"Collision" metastasis from unknown primary squamous cell carcinoma and papillary microcarcinoma of thyroid presenting as lateral cervical cystic mass.

We report an unusual case of "collision metastasis". In a single lymph node, we found a metastatic mass composed of two immunohistochemically distinct components originating from two primary tumors: a papillary microfollicular thyroid cancer and an unknown primary squamous cell carcinoma. The clinical features and immunohistochemical profile are reported. Collision phenomena in oncology are extremely rare and pose diagnostic and management challenges which are discussed.