CALCA and medullary thyroid gland carcinoma: In mixed medullary-follicular thyroid carcinoma (MMFTC), because of a common cell of origin, tumor cells are positive (individual cells or separate cell clusters) for both thyroglobulin and calcitonin within the same lesion [28], while TCTs composed of MTC and follicular components are positive for calcitonin only in the MTC component and thyroglobulin only in the follicular component, as a consequence of different pathogeneses (origin from two or more different cell progenitors separated by non-neoplastic tissue) [16].