Cancer of unknown primary (CUP) is the finding of a metastatic cancerous lesion without an established primary source localized within the body. CUP can be of any cancer cell type, however, adenocarcinoma is most often identified by histology. Up to 5% of all malignant diagnoses are classified as CUP. PET is an imaging modality often utilized to distinguish a primary source in the setting of CUP, yet often a primary is never identified. CUP can be further stratified using specific qualifiers as favorable and unfavorable, indicating the potential therapeutic response to treatment regimens. Treatment approach to CUP relies heavily on the cell type identified by histology, the location of the lesion, and the amount of spread within the body. In the typical setting and presentation, per current literature, CUP arises in the 7th decade of life in patients with multiple comorbidities, and often has a poor prognostic value. This case report identifies an atypical presentation of CUP, a 38-year-old Caucasian female with an axillary mobile mass, and no associated systemic symptoms. Biopsy of the node and immunohistochemical staining showed histology consistent with metastatic carcinoma. Mammography, MRI, and PET scan found no evidence of tumor primary or distant metastasis. Further staining confirmed metastatic carcinoma consistent with breast origin, without an established breast primary. As in this case, CUP may present in an atypical manner, warranting a thorough investigation aiming to identify the tumor primary to aid in identification of a proper treatment regimen and approach.
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