(P 343) STEREOTACTIC BODY RADIATION THERAPY (SBRT) IN ADVANCED/RECURRENT RETROPERITONEAL SARCOMA: PRELIMINARY RESULTS

Objective: The role of radiation in the treatment of retroperitoneal sarcomas (RPS) continues to be controversial. At our institution, we specifically reserve radiation therapy for cases involving borderline-resectable/unresectable primary or recurrent disease. The aim of this study is to evaluate the feasibility and toxicity of stereotactic body radiation therapy (SBRT) in this group of patients.

Methods: Between 1/2017 and 12/2021, 36 patients with nonmetastatic, primary borderline-resectable/unresectable (53%) or recurrent (47%) RPS received SBRT in this retrospective cohort study. Typical expansions from GTV to CTV was 5mm excluding adjacent bowel and other organs at risk, and typical expansions from CTV to PTV was 3mm.

Results: All 36 patients successfully completed the prescribed course of radiation therapy. Tumor size was larger than 10cm in 12 (37%) patients, and histology was dedifferentiated liposarcoma in 23 (64%) patients. The median PTV volume was 338.7 cm3 (range: 21.6-3439.6 cm3), to which a median dose of 30 Gy (range: 25-40 Gy) in 5 fractions was delivered. Nine patients received a simultaneous integrated boost (SIB) to a median volume of 111.7 cm3 (range: 17.2-412.1 cm3), to which a median dose of 40 Gy (range: 30-42Gy) was delivered. The intent of SBRT was preoperative in 64% of patients, and definitive (radiation alone) in 36% of patients.
The median follow-up was 14 months. No patient experienced acute grade 3-5 toxicities. Acute grade 1-2 fatigue was observed in 50% of patients, nausea 48%, vomiting 9%, diarrhea 6%, and radiation dermatitis 6%. The overall acute grade 1-2 gastrointestinal toxicity in those who received SIB was 33%, compared to 46% in those who did not (p=0.7). Overall, late complications were observed in 2 patients (6%). One patient who received SIB developed grade 2 persistent nausea and vomiting requiring Reglan. Another patient who did not receive SIB experienced grade 3 gastric bleed.
Of the patients treated with a preoperative intent, 14 out of 23 (61%) underwent complete gross resection. Of those 14 patients: 4 patients developed in-field local recurrence (1/8 primary, 3/6 recurrent), and 4 developed out-of-field recurrence (2/8 primary, 2/6 recurrent). The actuarial 2-year local control for those who completed preoperative SBRT followed by complete gross resection was 62%, and that for those who received SBRT alone was 48%.

Conclusion: This study demonstrates that SBRT administered up to 40 Gy in 5 fractions is feasible and well tolerated for patients with primary borderline-resectable/unresectable or recurrent RPS. All patients successfully completed the treatment without any acute grade 3-5 toxicities observed. Only 2 patients (6%) experienced late ≤ grade 3 toxicity. Despite the locally advanced nature of our cohort, in-field local recurrence was observed in only 1/8 patients who had primary borderline/unresectable disease. Further investigations regarding the role of SBRT in RPS, including potential dose escalation trials, are warranted.