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e23533 Background: The accurate classification of sarcomas remains a major challenge due to the diversity and rarity of this group of mesenchymal tumors. In addition to standard microscopy and immunohistochemical techniques, molecular analyses are often required to achieve an appropriate diagnosis. The limited availability of specialized pathology centers, multidisciplinary teams, and diagnostic tools may directly affect the initial evaluation and subsequent treatment plan. Therefore, we sought to characterize the impact of diagnostic review by reference pathologists and molecular assessments in a reference center in Brazil. Methods: We conducted a retrospective analysis of sarcoma samples initially analyzed at outside laboratories and subsequently revised in a reference center by two dedicated pathologists between 01/2014-12/2020. After extracting demographic and tumor characteristics, we assessed the presence of complete discordance (benign versus malignant, sarcoma versus other malignancy), partial concordance (similar diagnosis of connective tumor, but different grade/histological subtype/differentiation), and complete concordance. The concordance for histology or grade, as well as the role of molecular assessments supporting the diagnosis, were also independently determined. Statistical analyses were conducted through the kappa coefficient of agreement, adherence by χ2 test, χ2 test by Person, and Fisher's exact test. Results: A total of 197 cases were included, with a predominance of male patients (57.9%), localized tumors (86.8%), and the median age at diagnosis of 46.3 years. The most common primary sites were lower extremities (34.6%) and retroperitoneum (11.3%). Following the revision, the most frequent histologies were undifferentiated pleomorphic sarcoma (20.3%), liposarcomas (13.1%), and leiomyosarcoma (6.6%). Complete discordance was present in 13.2%, partial discordance in 45.2%, and complete concordance in 41.6% of reviews (p < 0.001). We found a concordance for histology or grade of 53.5% (p < 0.001) and 51.8% (p < 0.001) respectively. Molecular assessments, comprising NGS panels (79.5%) and FISH (20.5%), were performed in 44 (22.3%) cases, with findings of diagnostic relevance in 31.8% of those. Conclusions: In almost 60% of the cases, the initial diagnosis was at least partially modified when revised in a reference center, with a relevant impact of molecular assessments. These findings highlight the importance of managing sarcoma patients in reference centers and justify the assembly of referral networks in countries with limited healthcare resources.

e12595 Background: Preoperative breast MRI can provide important clinical information on the surgical management of BC patients (pts). However, there is no evidence that it can produce a statistical impact on surgical outcomes in the early BC setting. This retrospective study sought to evaluate whether preoperative MRI was significantly correlated with CSur among early BC pts. Methods: Digital and physical records from 532 pts treated in a specialized BC center in Brazil from 2005 to 2018 were analyzed. The institutional database registered pathological, radiological and clinical information. The categorical variables CSur and MRI were analyzed by Chi-squared coefficient of correlation, whereas significant correlations were included in a multivariate logistic regression model. All statistical analysis were performed in STATA version 15.1C. Results: The present analysis included 532 pts. The diagnosis was suspected through suspicious self-exam in 380 pts (71%), while 129 pts (24%) had altered mammary screening tests. Clinical T and N stages were: T0-1=154pts (29%); T2=179 pts (33%); T3=135 pts (25%); T4=49 pts (9%); N0=267 pts (50%); N1=172 pts (32%); N2=80 pts (15%). Neoadjuvant treatment was performed in 223 pts (42%). MRI was indicated in 248 pts (46%). CSur was performed in 149 pts (28%). Margins were positive in 15 cases. This study found a statistically significant correlation between CSur and MRI ( X2 = 8.07; p=0.018). Although, when controlled for neoadjuvant treatment, T, N, and age, the independent variable MRI was not a statistical predictor of CSur (R=0.008; t=0.25; p=0.8; 95%CI -0.57 – 0.74). Advanced T and neoadjuvant treatment were inversely correlated with CSur in the multivariate analysis (R= -0.11; t= -7.8; p

1059Background: Several phase III trials of breast IORT have recently been published with findings that have generated significant controversy and discussion. As different techniques were used in these trials in a broad range of patients, it is unclear if the results obtained were due to differences in technique, type of radiation or patient selection. The goal of the current study was to review findings from studies using only electron IORT in appropriately selected patients considered suitable for APBI. Methods: We performed a literature search addressing the use of electron IORT as the sole radiation treatment for early stage breast cancer. Publications that delivered IORT prior to tumor excision, and those that did not deliver the prescription dose as deeply as the chest wall, were excluded. The ASTRO and ESTRO Consensus guidelines were used to select patients from these studies based upon society derived criteria for the application of APBI. ESTRO Good category differs from ASTRO Suitable in that it ...

e11560 Background: To report single-institution series of breast intra-operative radiotherapy (IORT) using nondedicated linear accelerator. Methods: Prospective phaseII trial opened in May 2004. El...