Abstract P3-05-07: Primary neuroendocrine carcinoma of the breast: A case series by WHO classfication in 2019
Authors: Jeon Young Kyung , Kim Ji-Yeon , Ahn Jin Seok , Im Young Hyunk , Choi Kyue-Hee , Jeong Sun Young , Jung Yeji , Jang Jae Yeon , Choi Daeho , Hong Joohyun , Kim Hyo Jung , Cho Soo Youn , Park Yeon H. 
Published: March 2023

Introduction Neuroendocrine tumours (NET) are thought to arise from cells throughout the diffuse endocrine system and can occur almost everywhere in the body. Most NETs arise from the gastrointestinal tract, lung, thymus, and pancreas. Primary neuroendocrine carcinoma (NEC) of the breast is a rare and under-recognized subtype, accounting for less than 1% of breast carcinomas. Only a few small studies and case reports have been reported and there are no clear diagnostic criteria and established treatment options. World Health Organization (WHO) classification of tumor series’ fifth edition was published in 2019 and adopted ‘Neuroendocrine neoplasm (NEN)’ as a term encompassing all tumour classes with predominant neuroendocrine differentiation. NENs of the breast are classified into invasive ductal carcinoma (IDC) with neuroendocrine differentiation (NED), NET, and NEC of small cell or large cell types. Thus we aim to report pathologic reviews and treatment outcomes of patients with NENs of the breast at a single center. Methods We retrospectively screened the medical record of 34,370 patients diagnosed with breast cancer from 2007 to 2022 by Corporate Data Warehouse (CDW) and revealed there were 22 patients diagnosed with primary breast NEN. The pathologist reviewed the pathology slides and reclassified the diagnosis according to the WHO classification of tumor series’ fifth edition. Clinical characteristics, treatment modalities, and therapeutic outcomes were reviewed retrospectively. Results We reviewed pathology slides of 22 patients with histologically proven diagnoses of primary breast NEN from 2007 to 2022. We found only 8 patients meet the criteria of primary breast NEC (large cell 2, small cell 6), 3 patients with NET, and 3 patients with IDC with NED. We excluded 8 patients who did not fulfill the criteria of NEN. The median age of NEN was 48.5 years (range, 31-70) and 6 patients (42.9%) were postmenopausal women. The median follow-up duration was 25.3 months (Interquartile range(IQR), 15.0-54.7). All patients underwent surgery, 3 patients underwent a mastectomy and 11 patients underwent breast-conserving surgery (BCS) with a curative aim. Five patients had lymph node metastasis. There was no expression of the human epidermal growth factor receptor 2 (HER2) in all 14 cases. Hormone receptor expression was shown in 4 of NECs (50%) and all NETs or IDC with NED patients. Patients with primary breast NEC had a median recurrence-free period (RFP) of 14.6 months (95% confidence interval (CI), 11.0-18.2) and median overall survival (OS) of 52.1 months (95% CI, 0.0-120.0). Patients with NET or IDC with NED had an overall favorable outcome, none of the patients died and only one patient with IDC with NED experienced disease progression. The median PFS and OS were not reached in NET or IDC with NED subgroups. Conclusion NETs are rare tumours with a wide range of clinical presentations according to the site of involvement. Primary breast NENs are extremely rare and there are no specific guidelines for treatment. NENs are often underdiagnosed, as neuroendocrine markers are not routinely tested in breast cancer. In this retrospective single-center study, the incidence of primary breast NENs was 0.04% (14 of 34,370 patients) and primary breast NEC was associated with poor prognosis compared with breast NET or IDC with NED. Identifying innovative treatment strategies is needed to overcome poor outcomes of primary breast NEC.

Citation Format: Young Kyung Jeon, Ji-Yeon Kim, Jin Seok Ahn, Young Hyunk Im, Kyue-Hee Choi, Sun Young Jeong, Yeji Jung, Jae Yeon Jang, Daeho Choi, Joohyun Hong, Hyo Jung Kim, Soo Youn Cho, Yeon H. Park. Primary neuroendocrine carcinoma of the breast: A case series by WHO classfication in 2019 [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-05-07.