Thyroid collision tumor and Graves' disease: A case report and review of literature.

[BACKGROUND] Collision tumors of the thyroid are rare entities, defined by the coexistence of two histologically distinct tumors within the same organ, separated by intervening normal tissue. Graves' disease (GD) is a well-known risk factor for papillary thyroid cancer but has not been associated with thyroid collision tumor or medullary thyroid cancer (MTC).

[CASE SUMMARY] A 37-year-old female presented with palpitations, weight loss, and tremors. Thyroid function tests showed suppressed thyrotropin and slightly elevated free thyroxine. A thyroid ultrasound was performed with a report of a nodule in the right thyroid lobe. Thyroid scintigraphy revealed a goiter with increased radiotracer uptake. Ultrasound-guided fine needle biopsy suggested an MTC, Bethesda VI. Immunohistochemistry was positive for synaptophysin and negative for thyroglobulin. Calcitonin levels were elevated. Total thyroidectomy with central lymph node dissection was performed. Pathological exam revealed a medullary carcinoma in the right thyroid lobe and a 3-mm papillary microcarcinoma in the left lobe. Lymph nodes showed reactive hyperplasia without evidence of tumor involvement. The final diagnosis was synchronous medullary thyroid carcinoma and papillary thyroid microcarcinoma, staged as T1N0M0. Sequencing of the rearranged during transfection (RET) oncogene revealed no pathogenic variants, and multiple endocrine neoplasia was ruled out.

[CONCLUSION] Collision tumors are rare entities but thyroid collision tumors in GD are even more infrequent despite the known association between GD and differentiated thyroid cancer. The mechanisms by which this clinical entity occurs are unclear. Because it is a rare pathology, there are currently no guidelines for its treatment. Treatment must be guided separately or based on the more aggressive neoplasm.