(P 280) PERSONALIZED ULTRA-FRACTIONATED STEREOTACTIC ADAPTIVE RADIOTHERAPY (PULSAR) FOR ADULT AND PEDIATRIC SARCOMAS

Objective: Personalized Ultra-fractionated Stereotactic Adaptive Radiation Therapy (PULSAR) is a novel treatment which delivers high-dose pulses of radiation at extended intervals to allow treatment adaptation and optimize synergy with concurrent immuno- and other systemic therapies. Our institution has implemented PULSAR in a variety of clinical situations including oligo- metastatic or progressive disease and unresectable primaries in both pediatric and adult sarcoma patients. Here we report our initial clinical outcomes after PULSAR in sarcoma patients with a hypothesis that PULSAR provides excellent local tumor control with minimal toxicities.

Methods: This IRB-approved retrospective study evaluated pediatric and adult patients with primary or metastatic sarcomas treated with PULSAR at UT Southwestern from 2022-2025. We collected patient demographics, tumor characteristics, treatment details, toxicities, and clinical outcomes including local tumor control (LC), progression-free survival (PFS), and overall survival (OS). Kaplan-Meier analysis was used for survival outcomes.

Results: Among 58 irradiated sites in 31 patients, 38 sites in 20 patients had adequate follow-up imaging (median 8 months). 76% of irradiated sites were metastatic, while 24% were primary tumors. The most common histologies in adults were leiomyosarcoma and undifferentiated pleomorphic sarcomas, while in pediatrics were osteosarcoma, Ewing sarcoma, and rhabdomyosarcoma. Concurrent systemic therapy was administered in 92% of cases: immunotherapy (45%), chemotherapy (32%), targeted therapy (16%). The most common radiation dose 8 Gy/pulse given every 3 weeks x 5 total (cumulative 40 Gy). Overall, LC of irradiated sites was 90%. At 6-month follow-up, metastatic sites showed 90% LC, 71% distant control, and 65% PFS, while primary sites showed 88% LC, 63% distant control, and 56% PFS. Treatment-related toxicity occurred in 35% of patients, with radiation pneumonitis most common (20%) with all cases occurring with concurrent immunotherapy.

Conclusion: In primary and metastatic sarcomas, PULSAR offers excellent local control with limited toxicity despite high rates of concurrent systemic therapy integration (92%). These promising findings warrant further prospective study.