TWO CASES OF THYROID COLLISION TUMORS WITH DIFFERENT DIFFERENTIATION PATHWAYS.

Collision tumors of the thyroid gland are rare and characterized by the coexistence of two distinct neoplasms with different histopathological origins within the same anatomical region. We report two cases of synchronous thyroid carcinomas demonstrating divergent differentiation pathways: papillary thyroid carcinoma (PTC) combined with follicular carcinoma in one case, and PTC combined with a poorly differentiated component in the other case. Both cases presented with a long-term history of multinodular goiter but displayed rapid recent growth. Surgical treatment with total thyroidectomy was performed in both cases. Histopathology confirmed dual tumor composition with sharp topographic demarcation, supporting the diagnosis of collision tumors. We discuss clinicopathological features, diagnostic challenges, and potential mechanisms underlying synchronous tumorigenesis. While the exact pathogenesis remains unclear, environmental and molecular factors may contribute to divergent malignant transformation within a susceptible thyroid gland. Awareness of this phenomenon is crucial for accurate diagnosis and management.