Retrospective analysis of intraoperative radiation therapy with low-kilovoltage x-rays (IORT-LK) in soft tissue sarcomas (STS).

e23575 Background: STS include a heterogeneous group of malignancies that arises from or into connective tissues and limbs are the preferential primary sites. An adequate local control with multimodality therapy can be obtained in most cases. An additional approach, that is, the IORT-LK is able to deliver high dose to risky areas for recurrence, minimizing the toxicity to structures around the target. Such an importance is even greater in abdominal sarcomas, because there is some difficulty in removing the whole neoplasia with clear margins. IORT-LK has the advantage of higher Relative Biological Effectiveness (RBE), a mathematical model that measures the potential for tumour double-strand break (radiation effectiveness), that is close to Proton beam and higher than electron beam. Nevertheless, IORT-LK is not free of complications. It has been reported peripheral neuropathy and enteritis, in cases of limb and abdominal STS, respectively. Methods: In this retrospective study from a single Brazilian institution (Hospital Alemão Oswaldo Cruz), we reported 6 patients with histologically confirmed STS, that underwent IORT-LK (Intrabeam - Carl Zeiss). There are 3 male and 3 female patients, with a median age 48 years, and mean of 39 years. The histological subtypes are as follows: fibromyxoid sarcoma, high grade pleomorphic sarcoma, desmoid tumor, chondrosarcoma, dedifferentiated liposarcoma and malignant schwannoma - one subtype for each patient. The primary sites are: inferior limb (3), superior limb (1) and abdomen (2). Results: IORT-LK was administered in doses that ranged from 14 to 15 Gy to the involved area. Two patients, each one with a limb STS, received also neoadjuvant chemotherapy. With a mean follow-up of 30 months, there is no local recurrence among the 6 patients reported. The first patient received the IORT-LK in April, 2017 and the last one in July, 2019. All of them had clinical observation up-to-dated on January, 2020. No serious complications were observed, but one patient had a moderate local infection. Conclusions: The therapy was well tolerated. We must emphasize that dose previously used by Brussiers was slightly superior than ours but with electrons and not Low-Kylovoltage. In his pilot study it was used 17 Gy for treatment of abdominal sarcomas. In sum, we believe that IORT-LK has higher RBE than previous available equipment and can improve local control of STS, mainly those that arise on the extremities. Future studies must be carried out to confirm our initial impression.