Room: Exhibit Hall | Forum 7
Purpose: To develop an array of proton minibeams of 2-3 mm diameter based on the existing pencil beam scanning, PBS, system. The end goal is to maximize the benefit of the dose-volume effect in radiobiology and offer these minibeams for proton GRID therapy at existing proton therapy clinics.
Methods: Since full width at half maximum, FWHM, of modern cyclotron-based proton therapy machines is somewhere in range of 10-18 mm at isocenter, additional collimation is required to obtain minibeams of 2-3 mm. This study has two components. In the first part, using Monte Carlo code we simulated proton-pencil beams that mimic pencil beams from cyclotron-based clinical facility. We achieved parallel beams of 2-3 mm diameter using collimator made of brass. Beams with hexagonal spatial pattern were produced via the open holes of the collimator. To optimize the design we considered different combinations of parameters: center-to-center distance of collimator holes, air gap between the collimator and the phantom, the collimator thickness. The optimal design of the proton minibeam array is based on the figures-of-merit parameters: peak-to-valley dose ratio (PVDR), dose rate and secondary neutron dose. In the second part, EBT3 film measurements were performed to verify simulation results.
Results: Thicker collimator, larger center-to-center distance and smaller air gap is necessary for high PVDR values (>30). There is a trade-off between PVDR and dose rate, since increasing center-to-center distance decreases dose rate. Regardless, reasonable instantaneous dose rate (~2 Gy/s) and negligible (< 1% of the treatment dose) secondary neutron dose were achieved.
Conclusion: Results suggest that it is feasible to develop an array of proton minibeams using existing PBS system to further explore the benefit of GRID therapy. Results are promising because new generations of proton therapy machines promise even smaller spot sizes, meaning even better results than those obtained in this project.