Low-Grade Serous Carcinoma in Association With Endosalpingiosis in a Cervical Lymph Node: A Case Report and Literature Review.

Endosalpingiosis is a benign Müllerian inclusion that rarely involves supradiaphragmatic lymph nodes. Malignant transformation in the setting of nodal endosalpingiosis is exceptional. We report a case of low-grade serous carcinoma arising in a cervical lymph node in the setting of endosalpingiosis in a 41-year-old woman presenting with Horner syndrome. Imaging revealed a densely calcified, hypermetabolic supraclavicular lymph node. Fine needle aspiration and core biopsy demonstrated tubal-type epithelium consistent with endosalpingiosis adjacent to a neoplastic micropapillary and cribriform proliferation with psammoma bodies. Immunohistochemistry showed positive staining for PAX-8, estrogen receptor, and WT1, supporting Müllerian origin. The patient had a remote history of serous borderline tumor with noninvasive implants and KRAS and MAPK1 mutations. No recurrent pelvic disease was identified on positron emission tomography-computed tomography. Neck dissection confirmed low-grade serous carcinoma arising in association with nodal endosalpingiosis. This rare presentation underscores the importance of recognizing Müllerian proliferations in cervical nodes to avoid misdiagnosis as metastatic carcinoma.