Cervical lymph node metastasis mimicking stage IV breast cancer: a diagnostic pitfall revealed by fine-needle aspiration cytology: a case report

Background: Nonregional cervical lymph node metastasis in patients with breast cancer is generally considered indicative of stage IV disease and often leads to systemic treatment. However, it may occasionally represent metastasis from a synchronous second primary malignancy, leading to diagnostic misclassification. Presentation of case: A 46-year-old premenopausal woman presented with right cervical lymph node swelling. Computed tomography (CT) incidentally revealed a small right breast mass. Core needle biopsy confirmed invasive ductal carcinoma. Fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) demonstrated uptake in the right breast and in the bilateral axillary, right supraclavicular, and right cervical lymph nodes, with no abnormal FDG uptake in the thyroid gland or other distant lesions. Although the primary tumor was small, FDG uptake in multiple lymph node regions raised suspicion of advanced breast cancer. However, fine-needle aspiration cytology (FNAC) of the right cervical lymph node demonstrated cytological features suggestive of papillary thyroid carcinoma, and FNAC of the bilateral axillary lymph nodes revealed no malignancy. The patient was ultimately diagnosed with synchronous breast cancer and papillary thyroid carcinoma, both amenable to curative treatment. At the 15-month follow-up after breast cancer surgery, no recurrence of either malignancy was observed. Discussion: This case highlights a diagnostic pitfall in breast cancer staging. FNAC of the cervical lymph node suggested papillary thyroid carcinoma, despite the absence of abnormal FDG uptake in the thyroid gland on FDG-PET/CT. Conclusions: FNAC played a crucial role in preventing misclassification and enabling curative treatment. Tissue diagnosis should be considered when clinical and radiological findings are discordant.