(P 207) SPATIALLY FRACTIONATED RADIATION THERAPY IN SARCOMAS: THE MAYO CLINIC EXPERIENCE

Objective: Local control for advanced/metastatic sarcomas with radiation therapy alone yields suboptimal results. Integration of spatially fractionated radiation therapy (SFRT) may enhance local control and symptom management for these patients. We describe outcomes of sarcoma patients treated with modern SFRT techniques.

Methods: Sarcoma patients treated with SFRT from 12/2019-5/2022 were identified from a prospectively maintained database. A total of 53 patients with 58 treatment courses were identified. Medical records were retrospectively reviewed for clinical characteristics, treatment plan parameters, toxicity, and outcomes under an IRB approved protocol. Descriptive statistics were used to collate patient and treatment characteristics as well as treatment response and toxicity. Patients with < 30 days of follow up from treatment start were not included in the treatment response and toxicity analyses.

Results: 53 patients with 58 treatment courses were included. Median age at treatment was 61 years (range:15-90). Both soft tissue and bone histologies were treated, with leiomyosarcoma the most common histology (n=11, 21%). Abdomen/pelvis soft tissue was the most frequently treated site (n=23, 40%). Treatment was focused on the primary disease site in most patients (n=38, 66%) and treatment intent was palliative in 47 (81%) treatment courses. Median GTV volume was 659 cc (range:36-10898). Most patients received an SFRT dose of 20 Gy (n=44, 79%, range:16-20) with a VMAT lattice technique (n=51, 93%). External beam radiation (EBRT) was delivered post-SFRT in 52 (90%) treatment courses with a median time interval from SFRT of 5 days (range: 0-14). Median EBRT dose and fractionation was 40 Gy (range: 9-73.5) and 5 fractions (range: 3-33), respectively. With an alpha/beta of 10, this equates to a median EQD2 dose of 46.7 Gy (range: 9.9-76.3) and BED of 56 Gy (range: 11.7-91.5). Median survival was 5.6 months (range:0.2-28.1), with 21 patients dead of disease (40%), 27 alive with disease (51%), and 5 alive with no evidence of disease (9%). No treatment related toxicities were documented in 34 treatment courses (59%). Five grade 3-4 acute and subacute toxicities were attributable to SFRT. Symptom relief was documented with 27 treatment courses (45%) and stable/partial imaging response noted for 36 treatment courses (62%). Figures 1 and 2 show representative treatment courses for 2 sarcoma patients treated with SFRT + EBRT with imaging responses and complications (if applicable).

Conclusion: SFRT is an innovative treatment strategy for advanced/metastatic sarcomas. Combined SFRT with EBRT is associated with a favorable toxicity profile and high rates of symptomatic and radiographic responses. Survival is generally poor for this population indicating a shorter course of EBRT is preferred. Future studies will need to better identify sarcoma patients most likely to benefit from SFRT±EBRT.