Papillary thyroid carcinoma with extensive squamous dedifferentiation metastatic to the lung: BRAF mutational analysis as a useful tool to rule out tumor to tumor metastasis.

Tumors containing elements of both papillary thyroid carcinoma (PTC) and squamous cell carcinoma (SCC) are rare but well documented. When they present initially as metastatic disease in an organ that can harbor a primary SCC, the possibility of a tumor to tumor metastasis (TTM) must be considered. The aim of this case study is to illustrate how BRAF mutational analysis can be used to help differentiate between these two diagnoses. We report a 63-year-old male with a longstanding history of PTC metastatic to the brain and lymph nodes who presented to our institution with a right lower lobe lung mass after a 2-year recurrence-free interval. Histopathologic and immunohistochemical analysis revealed a composite neoplasm with distinct elements of both PTC and SCC. We performed BRAF (V600E) (c.1799 T > A) mutational analysis to help elucidate the origin of each component. This is the first time that BRAF sequencing has been used to discriminate between dedifferentiated PTC and TTM, to the best of our knowledge. In the context of metastatic PTC with SCC dedifferentiation, the presence of the identical BRAF (V600E) (c.1799 T > A) mutation in both components might help rule out tumor to tumor metastasis.