S1040
Clinical – Sarcoma, skin cancer, malignant melanoma
ESTRO 2026
1 Radiation Oncology, Massachusetts General Hospital, Boston, USA. 2 Radiation Oncology, Brigham and Women's Hospital, Boston, USA. 3 Medical Oncology, Massachusetts General Hospital, Boston, USA. 4 Radiation Oncology, Mayo Clinic, Rochester, USA. 5 Radiation Oncology, MD Anderson Cancer Center, Houston, USA. 6 Surgical Oncology, Columbia University Irving Medical Center, New York, USA. 7 Surgical Oncology, Dell Medical School, Austin, USA. 8 Surgical Oncology, MD Anderson Cancer Center, Houston, USA. 9 Surgical Oncology, Massachusetts General Hospital, Boston, USA. 10 Pathology, Massachusetts General Hospital, Boston, USA. 11 Radiation Oncology, Rush University Medical Center, Chicago, USA. 12 Surgical Oncology, Dartmouth-Hitchcock Medical Center, Lebannon, USA. 13 Radiation Oncology, Dartmouth- Hitchcock Medical Center, Lebanon, USA Purpose/Objective: Local recurrence continues to be a significant contributor to treatment failure in retroperitoneal sarcomas (RPS) despite aggressive surgeries. The role of pre-operative radiotherapy (RT) remains controversial, and there is limited prospective data investigating whether different radiation techniques improve local control. Thus, we conducted a multi- institutional, prospective Phase II study to assess efficacy and tolerability of intensity modulated proton radiation therapy (IMPT) with selective dose escalation to 63 GyRBE to the posterior RPS margin (clinical target volume [CTV] 2) at high risk for positive margins in decreasing the risk of LR. Material/Methods: Patients (pts) >18 years at diagnosis and pathologic confirmation of RPS were enrolled from January 2016 to April 2021. Using IMPT, pts received 50.4 GyRBE in 28 fractions to CTV1 (tumor plus adjacent tissue at risk of subclinical disease) with simultaneous integrated boost to CTV2 to 63.0 GyRBE in 28 fx. Due to late grade 2 neuropathy in a pt with Type II diabetes, study was amended to reduce IMPT dose in pts with diabetes to 57.4 GyRBE. To avoid treatment delay, 11 fx of photon IMRT could be substituted for IMPT. Pts had restaging and surgery 4-8 weeks after completion of RT. Primary objective was local failure (LF). Secondary endpoints included surgical margin status, disease-free survival (DFS) and overall survival (OS). Results: Of the 60 patients enrolled on the study, 34 had liposarcoma (LPS), 21 leiomyosarcoma (LMS), and 4 undifferentiated pleomorphic sarcoma. IMPT was delivered per protocol in all pts. Four pts did not have surgery due to metastases on post-RT and pre- operative imaging. Of the 56 pts who underwent resection and had surgical pathology reports, 22 had positive margins and 1 patient had gross residual disease. With a median follow-up of 50.3 months
(range: 1.9-99.5) after RT start, 4-year LF, DFS, and OS were 11.4%, 53.3%, and 75.9%. Surgical Clavien-Dindo morbidity scores: 0(21), 1(16), 2(6), 3a (4), 3b(4), 4a(2), 4b(0), 5(2). Two patients died due to surgical complications. Eight patients had late radiation- associated grade ≥ 3 toxicities including lymphopenia (n=5), bowel obstruction (n=2), back pain (n=1), colonic anastomosis ulcer (n=1), duodenal ulcer (n=1), and fatigue (n=1). Conclusion: Preoperative IMPT with dose escalation to 63.0 GyRBE to the high risk posterior RPS margin results in a promising LF rate of 11.4% and DFS and OS in line with published studies. Late toxicities can occur, highlighting importance of long-term follow-up. Keywords: retroperitoneal sarcoma, proton radiotherapy Digital Poster 2597 An Evaluation of Radiotherapy and Response in the Management of Perivascular Epithelioid Cell Tumors: A CanSaRCC Study Reinhardt Krcek 1,2 , Kristina Nesterova 3,4 , Anna T. Santiago 5 , Rachel Aubrey 6 , Michelle J. Michael 6 , Siyer Roohani 1,7 , Hagit Peretz 8,6 , Abha Gupta 8,6 , Peter W. Chung 9,3 1 Radiation Medicine Program, Princess Margaret Cancer Centre, University Health Network, ON M5G 2C4, Toronto, Canada. 2 Department of Radiation Oncology, University Hospital Zurich, University of Zurich, Zurich, Switzerland. 3 University of, Toronto, Toronto, Canada. 4 Division of Medical Oncology, , Princess Margaret Cancer Centre, University Health Network, , ON M5G 2C4, Toronto, Canada. 5 Department of Biostatistics, University Health Network, ON M5G 2C4, Toronto, Canada. 6 Division of Medical Oncology, Princess Margaret Cancer Centre, University Health Network, ON M5G 2C4, Toronto, Canada. 7 Department of Radiation Oncology, Charité - Universitätsmedizin Berlin, corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin, 13353 Berlin, Germany. 8 Division of Hematology/Oncology, Hospital for Sick Children, Toronto, Ontario, Canada. 9 Radiation Medicine Program, University Health Network, ON M5G 2C4, Toronto, Canada Purpose/Objective: Perivascular epithelioid cell tumors (PEComas) are ultra-rare soft tissue tumors with heterogeneous treatment approaches. While surgery is the standard for localized disease, the role of radiotherapy (RT) remains unclear. Material/Methods: This retrospective study reviewed data from the
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