Intra- and Inter-Fraction Relative Range Verification in Heavy-Ion Therapy Using Filtered Interaction Vertex Imaging

TL;DR

This paper introduces and validates a new imaging method for precise intra- and inter-fraction range verification in heavy-ion therapy, ensuring accurate dose delivery by detecting sub-millimeter range shifts using secondary particle tracking.

Contribution

The study presents a novel filtered Interaction Vertex Imaging technique for reliable relative range verification in heavy-ion therapy, validated with experimental data demonstrating sub-millimeter accuracy.

Findings

Range shift detection with 220 μm standard deviation.

Validated method for intra- and inter-fraction monitoring.

Potential for real-time, precise dose verification in clinical settings.

Abstract

Heavy-ion therapy, particularly using scanned (active) beam delivery, provides a precise and highly conformal dose distribution, with maximum dose deposition for each pencil beam at its endpoint (Bragg peak), and low entrance and exit dose. To take full advantage of this precision, robust range verification methods are required; these methods ensure that the Bragg peak is positioned correctly in the patient and the dose is delivered as prescribed. Relative range verification allows intra-fraction monitoring of Bragg peak spacing to ensure full coverage with each fraction, as well as inter-fraction monitoring to ensure all fractions are delivered consistently. To validate the proposed filtered Interaction Vertex Imaging method for relative range verification, a ${}^{16}$O beam was used to deliver 12 Bragg peak positions in a 40 mm poly-(methyl methacrylate) phantom. Secondary particles produced in the phantom were monitored using position-sensitive silicon detectors. Events recorded on these detectors, along with a measurement of the treatment beam axis, were used to reconstruct the sites of origin of these secondary particles in the phantom. The distal edge of the depth distribution of these reconstructed points was determined with logistic fits, and the translation in depth required to minimize the $\chi^2$ statistic between these fits was used to compute the range shift between any two Bragg peak positions. In all cases, the range shift was determined with sub-millimeter precision, to a standard deviation of the mean of 220(10) $\mu$m. This result validates filtered Interaction Vertex Imaging as a reliable relative range verification method, which should be capable of monitoring each energy step in each fraction of a scanned heavy-ion treatment plan.