ESTRO 2025 - Abstract Book

S3595

Physics - Quality assurance and auditing

ESTRO 2025

Purpose/Objective: We investigated variations in dose accumulation for reirradiation due to (i) image registration variation, and (ii) local radiobiological dose considerations among eight Danish radiotherapy centers for patient cases covering different anatomical regions. Material/Methods: Eight patient cases (Table 1) were evaluated. Organs at risk (OAR) delineations were validated by expert clinicians. Seven cases had direct overlap of irradiated volumes (ESTRO-EORTC reirradiation Type 1), while the lung case had concern of toxicity from cumulative doses (Type 2) [1] . Each center mapped dose from the previous CT scan to the reirradiation CT scan using either rigid (RIR) or deformable (DIR) image registration based on local clinical practice. Subsequently, all centers accumulated the mapped dose from the previous treatment with the reirradiation dose in (i) physical dose, (ii) equieffective dose in 2 Gy fractions (EQD2), and (iii) with dose scaling factors (DSFs) to account for tissue recovery (if used in clinical practice). We used institutional / values and DSFs to reflect variation in local clinical practice. We evaluated dose accumulations for one clinically relevant OAR in high dose overlap regions for each patient case.

Results: RIR was used for dose mapping by three centers for anal and rectum cases, by two centers for brain and abdomen cases, and by one center for the breast case. DIR was performed using MIM (two centers, all registrations intensity based), Velocity (five centers, two structure-based, remaining intensity-based), and RayStation (one center, all intensity-based). The inter-center range (maximum-minimum) in near-maximum dose (D 1% ) to the selected OARs ranged from 0.7 Gy for aorta in the lung case (Lung: Aorta) to 16.7 Gy (Abdomen: Kidney_L) for physical dose accumulations (Figure 1A) and from 1.1 Gy (Lung: Aorta) to 24.3 Gy (Abdomen: Kidney_L) for EQD2-rescaled dose accumulations (Figure 1B). For the eight selected OARs, / values ranged from 2 Gy to 5 Gy. One center applied DSFs to bowel bag in the rectum case (DSF=0.75), chiasm in the brain case (DSF=0.75), and for the bladder in the