Metastatic Thyroid Osteosarcoma With Concomitant Multifocal Papillary Carcinoma Presenting as a Collision Tumor

Metastatic involvement of the thyroid occurs rarely, by either hematogenous spread or direct extension from adjacent organs. The most frequent metastatic tumors are clear cell, renal cell, lung, breast, and squamous cell carcinoma. The occurrence of osteosarcoma and papillary thyroid carcinoma in the same patient is rare, with only a few reported cases in the literature. On the other hand, only one case of osteosarcoma thyroid metastasis has so far been reported. We herewith present another case with metastatic osteosarcoma and multifocal papillary thyroid carcinoma presenting as a collision tumor and review the relevant literature.