The simultaneous occurrence of two distinct neoplasms derived from different cells of origin is a recognized entity. It presents uncommonly in all organs,and very rarely in the thyroid gland.

Six patients with concurrent medullary thyroid carcinoma (MTC) and papillary thyroid carcinoma (PTC) were seen between January 2000 and June 2007. Clinicopathological features were evaluated, and follow-up for the patients was obtained.

All patients were female, with ages ranging from 39 to 63 years (mean age, 55.4 years). Five of six patients were incidentally detected during a medical check-up. The medullary carcinomas measured from 0.5 to 4.0 cm in diameter (mean diameter, 1.72 cm), while the papillary carcinomas ranged from 0.2 to 1.4 cm in diameter (mean diameter, 0.72 cm). Treatment consisted of a total thyroidectomy (n=5) and hemithyroidectomy (n=1) with central compartment node dissection. In twopatients, prophylactic bilateral neck dissection was also added. Supplemental radioactive iodine treatment was administered in five patients. During a mean follow-up period of 41.8 months, two patients showed recurrence in the lateral neck nodes, one recurrence from a papillary carcinoma and the other recurrence from a medullary carcinoma. After re-operation for the recurrent lesions, allof the patients were alive andfree of disease.

Attention should be given to the possibility of the simultaneous occurrence of MTC and PTC. Treatment for concurrent MTC and PTC should be based on the treatment for MTC, considering the more aggressive behavior of the tumor than PTC.