Origin and impact of multifocal growth in sporadic vs. hereditary medullary thyroid cancer.

Multifocal growth is characteristic of hereditary medullary thyroid cancer (MTC), whereas origin and impact of multifocal growth is enigmatic for sporadic MTC. To address this, 460 RET-negative patients with sporadic MTC, stratified by 1 (93.3 %), 2 (5.7 %) and 3 (1.1 %) thyroid tumor foci, were compared with 219 RET-positive patients with hereditary MTC, stratified by 1 (38.4 %), 2 (45.7 %), 3 (6.4 %), 4 (6.8 %) and ≥5 (2.7 %) thyroid tumor foci. For sporadic MTC, significant associations were identified with bilateral thyroid lobe involvement, microscopic lymphatic invasion, extrathyroid extension, node and distant metastases, number of node metastases, preoperative basal calcitonin level, and decreasing biochemical cure. For hereditary MTC, significant associations were limited to bilateral thyroid lobe involvement, largest thyroid tumor diameter, and preoperative basal calcitonin level. In sporadic MTC, multifocal growth is due to lymphatic invasion with frequent node metastases, whereas in hereditary MTC, it reflects malignant progression from C-cell hyperplasia to cancer.