418 The Impact of Tumor Location on Neoadjuvant Proton Beam Radiotherapy Response and Perioperative Blood Loss Management in Sacral Chordoma Patients

INTRODUCTION: Neoadjuvant proton beam radiotherapy (PBRT) reduces tumor volume and improves surgical resectability for sacral chordoma, yet its impact on surgical complexity has not been elucidated. As sacral chordoma resection is associated with extensive blood loss, identifying tumor characteristics with greater response to radiation may reduce operative complexity. METHODS: This prospective study included adult patients with biopsy-proven sacral chordomas treated with neoadjuvant PBRT (5040 cGy, 28 fractions) followed by en bloc resection at the Massachusetts General Hospital between October 2008-March 2024. Baseline clinical and imaging data within 3 months of treatment were obtained from the electronic health record. The two-sample t-test was used to examine associations between tumor characteristics and perioperative variables. RESULTS: This study included 40 patients with 29 (72.5%) sacrococcygeal chordomas and 11 (27.5%) sacral chordomas. Following PBRT, sacrococcygeal chordomas demonstrated smaller and more negative anterior-posterior dimensions on imaging scans than sacral tumors (mean±SD, -0.53±18.0% versus 18.1±32.8% of original, p=0.027) resulting in lower overall volume difference (7.04±24.2% versus 51.6±77.6% of original, p=0.009) independent of original tumor volume and number of vertebral levels involved. No significant differences were observed in operative time, blood loss, and intraoperative transfusion rates. However, among patients who developed acute anemia (44.8% sacrococcygeal, 63.6% sacral), those with sacrococcygeal chordomas required fewer blood product units (1.45±2.52 versus 5.55±8.70, p=0.025) during hospitalization regardless of length of stay. CONCLUSIONS: In this cohort, coccygeal involvement was associated with increased radiographic tumor response and reduced postoperative transfusion volume when acute anemia occurred. This finding suggests that tumor location may influence PBRT efficacy and perioperative outcomes in sacral chordoma patients.