ESTRO 2024 - Abstract Book

S4591

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

overdosage to OAR or unwanted target coverage compromises. Proton therapy is highly eligible for reirradiation as dose escalation to the recurrence is possible without increasing the surrounding (low and median) dose bath to OAR. These high reirradiation doses make correct dose summation and assessment of accumulated OAR doses even more important. The aim of this study was to determine the difference between EQD2-corrected and un-corrected physical dose summation.

Material/Methods:

The initial eighteen patients included in the prospective phase II trial, ReRad II (dose escalated proton therapy reirradiation for rectal cancer recurrences) were included. One patient was excluded due to no Dicom data available from first treatment with Papillon. All were treated with accelerated, hyperfractionated Intensity Modulated Proton Therapy (IMPT) to 55-65 Gy(RBE) in 1.25 Gy/fraction (fx) twice daily. Reirradiation IMPT plans were optimized in Eclipse. Primary radiotherapy plans ranged from 25Gy/5fx (5Gy/fx) to 78Gy/39fx (2Gy/fx) – all accessible in Dicom format. Both primary and reirradiation plans were voxel-by-voxel Equivalents Dose in 2 Gy Fractions (EQD2) corrected in Velocity AI. Subsequently, primary and reirradiation plans were accumulated in Eclipse as physical dose and EQD2-corrected dose, respectively. For the voxel-by-voxel EQD2-correction, we used an α/β=3 for delineated OARs, and for nerves α/β=2. A rigid 6D registration between primary and reirradiation therapy CT-scans where used, focusing on the dose overlap regions. Wilcoxon's signed rank test with Holm-Bonferroni corrected p-values was used for comparison between EQD2 corrected and physical dose. A p-value<0.05 was considered statistically significant.

Results:

The mean accumulated max doses for the scanned body outline varied from 107.2 Gy (range: 98.2-139.4) in EQD2 to 113.8 Gy (92.0-148.2) in physical dose and for the accumulated mean doses from 8.4 Gy (3.8-13.4) in EQD2 to 10.2 Gy (3.7-15.7) in physical dose – both statistically significant. Results for accumulated max and mean doses for individual OAR: bowel loops, bladder, ureter (contralateral and ipsilateral), sacral bone, cauda equina and genitalia varied from 4-9 Gy in max doses and 4-6 Gy in mean doses, as shown in Table 1. However, the variation was up to 16 Gy in max and 14 Gy in mean for individual OAR and patients. Figure 1 shows the max accumulated physical dose vs. EQD2 dose for all delineated OAR.