Coexistence of Two Different Thyroid Malignancies: A Collision Phenomenon.

The term "collision tumor" is described as the coexistence of two or more histologically distinct neoplastic morphologies separated by normal tissue in the same organ. Simultaneous papillary thyroid carcinoma (PTC) and follicular thyroid carcinoma (FTC) of the same thyroid lobe is a very rare pathology. Herein, we report a case of PTC and FTC of the same thyroid lobe. A 79-year-old man was evaluated at our hospital for the presence of left hip pain of two-month duration after sustaining a physical trauma to the left side of his body three days prior to admission. X-ray imaging of the left femur revealed a large lytic bony lesion at the proximal end of left femur. Biopsy of the bone lesion was suggestive of FTC. Computed tomography (CT) of the neck revealed an enlarged thyroid with a cystic lesion in the left lobe of the thyroid gland. Total thyroidectomy was performed. Histopathology revealed two separate primary malignancies of PTC and FTC. Genetic studies for RAS gene mutation were negative. He was initiated on suppressive doses of levothyroxine following thyroidectomy. Three months after surgery, thyrotropin alfa stimulated 204.5 mCi I-131 was administered. At seven months of follow-up, the thyroglobulin level was in the lower end of the normal range and anti-thyroglobulin antibody (anti Tg) remained negative (< 1.0 IU/mL). He was doing well and reported no symptoms. For each type of well-differentiated thyroid cancers, several genes have been identified. However, thus far, no specific gene mutation responsible for the pathogenesis of the different tumor types has been described. Management of thyroid collision tumor is usually complex due to the presence of different pathology in the tumor tissues and given the fact that literature on this condition is limited. Typically, the treatment needs to be individualized. Our report brings up a concept that the occurrence is a rare phenomenon of simultaneous mutation of different genes that could give rise to different thyroidal neoplasms.