Costal synostosis mimicking supraclavicular lymphadenopathy in a breast cancer patient: A diagnostic pitfall.

The discovery of a supraclavicular mass in patients with a history of breast cancer is highly suspicious for metastatic lymph node involvement. We report the case of a 68-year-old woman with a history of left breast carcinoma treated by mastectomy one year earlier. During routine follow-up, clinical examination revealed a firm mass in the left supraclavicular fossa suggestive of supraclavicular lymphadenopathy. Ultrasound examination showed a poorly defined structure with marked posterior acoustic shadowing, making characterization difficult. Further evaluation with computed tomography demonstrated a fusion between the first and second ribs consistent with costal synostosis. This anatomical variant explained the palpable supraclavicular prominence that clinically mimicked nodal disease. This case highlights a rare diagnostic pitfall and emphasizes the importance of cross-sectional imaging in differentiating benign anatomical variants from metastatic lymphadenopathy in oncologic patients.