(P 351) OSTEOSARCOMA STEREOTACTIC BODY RADIOTHERAPY: OUTCOMES AND PATTERN OF FAILURE

Objective: Radiotherapy is not a standard part of upfront treatment for osteosarcoma as this type of sarcoma is relatively radioresistant. Stereotactic body radiotherapy (SBRT) is a noninvasive ablative radiotherapy tool that utilizes multiple intersecting beams of focused high dose radiation which allows for increased biologic effective dose. Outcomes of stereotactic body radiotherapy (SBRT) for metastatic osteosarcoma are not well known. We sought to report outcomes of stereotactic body radiotherapy (SBRT) for metastatic osteosarcoma and identify factors associated with local failure.

Methods: We queried an IRB- approved registry of patients treated with SBRT for metastatic osteosarcoma between 2015-2023. Patients were assessed for local control, survival, and toxicity. Rates of overall survival and local control were estimated by actuarial analysis from the time of first SBRT. Dosimetric analysis was performed to determine patterns of failure. Definition of recurrences: In-field: >80% of recurrent volume within the 95% isodose line, marginal: 20% to 80% of recurrent volume within 95% isodose line, out of field: < 20% of recurrent volume within 95% isodose line. Estimates of local failure were determined using the Kaplan-Meier method. Local failure was determined on a per-lesion basis.

Results: A total of 21 patients with metastatic osteosarcoma received SBRT to a total of 71 lesions. Median age was 18 and KPS 90. Median number of lesions treated per patient was 4 (R:1-14). Median dose used was 40 Gy over 5 fractions, BED4 was 120Gy. Median imaging follow up was 9.5 months. Targets included: 47 bone (extremity: 12, spine: 24, other: 11); 11 lung; 12 soft tissue, and 1 liver. Median PTV was 36 cc (R: 3-623cc). Concurrent therapy was given for 57 lesions (75%). The median time from the development of local recurrence/metastatic disease to SBRT was 13.7 months (range 0-17.7 months). The 6- and 12-month local control rates were 89% (95% CI 0.8-0.99) and 78% (CI 66%-92%), respectively. Eleven of 71 lesions (15.5%) failed locally. Type of local failure included in-field in 4, marginal in 3, and out of field in 4. Median SBRT dose for lesions that failed was 35 Gy over 5 fractions. Spine targets and PTV size were significantly associated with local failure on both UVA and MVA. Five patients developed acute grade 2 toxicities consisting of esophagitis, pneumonitis, pain, and two patients with dermatitis. No acute grade ≥3 toxicity was observed.

Conclusion: SBRT for metastatic osteosarcoma results in durable control. Larger PTV size and spine targets are associated with local failure in our series. Further study is warranted.