Isolated Axillary Lymphadenopathy Presenting as Carcinoma of Unknown Primary in a Female Patient: A Diagnostic and Therapeutic Challenge.

Carcinoma of unknown primary (CUP) is a distinct and uncommon clinical entity characterized by metastatic disease without an identifiable primary tumor, despite thorough investigation. Isolated axillary lymphadenopathy as the sole manifestation is rare and diagnostically challenging, particularly in younger women. The patient was a 47-year-old woman with painless right axillary swelling with no breast or systemic lesions. Fine-needle aspiration cytology (FNAC) suggested metastatic carcinoma. PET-CT showed a hypermetabolic right axillary mass without evidence of disease elsewhere. Excisional biopsy confirmed poorly differentiated carcinoma; immunohistochemistry (IHC) was positive for PanCK, CK7, TRPS1, and PAX8 with a high Ki-67 index (70%). A multidisciplinary decision led to axillary lymph node dissection, which revealed no residual metastatic disease. The follow-up PET-CT was normal. This case highlights the importance of a thorough diagnostic approach. IHC, advanced imaging, and multidisciplinary input are critical for diagnosis and management. CUP localized to lymph nodes may have a more favorable prognosis.