Y. Bai, X. Gao, S. Qin, S. Li, M. Ma, S. Liu, and Y. Gao; Department of Radiation Oncology, Peking University First Hospital, Beijing, China
Purpose/Objective(s): The use of definitive radiotherapy in cases of unresectable soft-tissue sarcomas (STS) is a topic of debate. Additionally, the prognosis of large (= 5 cm) unresectable STS is much worse. This study aimed to evaluate the clinical outcomes of partial stereotactic ablative radiotherapy boost before conventional radiotherapy (P-SABR) for large unresectable STS. Materials/
Methods: Between 2012 and 2023, 28 patients with 33 sites of unresectable STSs who underwent P-SABR radiotherapy were retrospectively analyzed in this study. All the patients had locally advanced, recurrent, or metastatic stages with a tumor diameter of = 5 cm. Locations of the tumor were 16 (48.5%) in the retroperitoneal and abdomen, 5 (15.2%) in the extremities, 6 (18.2%) in the chest, 3 (9.1%) in the head and neck, and 3 (9.1%) in the truncal wall. The histologic grades of the tumors were 24.2% G1 and 75.8% G2 to G3. The median size of the tumor at P-SABR radiotherapy was 11.6 cm (range, 5.1- 27.1 cm). The P-SABR plan was combined with a partial SABR boost plan followed by a conventional radiotherapy (CFRT) part. In the SABR part, the prescription dose for PTV was 1.8 to 3 Gy per fraction over 3 to 5 fractions, and the artificially delineating GTV-boost (GTVb) within GTV received a simultaneously integrated SABR dose (6 to 12 Gy per fraction). In the following CFTR part, the median prescription dose for the entire PTV was 45 Gy (34 to 62 Gy) in 22 fractions (15 to 31 fractions). For the synthetic P-SABR plan, the median cumulative dose of PTV and GTVb was 55.2 Gy (range 40- 71.3 Gy) and 74 Gy (range 58- 98.1 Gy), respectively. Out of the 28 patients, 7 (21.2%) were treated concurrently with chemotherapy, 11 (33.3%) were concurrently treated with immunotherapy (PD-L1 or PD-L1 antibody), and 15 (45.5%) patients were treated with radiotherapy alone. Results: The median follow-up for patients was 30 months (with a range of 4.8 to 55.2 months). The overall survival rates at 1, 2, and 3 years were 87.1%, 42.8%, and 30.6%, respectively. The local control rates at 1, 2, and 3 years were 100%, 92.3%, and 92.3%, respectively. In addition, four cases became operable after radiotherapy, and the histologic assessment after neoadjuvant radiotherapy showed complete response (CR) for 3 cases and stable disease (SD) for 1 case. However, the out-of-field disease progression rates at 1, 2, and 3 years were 62.9%, 75.3%, and 81.5%, respectively. Only one (3.0%) patient experienced grade 3 leucopenia, and no other toxicity of grade 3 or higher was reported. One (3.0%) patient experienced late grade 2 pain. Conclusion: In conclusion, the high local control rate and well-tolerated toxicity of P-SABR (partial stereotactic ablative radiotherapy boost before conventional radiotherapy) in large unresectable soft-tissue sarcomas is encouraging. However, there is a need to explore combination therapy with P-SABR to decrease the out-of-field disease progression.
