Purpose: To develop and validate a novel collimator with for high-throughput hemithoracic irradiations of mice using 250 MeV protons delivered at FLASH-RT dose rates.
Methods: Varian ProBeam 250 MeV FLASH-RT protons have a range in water of ~38 cm. We designed a brass collimator of 7.62 cm width, corresponding to ~44 cm in water equivalent thickness. Six 13 mm apertures (approximate length of the mouse lung) were precision-machined to allow concurrent ipsilateral lung irradiation of six mice while sparing contralateral lung, head and neck, and abdominal organs. Radiochromic EBT-XD Gafchromic film was used to measure radiation dose profiles at various depths of solid water below the apertures to assess radiation penumbra and field size. A radiation survey of the neutron activation of the block was also performed to ensure that the dose exposure to technical staff would not exceed federal dose limits.
Results: A “halo effect” caused by scatter within the collimator was observed, as the dose at the surface nearest to the collimator edge exceeded the central axis (CAX) by ~50%. The homogeneity of the radiation profile improved with depth in solid water, but at the cost of a wider penumbra. At 20 mm depth, the radiation field edges delivered only 80% of the central axis dose (CAX), but showed a sharp profile edge, with a 80-20% penumbra measuring at ~0.4 mm. The four outer apertures had full width half-maxima (FWHM) of 13.5 mm while the two nearest to the isocenter had FWHM of 13.3 mm despite best attempts to match beam divergence. According to the radiation survey, radiation dose to technical staff would stay below federal limits if the workload stays below 240 Gy/hr.
Conclusion: We designed and constructed a brass collimator able to perform 6 concurrent hemi-thoracic proton irradiations while sparing other organs.
Not Applicable / None Entered.