Spindle epithelial tumor with thymus-like differentiation of the thyroid: a case report with pathological and molecular genetics study.

We report an unusual case of spindle epithelial tumor with thymus-like differentiation (SETTLE) of the thyroid present in a 6-year-old boy. The tumor, located at both the left lobe and isthmus, was a circumscribed mass with slightly gritty whorled appearance. Microscopically, the lobulated, highly cellular, spindle cell neoplasm was arranged in intersecting bundles and fascicles separated by fibrous bands. Benign-appearing glands entrapped within fibrous bands and foci of squamous differentiation within spindle cells were observed. Immunohistochemically, the spindle cells were diffusely positive for cytokeratins, vimentin, and alpha-smooth muscle actin and patchily reactive for muscle-specific actin and epithelial membrane antigen, exhibiting myoepithelial differentiation. The spindle cells were also patchily immunopositive for p53 protein. Molecular genetic analysis revealed Ki-ras gene mutations at codons 13 (GGC(gly) to AGC(ser)) and 15 (GGC(gly) to AGC(ser)) on the same allele. Mutation of the p53 gene was not detected. This is the first report on Ki-ras oncogene mutations in a case of SETTLE.