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11571 Background: Despite surgically resectable pulmonary metastases may lead to cure patients with B-STS (Chudgar NP 2017), a substantial proportion of patients will eventually relapse. Presently, patient selection is based on unique organ involvement, number of metastases, interval between previous surgery and pulmonary progression or relapse. We assessed the impact of anatomical site of metastasis into the lung (as if the pleural site might ease further tumor spreading) and nodule growth rate as additional predictive/prognostic factors of lung progression-free survival (L-PFS) and overall survival (OS). Methods: In our prospectively collected database, we retrospectively evaluated patients operated for B-STS pulmonary progression at 3 different centers from 2005 to 2019. Beyond patients’ clinical features at both baseline and disease progression in the lungs, we focused on whether the relapse occurred into the parenchyma or nearby the pleura (Welter S 2012); secondly, we estimated lung metastasis growth rate, defined as tumor doubling time (TDT) (Nakamura T 2011). Statistical analyses were carried out with IBM SPSS (v. 20.0). Survival outcomes were estimated by Kaplan-Meier method. Hazard ratios (HR) were estimated by Cox regression. Multivariate analysis was performed for both L-PFS and OS according to Cox proportional hazard model. All tests were 2-sided with their corresponding 95% confidence intervals (CI95%). Results: We identified 138 patients who underwent lung metastasectomy [(F=66 (48%); median age at surgery 50 (14-78)]. Median PFS and L-PFS were 8.7 months (CI95% 6.6-10.9) and 8.6 months (CI95% 6.2-11.0), respectively. Median OS was 40.6 months (CI 95% 32.8-48.5). Univariate analysis showed a statistically significant impact of the following variables for both L-PFS and OS: ECOG 0, nodule number 40 days. Disease-free interval ≤ 24 months and absence of metastases at diagnosis showed significant correlation with L-PFS and OS, respectively. At multivariate analyses the following variables retained statistical significance for L-PFS: TDT >40 days (HR 0.53, CI95% 0.31-0.93, p=0.028); nodule number 40 days (HR 0.36, CI95% 0.18-0.72, p=0.004), nodule number

e23546 Background: Although sarcomas may arise in any site of the body, the intrinsic challenge of BS is to affect the female most frequent cancer site with a completely different and rare set of heterogeneous tumors. Unfortunately, we lack BS-specific prospective trials to precisely guide the clinical decision-making and BS patients are often sent to sarcoma referral centers only after surgery without upfront BS MTB evaluation. Recent evidences reported a significant prognosis improvement with upfront MTB evaluation in retroperitoneal or extremity sarcomas, but whether this strategy might benefit also BS is still not defined. We retrospectively evaluated our prospectively collected database to test whether an upfront MTB evaluation at BS referral centers might impact prognosis and/or the need of subsequent surgeries. Methods: Detailed information on primary surgery, diagnosis, and ≥2 years of follow-up for non-recurring patients were required. We tested the following outcomes: event-free survival (EFS), local EFS (LEFS), distant metastases EFS (DMEFS), overall survival (OS), margin status, subsequent surgeries rate. We evaluated the different outcomes according to upfront MTB evaluation. Kaplan-Meier method was used for survival analyses. Comparisons used hazard ratios (HR). Results: We included 93 patients treated between 1/2000 and 10/2019 at 2 reference BS Centers, median age 53 years (IQR 41-70), ECOG (0;1;2) 67:16:10, high/low grade 60:33. Histotypes were: malignant phyllodes tumor 42 (45%), angiosarcoma 26 (28%), other STS 25 (27%). 27 (29%) cases were considered radiation-induced with a median time from RT of 105 months (IQR 70-127). 44 patients (47%) were discussed upfront in BS MTB, whereas 49 (53%) were not. With a median follow up of 43 months, 5-year EFS, LEFS, DMEFS, and OS for the whole population were 54%, 64%, 60%, and 62%, respectively. LEFS at 5 years had a strong association with margin status (R0 74%; R1 55%, R2 0%, p < 0.001). Regardless of histology, upfront MTB was associated with a significant lower rate of subsequent surgeries (OR 0.16, 0.06-0.41), and with better EFS, LEFS, DMEFS, and OS (HRs 0.57, 0.54, 0.63, and 0.43, respectively). Conclusions: Though retrospective, this series is one of the largest reported to date in BS. Our results highlight the need for early inter-tumor board sharing of BS to take into account sarcoma specificity in the overall disease planning. Given BS rarity, these data will hardly ever be verified in a prospective fashion. Therefore, validation of these results in the Italian Sarcoma Group database is planned.

e22508 Background: Advanced OS still represents an unmet medical need. Chemotherapy and targeted therapies have only limited activity in this disease. Moreover, OS is considered a radioresistant tumor at doses conventionally achieved by EBRT. We retrospectively evaluated the impact of palliative EBRT in patients (pts) with advanced OS. Methods: Main inclusion criteria: confirmed histological diagnosis of OS; EBRT for advanced/metastatic disease deemed unresectable after multidisciplinary tumor board discussion; ECOG performance status ≤3, age ≥18 years at RT start; progressive disease at site of irradiation with symptoms (e.g., pain or neurological deficit) or involving critical structures (e.g., superior vena cava compression). Endpoints: progression-free survival (PFS), PFS for RT-treated lesions (RT-PFS), best response with RT, overall survival. All survival endpoints were computed from RT start. We calculated the equivalent total dose in 2 Gy fractions (EQD2). Results: 23 pts, 15 males, median age at diagnosis 33 (17-82), median age at RT start 36 (20-83) were included. All pts had grade 3-4 OS (12 osteoblastic; 6 chondroblastic; 5 other), 7 were metastatic at diagnosis. At time of RT, median metastatic sites were 2 (1-5), 15 pts were symptomatic (11 pain, 4 neurological deficit) and were mainly heavily pretreated [median previous chemotherapies 2 (1-5), median previous surgery 1 (0-2)]. Overall, 45 lesions underwent EBRT. Median EBRT dose per lesion was 33 Gy (IQR 25 - 46.5) with a median dose per fraction of 3 Gy (IQR 3-7.5). Calculated median EQD2 was 50 Gy (IQR 36 - 88). Median PFS, RT-PFS, and overall survival were 5 (95%CI 2-9), 11 (5-15), and 12 months (8-16), respectively. Compared to systemic progression, median progression delay of irradiated lesions was 6 months (95%CI 2 - 12). In 40 lesions evaluable for dimensional response, we observed tumor shrinkage > 30% in 7 lesions, stability in 24, and progression in 9. Conclusions: EBRT might have a role in the treatment of advanced OS. In consideration of the limited activity of medical treatments, EBRT might help in disease control in the advanced setting, delaying progression especially at critical sites.