Author: Serdar Charyyev, Kaan Dere, Edgar Gelover, Mohammad Khurram Khan, Liyong Lin, Mark McDonald, Cristina Oancea, Alexander Stanforth, Sibo Tian, Suk Whan (Paul) Yoon, Mingyao Zhu 👨🔬
Affiliation: Department of Radiation Oncology, Stanford University, Stanford University, Emory University, Department of Radiation Oncology and Winship Cancer Institute, Emory University, Advacam 🌍
Purpose: The conventional implementation of proton spatially fractionated radiotherapy (SFRT) utilizes physical collimators with apertures to generate minibeams, creating alternating regions of high-dose peaks and low-dose valleys. SFRT effectiveness relies on peak-to-valley dose ratio (PVDR). While high PVDRs and low valley doses have been reported vital to normal tissue sparing, linear energy transfer (LET) spectra have never been investigated which could impact SFRT.
Methods: Using TOPAS Monte Carlo (MC) simulations, we simulated dose and LET at multi depths for 70-150 MeV proton minibeams passing through a brass collimator, which has 2 mm diameter holes with center-to-center distances of 6 mm. Deposited energy and track length are measured per proton interaction track/cluster using Minipix SPRINTER14x14 mm² detector of 55-μm pixels to derive LET spectra. The collimator modified the relative homogenous track map to a highly heterogenous map. LET distributions were MC scored in peak and valley regions and 14-mm square detector area.
Results: PVDR decreased with proton energy and depth in measurements as in simulation. LET spectra became non-Gaussian in collimated SFRT with its width increased with depth. Inverted dose and LET peaks are observed in collimated SFRT and are blurred by protons scattered from 2-mm collimation holes. When protons arrive at their distal ends, PVDR becomes smaller and the difference of LET spectra under blocked and open area become smaller. Non-Gaussian LET tails are more curtailed in measurement than MC due to non-divergent hole design and beam misalignment.
Conclusion: Proton counting detectors can visualize PVDR and non-Gaussian LET spectra in mini beam proton SFRT. The discrepancy of PVDR and non-Gaussian LET between measurement and MC suggest that the alignment/divergent design of mini beam collimator must be checked with the proposed novel LET QA method besides normal PVDR to achieve the desired proton SFRT dosimetry.