(P 282) LOCAL FAILURE ANALYSIS OF SOFT TISSUE SARCOMA TREATED WITH PREOPERATIVE ULTRA-HYPOFRACTIONATED RADIATION: UPDATE

Objective: While normo-fractionation using 50Gy applied in 25 fractions represents the historically most used and best-known conventional preoperative RT regimen, ultra-hypofractionated RT (uhRT) with 5 fractions is not (yet) accepted as a 'standard', however is based on a fast-increasing literature body derived from clinical results. There are no prospective randomized trials yet comparing normo- vs uhRT. This analysis focused on local recurrence in a single center cohort.

Methods: Between 03.2020-12.2024, 77 consecutive STS pats have been pre-operatively irradiated with 5x 5.0Gy/1w uhRT. PTVs/GTVs were defined according to the RTOG 0630 trial. Bolus material on skin was placed in 59/77 (77%). Induction chemotherapy was given in 2/77 patients (3%). The indication for RT was determined in all patients @ the weekly Swiss Sarcoma Network (SSN; swiss-sarcoma.net) multi-center sarcoma conference. The cohort was characterized by unfavourable features in >40%, Tab. 1.

Results: After a mean/median FU time of 31/29.5 months (2-62), 5/77 (6%) local recurrences were diagnosed 4/5/6/8 and 28 m. post treatment,
Tab. 2, Fig. 1. 6/77 patients presented with synchronous distant metastases. 2y LC/OS/DFS and distant metastasis free survival were 94%/88%/61% and 68%, respectively. Early RT tolerance was excellent, allowing for timely surgery, Tab. 3. R0-resection was achieved in 92%.

Conclusion: We found a 2y local/final local control rate of 94%/96% in our single institution STS cohort treated with 25 Gy / 5f / 1w uhRT with omission of systemic therapy. The results are well comparable to published reports on results following normofractionated RT +/- systemic treatment. Treatment tolerance was excellent.

Table 1. Unfavourable features of the cohort