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.