Standardising image registration and dose mapping for thoracic reirradiation: A national multi-centre benchmarking study

Background and purpose: Safe delivery of high-dose reirradiation depends on robust evaluation of previously delivered dose on current patient anatomy. This study aims to assess the variability of 1) rigid (RIR) and deformable (DIR) image registration, and 2) dose-mapping based on RIR or DIR, as part of the pre-trial quality assurance for CURE Lung, a Scandinavian thoracic reirradiation trial. Materials and methods: Seven lung cancer cases, treated with curatively intended reirradiation near a previously irradiated region, were distributed to six Danish centres. Organs-at-risk (OARs) were delineated on CT images of previous (CT_prev) and current treatment (CT_current). RIR and DIR from CT_prev to CT_current were performed for all cases at each centre, and doses were mapped from CT_prev to CT_current using both methods. Mapped and current physical doses were converted to equieffective doses (EQD2), and cumulative EQD2-doses were generated on CT_current. Inter-centre variation was assessed using volume coefficients and distance measurements. A linear mixed-effects model tested the effect of registration method on OAR alignment, near-maximum and volumetric EQD2-dose constraints from CURE Lung, and variability in DVH bands. Results: DIR significantly reduced the geometrical variations for all OARs, with mean surface distances <4.4 mm. Near-maximum EQD2-doses overall had less variability with DIR compared to RIR. No significant differences were found for the full DVH bands. Conclusion: DIR provides significantly better consistency in transfer of OAR from CT_prev to CT_current and reduces variability in EQD2-dose evaluations compared to RIR. This supports preferential use of DIR for reirradiation planning in lung cancer.