ESTRO 2025 - Abstract Book

S3652

Physics - Quality assurance and auditing

ESTRO 2025

after reirradiation depends on the total dose delivered to organs at risk (OAR) over all treatment courses. Image registration is crucial to evaluate accumulated doses, but uncertainties need to be considered due to the lack of a ground truth. Here we analyse the impact of different image registration methods on the uncertainties of accumulated OAR doses. Material/Methods: Eighteen head and neck cancer (HNC) reirradiation patients treated between 2008 and 2024 at our center were included in this retrospective planning study. All patients had an interval of at least 3 months between the treatment courses. An overall rigid and deformable registration (RIR and DIR) of the images was performed and doses were accumulated on the new CT and the old CT for all patients. Additionally, RIR based on the spinal cord and on the mandible was done for 9 patients each, depending on the region of dose overlap. Predefined dose volume-parameters (DVH) were recorded and compared between the different registration methods to estimate uncertainties. Results: The mean difference between overall RIR and DIR on CT2 was 0.0 ± 5.7Gy ( Figure 1). However, differences of up to 35.5Gy were recorded for individual organs. The mean deviation between the registrations on CT2 and CT1, analysed relative to our in-house tolerance doses for re-irradiation scenarios, was 2.8% ± 25.0% for RIR [range: - 97.5% to 83.3%] and -0.4% ± 16% [range: -44.8% to 81.0%] for DIR of the spine group. For the mandible group the mean deviation was 3.7% ± 24.8% [range: -88.1% to 116.6%] for RIR and 0.1% ± 20.9% [range: -66.8% to 92.5%] for the DIR ( Figure 2 ). For the OAR specific registrations, compared between the two CTs, the DVH parameters for the spinal cord were within [-5.6Gy and 4.4Gy] corresponding to [-5.9% and 4.0%] of the tolerance doses, for the oral cavity they were within [-4.8Gy and 25.7Gy] corresponding to [-10.7% and 57.2%] of the tolerance doses.

Figure 1: Differences between RIR and DIR.