Room: Davidson Ballroom A
Purpose: Investigate the secondary neutron production during pencil beam scanning proton therapy using a Dynamic Collimation System (DCS) and characterize the resulting secondary neutron dose equivalence distributions for an intracranial chordoma treatment using single field uniform dose (SFUD) and intensity-modulated proton therapy (IMPT).
Methods: Monte Carlo N-Particle transport code was used to simulate the production and transport of secondary neutrons generated from nuclear interactions between the primary proton beam and the DCS. Each beamlet was simulated separately based upon its unique collimation geometry. The individual beamlet distributions were then superimposed to acquire the composite secondary neutron dose distribution using an in-house treatment planning system. The neutron dose equivalence rates were calculated for both fields of the SFUD treatment and the composite IMPT treatment. The values were compared to neutron dose equivalence rates reported in literature by other authors.
Results: The healthy brain volume-averaged secondary neutron dose equivalence rates resulting from the use of a DCS during SFUD and IMPT treatment plans were 1.56, 1.18, and 1.32 mSv/Gy for the apex, right lateral, and combined IMPT fields, respectively. While small, this may indicate an elevated latent risk of secondary cancer development relative to the case of no collimation. However, in comparison to low-energy uniform scanning treatments with brass apertures, the DCS provides the benefits of a sharper penumbra for both SFUD and IMPT modalities but with nearly 58% less secondary neutron dose than uniform scanning.
Conclusion: These simulations reveal the effects that beam energy and collimation have upon the secondary neutron dose to the patient during SFUD and IMPT treatments with a DCS. While the DCS may increase the neutron dose relative to un-collimated treatments, the use of a DCS has been shown to provide superior target conformity relative to aperture-based collimation modalities with less than half of the neutron dose.