Thyroid carcinomas with mixed follicular and C-cell differentiation patterns.

Divergent endocrine-neuroendocrine differentiation in thyroid carcinoma occurs in mixed medullary-follicular carcinomas (MMFC). Less than 40 cases of MMFC have been reported having highly heterogeneous patterns of growth. Classical medullary carcinoma areas may be intermingled with follicles or papillae or even oxyphilic and solid areas. Calcitonin and thyroglobulin are expressed in different cell populations. Presence of the latter suggests a potential usefulness of radioiodine treatment. The clinical behavior of MMFC does not differ from that of ordinary medullary carcinoma. The histogenesis of MMFC is controversial. The genetic analysis of the 2 neoplastic components showed that they are not derived from a common precursor, but rather display remarkable differences in the genetic profile (RET mutations and allelic losses). In addition, in some cases the follicular component was found to be oligo/polyclonal and therefore possibly hyperplastic rather than neoplastic. The follicular cells may have grown into the medullary carcinoma, after acquiring some molecular defect, being 'hostage' of the true neoplastic (medullary) component.