Evaluation of an a priori scatter correction algorithm for cone-beam computed tomography based range and dose calculations in proton therapy

Abstract

Background and purpose: Scatter correction of cone-beam computed tomography (CBCT) projections may enable accurate online dose-delivery estimations in photon and proton-based radiotherapy. This study aimed to evaluate the impact of scatter correction in CBCT-based proton range/dose calculations, in scans acquired in both proton and photon gantries.

Material and methods: CBCT projections of a Catphan and an Alderson phantom were acquired on both a proton and a photon gantry. The scatter corrected CBCTs (corrCBCTs) and the clinical reconstructions (stdCBCTs) were compared against CTs rigidly registered to the CBCTs (rigidCTs). The CBCTs of the Catphan phantom were segmented by materials for CT number analysis. Water equivalent path length (WEPL) maps were calculated through the Alderson phantom while proton plans optimized on the rigidCT and recalculated on all CBCTs were compared in a gamma analysis.

Results: In medium and high-density materials, the corrCBCT CT numbers were much closer to those of the rigidCT than the stdCBCTs. E.g. in the 50% bone segmentations the differences were reduced from above 300 HU (with stdCBCT) to around 60-70 HU (with corrCBCT). Differences in WEPL from the rigidCT were typically well below 5 mm for the corrCBCTs, compared to well above 10 mm for the stdCBCTs with the largest deviations in the head and thorax regions. Gamma pass rates (2%/2mm) when comparing CBCT-based dose re-calculations to rigidCT calculations were improved from around 80% (with stdCBCT) to mostly above 90% (with corrCBCT).

Conclusion: Scatter correction leads to substantial artefact reductions, improving accuracy of CBCT-based proton range/dose calculations.

Keywords: A priori; APT; Adaptive proton therapy; Beam hardening; CB; CBCT; Cone beam; Cone beam computed tomography; Cone beam projections; Dose calculation; Dose recalculation; Inter-fractional motion management; Projections; Proton; Range; Scatter; Scatter correction; Shading correction; WEPL; Water equivalent path length.

Fig. 1

Median of CT numbers in HU of the catphan segmentations with SD as error bars. Seperated into groups of low, medium and high density to enhance the visibility of differences. The Materials on the x-axis reflect the naming in the Catphan 604 manual, with Uniform being “Uniform Housing” and Air and Air2 are just the upper and lower otherwise equivalent air segmentations.

Fig. 2

Posterior to anterior WEPL differences between rigidCT and the given CBCT with median ± SD for each sub-region, indicated by the horizontal blue lines, displayed on top. The color scale is cut off at 60 mm, and points above that value have the same red color. The Photon and Proton rows represent the given gantry, and the columns the CBCT scans.

Fig. 3

Upper: Gamma pass rates for a 2% over 2 mm tolerance from comparison of recalculated dose on the given CT with the original dose distribution on the rigidCT, for different tolerance criteria. Lower: The paired mean difference from the stdCBCT with 95% CI, as computed by the DABEST framework.

Figure S1

Left: Axial view of the Catphan phantom (rigidCT). Right: 3D volume rendering of the segmentations.

Figure S5

Heatmap of HU value differences from the deformCT, with axial and sagittal view of the center slices of the Alderson head (Photon Gantry), for the given (CB)CT. From left to right: rawCBCT, corrCBCT, rigidCT, stdCBCT and iterCBCT. A negative value indicates the given (CB)CT is too bright (dense) and vice versa.