EP09.03 OUR EXPERIENCE WITH A SILICON SPACER FOR PARTICLE THERAPY: SPACE-MAKING PARTICLE THERAPY (SMPT) OF PATIENTS WITH PRIMARY SACRAL TUMORS.

Abstract INTRODUCTION Sacral chordoma and hemangioblastoma have high local aggressiveness and a high percentage of local recurrence after macroscopically radical procedures. Therefore, it is necessary to apply a high dose to ensure a sufficient therapeutic effect. Proton beam radiotherapy allows the delivery of a high biologically effective dose to the tumor target volume, while offering better sparing of the surrounding organs. When the space between a tumor and the rectum or intestine is too narrow, it may be challenging to safely deliver effective radiation doses without risking organ damage. Inserting a surgical spacer before proton beam therapy creates a safe distance around the tumor and allows better coverage of the target volume. Our own experience with SMPT in adult patients will be presented. MATERIAL AND METHODS Between March 2021 and March 2025 we treated six patients with sacral tumors that were enrolled for surgical placement of a silicon spacer: five chordomas (CH) and one hemangioblastoma patient. Average volume of the spacer was 130 cm3. Spacers were inserted by experienced surgeons and patients were thereafter treated with IMPT (proton radiotherapy with modulated intensity) with doses of 74 GyE (37 fractions) for chordoma and 35 GyE (7 fractions) for hemangioblastoma. Three patients received first-time radiotherapy treatment, while one patient underwent radiotherapy for local recurrence following surgery and previous high-dose radiotherapy. RESULTS Five patients with SMPT are without disease progression, one with local recurence and none have severe toxicity. Acute toxicity was mild. CONCLUSION Placement of a silicon spacer for SMPT of patients with primary sacral tumors is feasible and enables delivery of a high radiotherapy dose with better sparing of pelvic organs.