Introduction: Inflammatory breast cancer (IBC) is a rare (<1%) and aggressive subgroup of breast cancer (BC), presenting in relatively young patients. Rapid onset of signs and symptoms, challenging diagnosis and delayed treatment contribute to poor outcome. IBC itself is an independent adverse prognostic factor, but IBC-specific targets for therapy are not available. The prospective Dutch INFLAME registry was set up to improve (molecular) IBC insights, increase awareness, improve diagnostic and treatment protocols in national guidelines, ultimately to improve outcome in IBC. Here, we report baseline clinicopathological- and treatment data from INFLAME. Methods: INFLAME was set up through the Dutch breast oncology research network BOOG. Access to the INFLAME PI team, in light of diagnostic challenges in IBC, was ensured by a study email address and phone number. Patients identified with IBC were included locally in participating hospitals. At baseline, clinical data, blood and if available tumor tissue for molecular analysis was collected. Tissue was also sampled at surgery after preoperative systemic therapy, if applicable. Follow up clinical data were collected after 6 months, 1, 2, and 3 years. Results: From Feb 2015 to Dec 2018, 125 IBC patients were enrolled in 43 hospitals, with a good geographic spread throughout the Netherlands. Patients from non-participating hospitals were referred to participating hospitals. Baseline clinical data was available for 122 patients. Median age was 56.7 years. Median interval between symptoms and diagnosis was 34 days, and 84 (68.9%) patients were diagnosed within three months. Among patients with known M status (n=112), 37 (30.3%) had metastatic disease of which 21 (56.8%) had visceral disease. Of 74 patients without distant disease and N status known, 59 (78.7%) had node positive disease. Baseline tissue was available for pathology assessment in 122 patients. Tumors expressed ER, PR or HER2 in 63, 47 and 41 of cases (51.6, 38.5 and 33.6% respectively). Intrinsic subtype distribution showed hormone receptor (HR)+/HER-, HR+/HER2+, HR-/HER2+ or HR-/HER2- subtype in 47, 17, 24 and 31 of cases (38.5, 13.9, 19.7 and 25.4% respectively). Subtype was unknown in 3 cases (2.5%). The vast majority of patients (n=103, 84.4%) received (anthracyclin and taxane based) chemotherapy. Median interval between diagnosis and start was 19 days and 32 (26,2%) patients started within 2 weeks. HER2. targeting was started in 38 patients (92.7%) with HER2+ IBC. Chemotherapy was followed by surgery in 72 patients, 11 of whom had M+ disease. 41 patients were not operated upon, mostly because of progression. In 70 patients, surgery was followed by locoregional radiotherapy. Endocrine therapy was started in 62 (98.4%) patients with ER+ IBC. Awareness was supported with information material, patient advocate involvement, (inter)national presentations, papers for general practitioners, and the first Dutch IBC guideline. Ongoing consultations of the INFLAME team (by patients, general practitioners, and oncology professionals) reflect this awareness. Conclusions: the INFLAME effort shows feasibility to raise awareness and set up a nationwide prospective registry in a very rare disease such as IBC, despite diagnostic challenges. The registry was accessible, with good Dutch coverage of participating hospitals, referrals, BOOG support, consultations, all supporting inclusion of most IBC patients. Nonetheless, the incidence was very low: <0.25% of the yearly Dutch breast cancer incidence. Further assessments of the INFLAME cohort will focus on molecular tissue assessment, and outcome. The extreme rarity of the disease and its dismal outcome support future joint international efforts for IBC specific intervention studies. Funding: Dutch Cancer Society grant RUG 2013-6267

Citation Format: Jasper JL van Geel, Gonneke van der Schoor, Elise van Leeuwen-Stok, John WM Martens, Saskia M Wilting, Jelle Wesseling, Gabe S Sonke, Carolien P Schröder. Clinicopathological characterization of inflammatory breast cancer in the Netherlands: First results of the prospective INFLAME registry [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-24-03.