Sporadic minute medullary thyroid carcinoma with a double RET mutation: A case report

We describe a 74-year-old man with a nodular goiter accompanied by an incidental sporadic minute medullary thyroid carcinoma (MTC). Histopathologically, the MTC was a well-defined 1.7 mm tumor in the upper one-third right lobe, with solid cell nests (SCNs) adjacent to the MTC. C-cells were scattered mainly around the SCNs, but C-cell hyperplasia was not evident in the background thyroid. The MTC cell phenotype was immunohistochemically identical to background C-cells, but was completely different from the SCN main cells. Direct DNA analyses of isolated MTC paraffin-embedded specimens revealed two RET proto-oncogene missense point mutations in exon 11 (i.e., C630R and C634W). The non-tumor thyroid tissue did not reveal any mutations. This study reports the smallest case of sporadic MTC with a double RET somatic mutation, substantiating that RET mutations can occur during a very early stage of carcinogenesis. The combined presence of C630R and C634W represent a novel somatic mutation in sporadic MTC. The present case indicates that the sporadic MTC originated from the surrounding C-cells of the SCNs without C-cell hyperplasia and that the SCN main cells may not be able to develop into an MTC.