Impact of collimator misalignments on dosimetric robustness in proton minibeam radiotherapy using an automated measurement setup

Abstract

Background

Proton minibeam radiation therapy is an emerging technique that combines the tissue-sparing benefits of protons with the spatial modulation used for minibeams to reduce toxicity in healthy tissue. Achieving accurate delivery of planned dose distributions depends on precise collimator-to-beam alignment, for which it is essential to understand the behavior of both multi-slit and single-slit configurations. Understanding single-slit behavior is particularly important, as it also linearly affects the heterogeneity (peaks and valleys) of the overall minibeam. While several studies have investigated misalignment effects with multi-slit collimators, there is limited experimental characterisation of single-slit behavior and its impact on multi-slit arrays.

Purpose

This study aims to quantify the dosimetric effects of translational and rotational collimator-to-beam misalignments for single-slit proton minibeams using our in-house automated measurement setup. Additionally, the robustness of lateral dose profiles for minibeam arrays will be investigated under varying collimator misalignments.

Methods

A single slit collimator with $150MeV$ protons was used to produce a single minibeam with adjustable collimator slit widths (0.4 and $0.7mm$). We introduced rotational and translational misalignments using motorized stages. We developed a quick measurement setup to measure lateral profiles using a microDiamond detector. The lateral profiles were measured at depths of 1 and $13cm$ using a water-equivalent phantom. We validated the setup by comparing the measurements to gafchromic film measurements using local gamma analysis. A custom fitting model combining two Gaussians and a Lorentzian was developed to fit the lateral profiles. From the fitted profiles, we determined the maximum dose, the lateral position of maximum dose, the full width at half maximum, and the symmetry. Minibeam arrays were simulated via superposition of single-slit profiles. Specific misalignment combinations were measured for 140, 160, and $170MeV$ protons, energies typically used in the clinic for deep seated tumours, to assess how misalignments affect the minibeam at different energies.

Results

Rotational misalignments caused substantial asymmetry and peak dose reductions up to 68.4% at shallow depths, with asymmetry reducing at greater depths. Translational shifts resulted in a translation of the minibeam profile in the same direction and magnitude without changing the minibeams shape. Despite these distortions, simulated minibeam arrays achieved acceptable dose homogeneity at $13cm$ depth for central 80% field-widths with flatness below a 3% threshold. The proposed measurement setup enabled profile acquisition of a single minibeam in under one minute with submillimetre resolution and strong agreement with gafchromic film (3%/$0.2mm$ gamma pass rates of 100%).

Conclusions

Collimator misalignments substantially affect the minibeam dose robustness and should be considered in clinical treatment planning and quality assurance (QA). The presented measurement setup enables, rapid, high-resolution profile measurements using commercially available detectors and can facilitate routine clinical QA.