Solitary Parotid Metastasis as an Atypical Site of Recurrence in Lung Squamous Cell Carcinoma: A Diagnostic Difficulty.

Lung squamous cell carcinoma (LSCC) rarely metastasizes to the parotid gland, with only a handful of cases reported in the literature. Its clinical and pathological presentations closely mimic those of primary parotid squamous cell carcinoma (PSCC). Differentiating solitary metastasis from metachronous primary squamous cell carcinoma (SCC) in patients with a history of smoking and lung cancer presents a significant diagnostic challenge. A 55-year-old man with a history of heavy smoking presented with a solitary parotid mass two years after curative LSCC resection. Excisional biopsy with histological and immunohistochemical analyses confirmed SCC. Based on (1) a clear history of a primary tumor with a known metastatic propensity, (2) the absence of other primary sites on whole-body imaging, (3) histological features (highly consistent with the primary tumor type and no evidence of squamous metaplasia and dysplasia in the ductal or acinar epithelium), and (4) an immunohistochemical profile that effectively ruled out common primary parotid neoplasms, we retained solitary parotid metastasis as LSCC recurrence. However, definitive comparative molecular profiling, which is crucial for distinguishing recurrence from a second primary tumor, could not be performed because of resource limitations. Beyond the diagnostic challenge, this case highlights the complex intersection between rare metastatic sites and the risk of developing a secondary malignancy. In such scenarios, the decision to proceed with systemic therapy without definitive molecular proof of clonal relatedness depends on a robust convergence of clinical context, consistent imaging, and highly suggestive histopathological and immunohistochemical findings. It is imperative that traditional pathology and advanced molecular studies converge to provide not only a diagnosis but also a precise biological understanding of the disease.