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About Secondary Neutrons in Spatially Fractionated Radiation Therapy with Protons

S Charyyev*, C Wang , Georgia Institute of Technology, Atlanta, GA


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: New technique is trending in spatially fractionated radiotherapy, SFRT, with protons: to utilize the spot scanning ability together with a physical collimator to obtain minibeams. The aim of this project is to address the concern of secondary neutron dose formed when a dense collimator is put directly in the beamline.

Methods: Monte Carlo code TOPAS was used to model the pencil beam characteristics like spatial beam spread, angular divergence, the coefficient of correlation and energy spread. Single energy iso-layer was simulated using multiple spots to create different field sizes. Minibeams with the hexagonal spatial pattern were achieved via the open holes of collimator made of brass. Brass collimator had parameters suitable for obtaining minibeams for SFRT. Proton dose was scored in a water phantom and the ambient dose equivalent from neutrons was scored at different angles (135, 180, 225 and 270 degrees) and distances (adjacent to phantom, 5 and 10 cm) from the phantom for two cases: with and without physical collimation.

Results: Without collimation ambient dose equivalent varied between 0.017-0.185 mSv/Gy at 135 degrees, 10 cm and 270 degrees, adjacent to water phantom scenario respectively. With collimation ambient dose equivalent varied between 0.092-2.6 mSv/Gy at 225 degrees, 10 cm and 135 degrees, adjacent to water phantom scenario respectively.

Conclusion: Without the collimation, most of the neutrons are created at the distal end of the phantom owing to the forward directedness of the secondary neutrons. With the collimation, most of the secondary neutrons are created in the periphery of the collimator. Results suggest that the secondary neutron dose will increase tenfold when the physical collimator is used. Regardless, even in the worst case scenario, negligible neutron dose is created as compared to uniform scanning.


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