174 - Stereotactic Body Radiation Therapy (SBRT) for Metastatic Osteosarcoma: Local Control Outcomes with Local Failure Dosimetric Analysis

J. Kocsis¹, M. Dylong², A. Patsko², C. A. Reddy³, E. Somasundaram⁴, T. Smile³, J. G. Scott⁵, C. S. Shah⁶, L. Angelov⁷, P. Anderson⁸, S. Zahler², M. Trucco², S. Thomas², S. Johnson⁶, P. Qi⁶, A. Magnelli⁶, S. R. Campbell⁹, and E. S. Murphy⁶; ¹Cleveland Clinic, Cleveland, OH, United States, ²Cleveland Clinic Foundation, Cleveland, OH, ³Department of Radiation Oncology, Taussig Cancer Institute, Cleveland Clinic, Cleveland, OH, ⁴Massachusetts General Hospital, Boston, MA, ⁵Department of Radiation Oncology, Cleveland Clinic, Cleveland, OH, ⁶Department of Radiation Oncology, Cleveland Clinic Foundation, Cleveland, OH, ⁷Department of Neurological Surgery, Cleveland Clinic Foundation, Cleveland, OH, ⁸Department of Hematology Oncoloigy, Cleveland Clinic, Cleveland, OH, ⁹Cleveland Clinic, Cleveland, OH

Purpose/Objective(s): Osteosarcoma (OST) is a radioresistant tumor that may benefit from SBRT for locoregional control. SBRT is increasingly used for patients (pts) with sarcoma, but outcomes continue to be underreported, especially in the pediatric population. We reviewed our practice patterns, outcomes, toxicity, and patterns of failure when treating OST pts with SBRT. Materials/

Methods: We queried an IRB approved registry of pts treated with SBRT for recurrent or metastatic OST between 2015-2023. Pts were assessed for local control (LC), overall survival (OS), and toxicity. Rates of LF were calculated using cumulative incidence analysis with death treated as a competing event, and rates of OS were calculated using actuarial analysis. Competing risk regression was used to identify factors associated with LF. Patterns of LF were assessed dosimetrically. In-field failures (IFF) were identified when =80% of the recurrent tumor volume was within the 95% prescription isodose line (IDL), marginal (MF) when 20%-80% was within the 95% IDL, or out-of-field (OOF) when <20% was within the 95% IDL.
Results: 27 pts met eligibility and a total of 71 lesions were treated. 6 pts (22.2%) were alive at time of analysis. Median follow-up time was 5.9 months (mo) (range 0.6-57.9 mo). Median age was 18.1 years (range 9.0-42.3). 47 (66.2%) lesions were osseous, 12 (16.9%) parenchymal, and 12 (16.9%) soft tissue. Of all 47 osseous lesions, 24 (51.1%) were in the spine, 12 (25.5%) in an extremity, and 11 (23.4%) in the pelvis. Of the 24 spine lesions, 10 (41.7%) had epidural disease. The median dose was 40 Gy in 5 fractions (fx) (range 16-60 Gy in 1-5 fx). More common fractionations included: 40 Gy in 5 fx (45.1%) and 21 Gy in 3 fx (12.7%). 53 lesions (74.6%) were treated with concurrent therapies that included chemotherapy (CHT) (60.4%), targeted agents (34.0%), and radionucleotides (26.4%). There was a total of 11 LF (15.5%). The 6- and 12-mo cumulative incidence of LF were 10.3% (95% CI 4.5-18.9) and 13.6% (95% CI 6.6-23.2) respectively. On univariate analysis, SBRT to the spine and PTV size was predictive of LF and remained so on multivariate analysis: spine (HR 3.39, 95% CI 1.03-11.14); PTV size (HR: 1.003, 95% CI 1.001-1.006). Of the 11 LF, there was 4 IFF, 3 MF, and 4 OOF. There were 7 spine LF, and 5 (71.4%) of them failed with epidural disease. The 6- and 12- mo rate of OS for individual pts from first treatment was 87.5% (95% CI 66.1-95.8) and 64.1% (95% CI 40.6-80.3). Grade 2 acute toxicity occurred in 4 (5.6%) cases which included esophagitis, pneumonitis, pain, and dermatitis. No acute grade=3 toxicity. One pt had a late toxicity requiring sacroplasty 9 mo after SBRT.
Conclusion: Our OST SBRT series demonstrates that SBRT for recurrent and metastatic disease is safe and effective with a 1-year cumulative incidence of LF of 13.6%. LF’s were evenly distributed amongst IFF, MF, and OOF. In our series, SBRT to the spine had the highest risk of local recurrence. This LF analysis suggests the need to modify strategy for tumors adjacent to critical structures.