2946 - Spatially Fractionated Radiotherapy (SFRT) for Large Tumors: A Single Institutional Experience of Dosimetry and Clinical Outcomes

K. Amarell¹, B. Guo², Y. B. Cho³, A. Ramalingam⁴, K. L. Stephans⁵, J. H. Suh², J. G. Scott⁵, and S. R. Campbell⁶; ¹Cleveland Clinic Foundation, Cleveland, OH, ²Department of Radiation Oncology, Cleveland Clinic Foundation, Cleveland, OH, ³Department of Radiation Oncology, Taussig Cancer Institute, Cleveland Clinic, Cleveland, OH, ⁴Akron General Medical Center, Akron, OH, United States, ⁵Department of Radiation Oncology, Cleveland Clinic, Cleveland, OH, ⁶Cleveland Clinic, Cleveland, OH

Purpose/Objective(s): SFRT is theorized to enhance the efficacy of radiotherapy by delivering ablative doses to create vascular damage, provoke inflammatory cytokines, and activate immune cells, all enhancing cell kill. We report our single institution experience utilizing SFRT in various cancer types, notably sarcomas. Materials/

Methods: An IRB approved database of external beam radiation patients was queried for those treated with SFRT with a minimum dose of 15 Gy. Records were evaluated for acute grade 3+ toxicity at least possibly related to SFRT within 90 days.
Results: From 2019-2023, 18 patients were planned with SFRT with 13 receiving SFRT. For treated patients, median follow up was 2.8 months (IQR 1.4 – 7.5) with 4 patients alive at last follow up with median follow up of 10 months (IQR 6.1 – 13.3). Of expired patients, median time to death from SFRT was 1.5 months (0.7 – 16.9). Sarcoma was the most common tumor type (N=12, 66.7%) and others included lung, GI, and GU. Treatment locations included thorax (4), abdomen (3), pelvis (4), and extremity (7). SFRT was planned with 2D GRID (6) and VMAT LATTICE (12). Of LATTICE, 16 were single fraction and 2 were 66.7 Gy SIB over 5 fractions with 20 Gy to the entirety of the tumor. In single fraction SFRT, the dose was 20 Gy (1), 18 Gy (9) and 15-16 Gy (6). For sequential patients, radiation was started on the subsequent day. Subsequent radiation plans included 40-50 Gy in 5 fx (4), 20 Gy in 5 fx (8), 30 Gy in 10 fx (2), 16 Gy in 4 fx (1), 57.5 Gy in 25 fx (1), and 37.5 Gy in 15 fx (1). GRID was delivered via single en face field or parallel opposed fields, 50% each; LATTICE was delivered with VMAT using 3-6 coplanar arcs with 6 or 10 MV FFF beam. GTV of the entire target was median 1063 cc (66.9 – 4485.79 cc) and median target volume for LATTICE spheres was 33.8 cc (3.7 – 85.5 cc) with median 17.5 (4 – 34) spheres per plan. There was favorable sparing of organs at risk, with median maximum dose to 0.03 cc of organs at risk of 3.89 Gy (0.269 – 8.508 Gy). Of the 13 patients who completed therapy, 4 were alive at last follow up. 8 patients had follow-up imaging available with median follow up of 6.9 months. 2 (25%) patients had progression of irradiated site with a mean time to progression of 12.9 months, 6 (75 %) patients had stable local disease on most recent imaging, and 5 patients were not assessed for progression (expired). In terms of Grade 3+ acute toxicity following SFRT, 3 patients (23.1 %) were hospitalized for uncontrolled pain or failure to thrive related to SFRT or tumor burden.
Conclusion: SFRT is a reasonable treatment option for palliation/definitive management of unresectable locally advanced tumors. Since patients are of limited life expectancy, it is common for early death prior to or after SFRT, so caution must be utilized and careful counseling of patients is paramount. For those who do not decompensate around the time of treatment, SFRT may aid in improved local control, as seen in this series.