Simple SummarySoft tissue sarcomas are rare malignant tumours that originate in the mesenchyme. Over 80 different entities have been identified to date. With an incidence of 4.7 and a median age of onset of 59 years, they account for about 1% of all malignant tumours in adults and 15% in children. The treatment regimen includes surgical resection, radiotherapy and, on a case-by-case basis, chemotherapy, which can achieve local control rates of about 90% for extremities and 50–80% for retroperitoneal sarcomas. In addition to external beam radiation, the use of intraoperative brachytherapy has for several decades offered the possibility of treating the exposed tumour bed directly with a single fraction of high-dose radiation, increasing its effectiveness while reducing toxicity to surrounding healthy tissue. This study investigated the rate of local recurrence in 35 individuals with soft tissue sarcomas of the extremity or retroperitoneum treated at the University Hospital for Orthopaedics Innsbruck from 2010–2016.AbstractIntroduction: According to the literature only sparse data are available on the use of high-dose-rate intraoperative brachytherapy (IOHDR-BT) as a boost to external-beam irradiation (EBRT) in combination with a wide resection in patients with high-grade soft tissue sarcomas (STS). Materials and Methods: Applying a retrospective study design, we investigated all patients who between 2010 and 2016 underwent marginal resection of a high-grade STS and intraoperative radiotherapy, followed by EBRT. We included only patients with a traceable follow-up time of at least two years. Of 89 patients, 35 met our inclusion criteria and showed an average follow-up of four years. Results: We found an overall 2-year local control rate of 94.3%. The local recurrence rate for R0 resections was 6%, whereas recurrences occurred in 13% of R1 resections and in 100% of R2 resections. One affected patient received only intraoperative radiotherapy. The recurrence rate by tumour entity was 36% for LPS, 11% for myxofibrosarcoma and 17% for undifferentiated pleomorphic sarcoma. Conclusion: The treatment regimen consisting of limb-preserving surgery, IORT and pre- or postoperative radiotherapy consistently shows excellent local control rates.