic clinical conditions for common anatomic sites using an anthropomorphic phantom, to assess its feasibility for clinical translation. <h3>Materials/Methods</h3> Our electron UHDR RT set-up utilizes a treatment delivery system with a configured program board of a decommissioned electron energy to adjust the beam output and control, without hardware modification. The electron cone is replaced with a flat applicator with custom cut-outs that lies in the accessory mount. We tested this set-up under realistic clinical conditions on a RANDO phantom for treatment of the chest/breast, leg, and scalp. We used 3 cm and 10 cm circular field sizes at 80 cm SSD with the gantry angled en-face to the targets. Tissue-equivalent bolus and flexible tungsten-silicone skin collimators (made from silicone sheets with tungsten powder cut into donut shapes of varying diameters) were used to mimic clinical treatment standards. Measurements were done using radiographic film and we evaluated the beam width, percent depth dose, and penumbra width. <h3>Results</h3> All results reported include 1 cm bolus and skin collimators unless otherwise specified. The values for R80 (depth at 80% dose), beam width of 80% dose (at R95), and penumbra width (at R85) for the 10 cm chest/breast field are 5.2, 8.7, and 0.9 cm, respectively. Without skin collimators, the values are 5.4, 9.0, and 1.2 cm, respectively. The values for the 3 cm chest/breast field are 2.9, 1.9, and 0.6 cm, respectively. Without skin collimators, the values are 3.2, 1.9, and 0.7 cm, respectively. The values for the 10 cm leg field are 4.0, 6.6, and 0.9 cm, respectively. The values for the 3 cm scalp field (with 4 cm bolus to minimize dose penetration to the brain) are 1.1, 1.6, and 0.9 cm, respectively. The steep curvature of the leg and scalp sites led to expected decrease in beam flatness but retained a treatment area that allows for a reasonable range of tumor sizes and depths. <h3>Conclusion</h3> Our configuration is able to deliver electron UHDR RT with clinically applicable beam profiles to various anatomic sites under realistic clinical conditions. Tungsten-silicone skin collimators are an easy-to-use tool to narrow the penumbra. Overall, these results show potential for treatment of superficial indications, including cutaneous lymphoma, extremity sarcoma, and partial breast irradiation, and place us one step closer toward the clinical translation of FLASH.