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ESTRO 37

radiotherapy was analysed: indication for SBRT/SRS, site, number of lesions, number of recurrences before SBRT/SRS, previous irradiation in-field, total dose (TD), dose per fraction (DF), method of dose prescription, treatment technique, volumes of GTVs and PTVs, early and late toxicities, best obtained SBRT/SRS result, in- field and field border progression. Additional parameters included, tumour grade, date of primary diagnosis, primary tumour site, systemic therapy received before and after SBRT/SRS, and date of overall disease progression, if occurred. The Kaplan-Meier estimator was used to calculate progression-free survival (PFS). Results Totally n=31 patients who underwent 1-3 SBRT/SRSs on 1- 5 target lesions were included. The indications for irradiation were: oligometastatic disease (59.5%), oligoprogression (21.6%), primary definitive treatment (5.4%), recurrence definitive treatment (10.8%), and adjuvant setting (2.7%). The irratiated sites were: lungs (43.2%), head&neck (21.6%), bones (16.2%), central nervous system (8.1%), liver (5.4%), lymph nodes (2.7%), and soft tissues (2.7%). The dose was delivered by linacs (IMRT: 21.6%, VMAT: 54.1%), CyberKnife (21.6%) and GammaKnife (2.7%). DF varied from 4 to 18 Gy, and TD from 8 to 60 Gy. GTVs and PTVs were between 0.5-110.94 cm 3 (mean GTV 17.16 cm 3 ) and 3.29-138.1 cm 3 (mean PTV 38.64 cm 3 ), respectively. SBRT/SRS allowed to obtain complete response in 18.9%, partial response in 10.8%, stable disease in 62.1% and progressive disease in 8.1%. No acute or late toxicity grade 3 or above was observed. In-field progression (at any time) was identified in 18.9% of patients. Median PFS was 9.4 months (95% CI: 2.7- 16.0).

resection. Postoperative radiotherapy (PORT) had been carried out to improve local control, but nowadays it has been reluctant to use due to its higher complication risk. So far, most of the studies investigating PORT are based on the old RT technique. We investigated the efficacy of PORT using modern RT technique in patients treated with retroperitoneal soft tissue sarcoma. Material and Methods A total of 80 patients who underwent surgical resection for non-metastatic primary retroperitoneal soft tissue sarcoma at Yonsei Cancer Center between 1993 and 2015 were retrospectively reviewed. The most common histologic subtype was liposarcoma (52.5%) followed by leiomyosarcoma (22.5%), undifferentiated pleomorphic sarcoma (11.2%) and others (13.8%). Resection margin status was R0 in 45 patients (56.2%), R1 in 24 patients (30%), and R2 in 11 patients (13.8%). PORT was delivered in 38 patients (47.5%) with a median dose of 54 Gy in 1.8 Gy per fraction using 3-dimensional RT or intensity modulated RT. Adjuvant chemotherapy was performed in 23 patients (28.8%) with a regimen of adriamycin/ifosfamide. The effect of PORT on local failure-free survival (LFFS), overall survival (OS) as well as RT-related toxicities was investigated. Results The median follow-up was 37.1 months (range, 5.8-207.9 months) for entire patients. The median LFFS and OS were 56.3 and 146.9 months for all patients, respectively. Treatment failure was occurred in 47/80 patients (58.8%); local recurrence in 33 patients (41.3%), distant metastasis in 8 patients (10%), and both in 6 patients (7.5%). While PORT was performed more frequently in patients who underwent R1 or R2 resection (P = 0.039), there were no significant differences in other clinical characteristics between PORT group and no-PORT group. The LFFS was significantly better in PORT group; the 5-year LFFS rate was 74.2% for PORT group and 24.3% for no-PORT group (P < 0.001). In multivariate analysis, the use of PORT were the only independent prognostic factor for LFFS (P=0.002). However, there was no significant correlation between RT dose and the LFFS. OS showed no significant difference between two groups. Grade ≤ 2 acute toxicities were observed in 63% of patients, but no acute toxicity of grade > 3 was observed. Only 1 patient who received 65 Gy developed a neuropathy as a late complication. Conclusion PORT markedly reduced a local recurrence in retroperitoneal sarcoma patients. Using modern RT technique, there was no severe RT-related complication. Optimal RT technique in terms of RT dose and target volume should be further investigated.

Fig. 1. Progression free-survival of sarcoma patients after SBRT/SRS

EP-1632 Clinical Outcome after Multimodality Approach including Radiotherapy in Rhabdomyosarcoma J.J.B. Lee 1 , S.H. Choi 1 , H.I. Yoon 1 , C.O. Suh 1 1 Yonsei Cancer Center, Deparment of Radiation Oncology, Seoul, Korea Republic of

Conclusion SBRT/SRS allows to obtain high LC with excellent treatment tolerance in patients with sarcomas. This treatment modality may be considered as an alternative to surgery in selected clinical situations, especially in oligometastatic and oligoprogressive disease. EP-1631 Efficacy of Postoperative Radiotherapy (RT) using Modern RT technique in Retroperitoneal Sarcoma H.J. Kim 1 , W.S. Koom 1 , J. Cho 1 , C.O. Suh 1 1 Yonsei University, Radiation Oncology, SEOUL, Korea Republic of Purpose or Objective Local recurrence is the most common cause of failure in retroperitoneal soft tissue sarcoma patients after surgical

Purpose or Objective To report Yonsei Cancer Center’s experience in treating rhabdomyosarcoma and evaluate treatment outcome and prognostic factors in subgroups. Material and Methods Sixty-two patients who received multimodal treatment including radiotherapy at the Yonsei Cancer Center from March, 2000 to November, 2015 were included. Clinical and treatment characteristics, pathologic features