Room: AAPM ePoster Library
Purpose: FLASH is an ultra-high dose rate radiotherapy that demonstrates great promise pre-clinically in sparing normal tissues while maintaining tumor control. The purpose of this work is to enable the exploration of possible proton machine and field parameters required to achieve FLASH dose rates with pencil beam scanning (PBS).
Methods: The tool allows the user to control parameters such as field size, field dose or spot MU, spot spacing, minimum spot irradiation time, scanning magnet speed, and nozzle current. The tool uses the Eclipse treatment planning system dose calculation engine to generate dose distributions for individual spots. The detailed dose information is then combined with delivery timing information to produce a 4D “movie” of the dose delivery. For each voxel, the PBS dose rate is computed resulting and a 3D dose rate distribution. The PBS dose rate is defined as the dose delivered to a voxel divided by the irradiation time experienced by that voxel (elapsed real time). The tool reports various properties of the resulting field such as the volume receiving more than 40 Gy/s, the volume receiving more than 50% of the field dose, and the volume of intersection of these two regions. The tool was evaluated on uniform fields sized from 2 cm up to 30 cm at dose levels from 0.0 Gy to 12 Gy.
Results: For the evaluated uniform fields, the % volume inside the 10% isodose line receiving more than 40 Gy/s was maximized for smaller field sizes and larger field doses. Above a certain field dose, the dose rate becomes independent of the dose since the maximum beam current is reached.
Conclusion: We successfully implemented and evaluated a tool for calculating PBS dose rates in 3D, allowing us to explore theoretical delivery parameters that maximize volumes of healthy tissue receiving FLASH dose rates.
Funding Support, Disclosures, and Conflict of Interest: All authors are full-time employees of Varian Medical Systems.