1070 - Long-Term Outcomes following Proton-Based Radiation Therapy for Pediatric Chordomas and Chondrosarcomas of the Mobile Spine and the Sacrum

M. Ioakeim-Ioannidou¹, A. Niemierko², D. J. Konieczkowski³, D. W. Kim⁴, P. Nielsen², G. M. Cote⁵, J. H. Shin², T. F. DeLaney⁶, Y. L. E. E. Chen⁶, F. Giap⁶, and S. M. MacDonald⁶; ¹Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA, ²Massachusetts General Hospital, Boston, MA, ³Department of Radiation Oncology, James Cancer Hospital, The Ohio State University, Columbus, OH, ⁴Department of Radiation Oncology, Massachusetts General Hospital, Boston, MA, ⁵Division of Hematology-Oncology, Massachusetts General Hospital, Boston, MA, ⁶Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA

Purpose/Objective(s): We report the first cohort of children with spinal and sacrococcygeal chordomas (CH) and chondrosarcomas (CHS) treated with proton-based radiotherapy (PRT). Materials/

Methods: Between 1988 and 2019, 52 pediatric patients with CH and CHS of the mobile spine and sacrum were treated with PRT at a single institution. Primary endpoints were overall survival (OS), cancer-specific survival (CSS), and progression-free survival (PFS). A univariate was performed to identify prognostic factors.
Results: There were 31 (60%) females and 21 (40%) males. Median age at diagnosis was 12.8 years (4-21). 43 (83%) patients were diagnosed with CH and 9 (17%) patients with CHS. The primary tumor originated in the C-spine (n=37, 71%), the T-spine (n=6, 12%), the L-spine (n=7, 14%), or the sacrococcygeal region (n=2, 3%). The majority of the CH patients were conventional type (n=33) while 10 had atypical/poorly differentiated CH. CHS patients included 4 mesenchymal, 3 mixed hyaline/myxoid, and 2 hyaline CHS. 32 (61%) patients underwent gross total resection, 19 (37%) patients underwent subtotal resection while 1 (2%) patient had a biopsy only. 8 (15%) patients were treated with pre- and post-op RT, while 44 (85%) patients received only post-op RT. 34 (65%) patients had spinal stabilization in place at the time of RT. 13 (25%) patients with high-risk pathological features received pre-RT chemotherapy. Due to radiation avoidance, 13 (25%) patients developed progressive disease prior to RT. 11 (21%) patients were treated with protons only and 41 (79%) patients received combined proton/photon RT. The median total dose was 74.5 Gy (RBE) [IQR, 69.8 - 76 Gy (RBE)] delivered in 1.8-2.5 Gy (RBE) daily fractions. The median maximum dose to the spinal cord was 62.6 Gy (RBE) [range, 30.6-69.2 Gy (RBE)]. At a median follow-up of 11.4 years (IQR, 5.7 – 19.8) from the date of diagnosis, 17 (32.7%) patients recurred (8 local, 7 distant and 2 iatrogenic seeding along the surgical tract). 15 of these patients died of disease. Median follow-up for surviving patients was 15.4 years (IQR, 10.9 – 22.9). The 5-, 10-, and 20-year OS were 82.7%, 72.3% and 72.3%, respectively. The 5-, 10-, and 20-year DSS were 86.1%, 77.5%, and 77.5%, respectively. The 5-, 10-, and 20-year PFS were 72.3%, 70.1% and 70.1%, respectively. The 10-year OS, DSS, and PFS for conventional CH were 84.3%, 90.5%, and 81.5%, respectively. The 10-year OS, DSS, and PFS for CHS were 77.8%, 77.8%, and 77.8%, respectively. Factors significantly associated with worse outcomes were poorly-diff CH subtype, receipt of pre-RT chemo, and low KPS (p < 0.05). Pre-RT tumor progression was found to be a significant prognostic factor for PFS (p=0.05). No patients experienced late = grade 2 toxicities.
Conclusion: This is the largest study of pediatric spinal and sacrococcygeal CH/CHS in the literature. High-dose PRT following surgical resection offers high rates of disease control for conventional chordomas and low-grade chondrosarcomas.