Room: Exhibit Hall | Forum 3
Purpose: To investigate the dose distributions of photon minibeams arrays to assess the possible clinical applications in radiation therapy.
Methods: MCNP6 was used to model the effect of various parameters (photon source, collimator thickness, beam diameter, and beam spacing) on the dose distribution in a water phantom. Specifically, the sources investigated were Co-60, Cs-137, Ir-192, and 1-2 MV bremsstrahlung beams. The parameters were optimized to maximize the expected skin tolerance, dose rate and the peak-to-valley dose ratio (ratio of the dose in beam to out of beam) and to minimize the shielding requirements. Both parallel and convergent beam geometries were considered.
Results: A beam diameter of 3 mm was chosen as the best compromise between radiobiological benefit and feasibility. Of the radioisotopes, Ir-192 was the best photon source due to its high specific activity and low shielding requirements. The parallel case gave a high peak-to-valley dose ratio, but was limited by its small treatment area and low dose rate. The convergent case gave an acceptable dose rate with excellent beam separation above the treatment target and a sharp dose fall off below. The 1-2 MV bremsstrahlung source offers a way to increase the dose rate while maintaining the advantages of the Ir-192 due to the similar mean energy.
Conclusion: This work shows the potential use for photon minibeams in clinical treatments. Current results show the limitations of Ir-192 for parallel beams, but a 1-2 MV bremsstrahlung source could allow for a larger area to be treated at a higher dose rate. The convergent geometry could be used in SBRT treatments for small tumors, especially those in close proximity to organs at risk due to its rapid dose fall off and the reduced need to treat from multiple angles to avoid late effects.
TH- External beam- photons: Small/nonstandard field Monte Carlo dosimetry