Sensitivity of coaxial prompt gamma-ray monitoring in heterogeneous geometries: A Monte Carlo simulation study

Proton beams offer significant advantages over conventional radiotherapy due to their unique interaction with matter. Specifically, the ionization density caused by these beams is higher in a well-defined region (the Bragg peak) with a sharp decline in intensity beyond a specific depth. However, variations in proton range – often caused by changes in patient anatomy and morphology during treatment – can introduce uncertainties in dose distribution. To account for this, clinicians apply conservative margins, which limit the full potential of proton therapy. Efforts have been focused on developing proton range and dose distribution monitoring systems to reduce the need for large safety margins. These systems are based on detecting and analyzing the byproducts that result from the interaction between the proton beams and tissue. In this article, we focused specifically on a system that aims to detect photons called prompt gamma (PG) rays. We conducted Monte Carlo simulations of proton beams interacting with anthropomorphic phantoms of varying densities to simulate morphological changes. A single scintillation detector was positioned coaxially with the beam and behind the phantom to capture the emitted PG rays in each scenario. Our analysis focused on discrepancies in proton range that resulted from irradiating an anthropomorphic head phantom with varying brain tissue densities and detecting secondary particles resulting from these interactions. We observed potential correlations between gamma-ray signatures and variations in proton range and energy deposition, suggesting that this monitoring technique could be effective for real-world clinical applications.