Validation of a Novel Cone-Less Set-up for Electron FLASH Radiation Delivery on a Clinical-Use Linear Accelerator

Abstract

<h3>Purpose/Objective(s)</h3> Ultra-high dose rate (FLASH) radiotherapy (RT) at ≥40 Gray per second has been shown preclinically to achieve improved normal tissue sparing while maintaining tumor control similar to that of conventional dose-rate RT. Many published FLASH-RT delivery models are inaccessible for widespread use and/or not applicable for clinical translation. We previously described a model for reversible configuration of a clinical linear accelerator (LINAC) that can deliver ∼16-18 MeV electron FLASH-RT at source-to-surface distances (SSDs) < 95 cm. However, it precludes the use of a standard electron cone. This proof-of-concept study will characterize the beam profiles from a novel cone-less set-up, to verify its feasibility as a widely accessible method of electron FLASH-RT delivery for the intention of clinical treatment. <h3>Materials/Methods</h3> This novel set-up employs a flat 25 × 25 cm applicator with custom cut-outs that lies within the accessory mount at 66.6 cm SSD of a standard treatment delivery system. We tested a 10 × 10 cm circular field size at a target distance of 80 cm SSD (selected to maintain FLASH dose rates). Radiographic film and an IC Profiler were used to evaluate penumbra width, beam flatness, symmetry, and percentage depth dose (PDD). FLASH-RT was delivered in 20 pulses with 3 independent measurements each using film and the profiler. This was compared to a conventional 16 MeV beam using a standard cone with a 10 × 10 cm field size at 100 cm SSD. <h3>Results</h3> Our novel cone-less FLASH set-up achieved a beam profile that was similar to the conventional electron set-up. Respectively, for the cone-less and conventional set-up, measured at a standardized depth of half R85, penumbra width was 1.13 cm and 1.15 cm, and uniformity index was 0.72 and 0.74 (defined as the ratio of the areas inside the 90% and 50% isodose lines); measured at a standardized depth of R95, flatness (defined as the maximum dose deviation from the central axis at a specific depth over an area confined within lines 2 cm inside the geometric edge of fields) was 1.5% and 4.1%, beam diameter containing dose > 80% of the central axis was 10.6 cm and 9.6 cm, and symmetry (defined as the maximum dose difference between any two symmetric points about the central axis in the flat portion of the beam, as a percentage of central dose) was 2.24% and 2.20%. PDD values for the cone-less set-up at Dmax, R95, R85, and R50 were, respectively, 3.1, 4.1, 5.3, and 6.8 cm. <h3>Conclusion</h3> We have developed a novel cone-less set-up that allows for the delivery of electron FLASH-RT utilizing a clinical LINAC at practical SSDs and verified that its beam profiles are clinically feasible. Further work will be done to characterize additional SSDs and field sizes, along with the addition of a custom skin collimator to further shape the beam. This will pave the way for practical treatment using electron FLASH-RT to a host of oncologic indications, including cutaneous malignancies and partial breast irradiation in the future.