(P 290) CLINICAL OUTCOMES AFTER RADIOTHERAPY FOR PATIENTS WITH POORLY DIFFERENTIATED CHORDOMA

Objective: Poorly differentiated chordoma (PDC) is an aggressive malignancy , often requiring intensive multimodal therapy, including surgery, radiotherapy (RT), and systemic therapy. Little is currently known about the optimal treatment regimen and outcomes after RT for this disease.

Methods: This was an Institutional Review Board-approved retrospective study for patients diagnosed with PDC and who received RT at Massachusetts General Hospital between 2007-2024. Clinical and treatment characteristics were recorded. Kaplan-Meier method was used to assess overall survival (OS), progression-free survival (PFS), and local recurrence-free survival (LRFS). Cox regression model was used for univariate analyses (UVA). Toxicity was graded using CTCAE v5.

Results: We identified 22 patients with median age at diagnosis of 11.0 years (range: 2-34.1). Sixteen patients (72.7%) were female. Primary tumor location included clivus/craniocervical junction (n=10), cervical/thoracic spine (n=11), and sacrum (n=1). Seven patients had metastases at diagnosis. Seven patients received gross total/near-total resections, 9 had subtotal resections, and 6 had biopsy alone prior to RT. Sixteen patients received pre-RT chemotherapy with median of 6.5 cycles (range: 1-17 ). Two patients had a complete response to pre-RT chemotherapy at the primary site and 10 had a partial response. Two patients had pre-operative RT and all received post-operative RT. Median RT total dose was 74.6 Gy (range: 66.6-79.4). The median time from diagnosis to RT was 8.6 months (range: 1.4-42.2). With a median follow-up time from end of RT of 39.8 months (range: 0.5-144.6), 3-year OS, PFS and LRFS after RT was 69.8%, 25.6% and 63.5%, respectively. Three patients (13.6%) had acute and 2 patients (9.1%) had late grade ≥3 toxicity, respectively. On UVA, metastatic disease at diagnosis (HR: 4.55, 95% CI: 1.42-14.59, p=0.011) and biopsy alone (HR: 4.42, 95% CI: 1.02-19.16, p=0.047) was associated with progression post-RT.

Conclusion: Patients with PDC remain at high risk of metastatic and local recurrence despite multimodality treatment, including RT. Patients with metastatic disease and biopsy alone are at higher risk of relapse after RT. Future studies with larger cohorts and multivariable analyses remain needed to better characterize outcomes for patients with PDC.