191P Automated contouring (AC) of cardiac substructures using AI-based algorithms on whole breast radiation therapy (RT) CT-scans for NRG oncology RTOG 1005 (R1005)

Background: Breast-conserving surgery (BCS) followed by radiotherapy remains the standard treatment for early-stage breast cancer, achieving oncologic outcomes equivalent to mastectomy while providing superior cosmetic results and quality of life (QOL).Complete tumor excision with negative margins is essential, as margin positivity represents the strongest predictor of local recurrence.The present prospective, interventional, multicenter study assesses specimen PET-CT imaging in the largest cohort reported to date.In addition to evaluating diagnostic performance, the study compares specimen PET-CT with each participating institution's standard-ofcare intraoperative margin assessment (SOC-IMA) techniques and examines interobserver agreement and safety. Methods:The study was conducted at six centers in Belgium, Germany, and Italy and included women aged 18 years undergoing BCS for various breast cancer subtypes, including invasive ductal carcinoma (IDC), invasive lobular carcinoma (ILC), and ductal carcinoma in situ (DCIS).Patients received a low-dose 18F-FDG injection prior to surgery.Surgeons assessed images in real time and performed additional resections when margins were suspected to be positive.Final histopathology served as the reference standard.The primary endpoint was the intraoperative success of specimen PET-CT in addressing positive margins in IDC.Results: Site-specific results from UZ Gent are reported for a cohort of 58 included patients.The overall positive margin rate in the total study population was 32.7% (19/58).The intraoperative success rate was 67.2% (39/58) when no intraoperative margin assessment (IMA) was performed and 74.1% (43/58) with standard-of-care IMA (SoC IMA).In contrast, specimen PET-CT achieved a markedly higher success rate of 93.1% (54/58).Only 2 patients underwenst a second surgery due to positive surgical margins in main specimen.Conclusions: Specimen PET-CT imaging improved the assessment of positive margins for invasive component in patients undergoing conserving surgery for early breast cancer in our hospital.The use of this integrated approach might lead to a substantial reduction of re-excision rates after breast conserving surgery.