S1114
Clinical – Upper GI
ESTRO 2026
Material/Methods: We retrospectively evaluated 135 patients with distal esophageal cancer consecutively-treated with neoadjuvant chemoradiotherapy and IMPT between 2020–2024. IMPT was selected using the Dutch model- based approach, with a prescribed ITV dose of 41.4 Gy (3), where ITV included GTV and elective nodal regions with respiratory motion margins. ITV dose deposition was assessed weekly using repeat CTs and a voxel-wise minimum dose (VWmin) criterion: ≥ 95% of the prescribed dose to ≥ 98% of the ITV. Cumulative dose deposition was assessed using robust dose summation based on multi-plan accumulation, incorporating adapted plans where applicable (4). Baseline diaphragm amplitude was measured. Primary endpoints were clinical response at one year and pathologic response in resected patients. Secondary endpoints included post-treatment clinical response and two-year overall survival (OS). Results: Median baseline diaphragm amplitude: 1.6 cm (range 0.54 – 8.68 cm). Median GTV and ITV coverage (D98%, VWmin): 41.0 Gy (range 38.8 – 43.5 Gy) and 39.7 Gy (range 21.8 – 41.4). GTV underdosage occurred in 1 patient (<1%); ITV underdosage in 47 patients (35%; Figure 1), of whom 20 (43%) underwent plan adaptation based on ITV coverage. Of 132 patients evaluable for radiologic response, 52 (39%) achieved clinical complete response (cCR); 50 were evaluable at 12 months, with persistent cCR in 19 (38%). Persistent cCR rates at 12 months were comparable between patients with and without ITV underdosage (47% vs. 33%, p=0.37; Figure 2B). Among 94 resected patients (70%), pathologic complete response (pCR) rates were not significantly different (12% vs. 23%, p=0.27; Figure 2C). No significant difference in cCR rates was observed (p=0.61; Figure 2A). Large baseline diaphragm amplitudes (>2 cm) were not correlated with underdosage. Two-year OS was 49% with ITV underdosage vs. 37% without (p=0.30; N=114).
Conclusion: ITV underdosage occurred in over one-third of patients, but was not associated with tumor response or OS. GTV dose remained adequate despite ITV underdosage, and large baseline diaphragm amplitudes did not affect target dose deposition. References: 1. Li H, et al. AAPM Task Group Report 290: Respiratory motion management for particle therapy. Med Phys. 2022;49(4):e50-e81.2. Canters R, et al. Robustness of intensity modulated proton treatment of esophageal cancer for anatomical changes and breathing motion. Radiother Oncol. 2024;198:110409.3. NVRO. Addendum Slokdarmcarcinoom bij Landelijk Indicatie Protocol Protonentherapie Longcarcinoom. 2021.4. Taasti VT, et al. Treatment planning and 4D robust evaluation strategy for proton therapy of lung tumors with large motion amplitude. Med Phys. 2021;48(8):4425-37. Keywords: target coverage, proton therapy, tumor control Effectiveness of Stereotactic Body Radiotherapy for Small Pancreatic Neuroendocrine Tumors Mercedes López Gonzalez, Ovidio Hernando, Emilio Sánchez, Xin Chen, Raquel Ciervide, Beatriz Alvarez, Angel Montero, Jeannette Valero, Mariola Garcia- Aranda, Bruno Zambrana, Raquel Sanchez, Carmen Rubio Radiation Oncology, HM HOSPITALES, MADRID, Spain Purpose/Objective: Surgery remains the standard treatment for Pancreatic Neuroendocrine Tumors (pNETs), providing favorable outcomes but often associated with significant morbidity and mortality. This study investigates Stereotactic Body Radiation Therapy (SBRT) as a potential radical treatment for small (<2 cm), nonfunctioning pNETs. Digital Poster 4378 Prospective Analysis of Tolerance and
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