Residual range-based quenching correction for Gafchromic EBT3 film in proton therapy patient-specific QA

Gafchromic EBT3 film is a widely used dosimeter in radiotherapy due to its high spatial resolution and near tissue equivalence. However, its application in proton therapy is limited by the linear energy transfer (LET) quenching effect, particularly in the Bragg peak region.

This study proposes a correction method based on residual range (R_res) to quantify and mitigate this quenching effect for improved accuracy in patient-specific quality assurance (PSQA) and related research.

We defined a quenching factor (QF) as the ratio of dose measured with a Markus ionization chamber to that measured with EBT3 film. The EBT film calibration curve is established by measurements in the plateau region of the depth-dose distribution of a monoenergetic proton reference beam. A series of proton treatment plans with varying spread-out Bragg peaks (SOBPs) were designed to examine QF across different residual ranges. A correction function correlating R_res to QF was derived. Clinical verifications of patient-specific QA were performed by comparing EBT3 measurements, with and without QF correction, to MatriXX-PT and Eclipse TPS calculations.

The EBT3 film underestimated dose by approximately 7% at an R_res of 1.4 cm. A clear dependence of film response on R_res was observed: as R_res decreased, the degree of under-response increased. A correction function was established based on the relationship between R_res and QF, enabling accurate dose reconstruction. The dose map measured using QF-calibrated EBT3 film showed good agreement with the TPS-calculated data, and the discrepancies between the EBT3 film measurements, MatriXX-PT measurements, and TPS calculations were significantly reduced in regions with smaller R_res values.

We proposed and validated a clinically applicable correction function for EBT3 film quenching in proton PSQA. This approach offers a practical and cost-effective solution for both clinical and research applications involving proton dosimetry.