S1734
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
Purpose/Objective: To assess the robustness of extended-field intensity- modulated proton therapy (EF-IMPT) against interfractional anatomical variations in patients with locally advanced cervical cancer (LACC), by simulating 25 real-world daily treatment scenarios and comparing predicted hematologic adverse events with extended-
field VMAT (EF-VMAT). Material/Methods:
A total of 270 treatment plans from 10 patients with LACC previously treated with EF-VMAT were analyzed. All received 45 GyRBE in 25 fractions to the pelvis and para-aortic regions, with a simultaneous integrated boost to 55 GyRBE for involved nodes. Each case was replanned using EF-IMPT with a four-field, multi-field optimization technique and a 3 cm range shifter. Daily recontouring of clinical target volumes (CTVs) and organs of interest, including bone marrow, was performed on CBCT images using deformable registration or manual adjustment. For each patient, one nominal EF-IMPT plan and 25 CBCT-based scenario plans simulated interfractional variations. Grade ≥ 3 hematologic adverse events (Gr ≥ 3 HAE) for lymphocytes, erythrocytes, and platelets were evaluated per CTCAEv5.0. Normal tissue complication probabilities (NTCPs) for Gr ≥ 3 HAE were estimated using the Lyman–Kutcher–Burman model ( α / β =3, n=0.7) and compared between EF-VMAT and EF-IMPT, including 25 daily proton scenarios. Results: The low-dose bath (0–20 GyRBE) to pelvic bone marrow was significantly lower with EF-IMPT than with EF-VMAT across nominal and 25 daily scenarios. EF- IMPT reduced cumulative low-dose exposure by approximately 12.6%, as indicated by a smaller area under the dose–volume curve (21,118 vs 24,158 mL·GyRBE). The PBT plan exhibited a right-shifted sigmoid NTCP curve, indicating a lower probability of Gr ≥ 3 HAE compared with VMAT. Across 25 PBT robustness scenarios, NTCP curves were tightly clustered, showing minimal variation in predicted adverse events risk. (Figure1) The average Δ NTCP between VMAT and nominal PBT plans was 9.9% (95%CI: 0.1–16.9), while the median average Δ NTCP across 25 daily PBT scenarios was 13.5% (range:9.6– 17.3). The Δ NTCP difference peaked at 45–47 Gy, with a maximum NTCP reduction of approximately 40–60% for PBT. (Figure2) All PBT scenarios consistently yielded lower NTCPs than VMAT, and the narrow interquartile range across scenarios confirmed the stability and robustness of PBT in reducing predicted hematologic adverse events.
Conclusion: EF-IMPT demonstrated superior and consistent hematologic sparing compared with EF-VMAT in locally advanced cervical cancer. Despite interfractional anatomical variations across 25 fractions, EF-IMPT maintained robust Δ NTCP reductions for Gr ≥ 3 HAE of approximately 10–14%, with maximum NTCP decreases reaching 40–60%, highlighting its robustness and clinical potential for extended-field irradiation.
Made with FlippingBook - Share PDF online