S1835
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
References: [1]Varian Medical Systems. Halcyon and TrueBeam Radiotherapy Systems: IEC Accompanying Documents — Tests and Procedures. Palo Alto, CA: Varian Medical Systems; 2024. User Manual[2] National Library of Medicine. PubMed search: “HyperSight” [Internet]. Bethesda (MD): U.S. National Library of Medicine; 2025 [cited 2025 Nov 11]. Available from: https://pubmed.ncbi.nlm.nih.gov/?term=HyperSight[3] National Library of Medicine. PubMed search: “AEC + CBCT” [Internet]. Bethesda (MD): U.S. National Library of Medicine; 2025 [cited 2025 Nov 11]. Available from: https://pubmed.ncbi.nlm.nih.gov/?term=AEC + CBCT Keywords: Dose optimisation, CBCT Accumulated dose evaluation and correlation with gastrointestinal toxicities in pancreatic cancer patients treated with SBRT Youssef Ez-zyouy 1 , Sara Poeta 1 , Zelda Paquier 1 , Christelle Bouchart 2 , Nick Reyanert 1 , Younes Jourani 1 1 Department of Medical Physics, Université Libre de Bruxelles (ULB), Hôpital Universitaire de Bruxelles (H.U.B), Institut Jules Bordet, Brussels, Belgium. 2 Department of Radiation Oncology, Université Libre de Bruxelles (ULB), Hôpital Universitaire de Bruxelles (H.U.B), Institut Jules Bordet, Brussels, Belgium Purpose/Objective: Pancreatic cancer ranks among the deadliest cancers worldwide, with a 5-year survival rate of about 12%1. Its treatment remains challenging due to the proximity of radiosensitive organs and continuous anatomical variations. This study aimed to (1) evaluate correlations between gastrointestinal toxicities and Mini-Oral 1477 planned dosimetric parameters, (2) quantify anatomical changes during stereotactic body radiotherapy (SBRT), and (3) assess correlations using accumulated dose data. Material/Methods: 61 patients with pancreatic cancer treated with SBRT (50 Gy in 5 fractions with a full isotoxic dose prescription2) between January 2021 and July 2023 on a conventional SBRT dedicated Linac were included. For the first objective, univariate Student’s t-tests and multivariate logistic regression analyses were performed to assess correlations between grade2+ toxicities (CTCAE v5) and planned dosimetric parameters. For the second objective, anatomical variations were evaluated in 42 patients using deformable image registration between daily CBCTs and the planning CT, based on a B-spline algorithm. The deformation accuracy was verified both visually and quantitatively using the Jacobian determinant to ensure anatomical consistency across the GTV and
OARs. Deformed doses were accumulated over all fractions to estimate the total delivered dose. Dose– volume histograms (DVHs) from accumulated and planned doses were compared using paired t-tests (p < 0.05), and correlations between toxicity and accumulated dosimetric parameters were then assessed using the same statistical approach. Results: Among 61 patients, 11 developed grade2+ epigastralgia and 8 cases of gastroparesis/dyspepsia, acute or late. No clear predictors were identified for epigastralgia, though the duodenum PRV D0.5cc was the most likely factor (OR = 2.8; p = 0.125). For gastroparesis/dyspepsia, the Colon PRV V40Gy was significant (OR = 1.29; p = 0.037). From 210 CT–CBCT deformations (42 patients), registration accuracy was satisfactory with Jacobian within reasonable limits (Figure 1). Accumulated dose analysis showed a median of 3 Gy underdosage for D50% GTV and a 7.6 Gy increase in duodenum D0.5cc, while other OAR doses were consistent with planning (Figure 2). No significant predictors emerged from accumulated dose data, but duodenum D0.5cc remained the most plausible toxicity-related parameter.
Conclusion: In pancreatic SBRT, the maximum duodenal dose appears to be the main factor linked to grade2+ gastrointestinal toxicities. The observed GTV underdosage and duodenal overdosage illustrate the challenges of achieving precise dose delivery in the abdominal region. These results highlight the importance using daily of adaptive treatment and
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