Aspiration cytology of ectopic cervical thymoma mimicking a thyroid mass: A case report

BACKGROUND: Ectopic cervical thymoma, first described in 1941 by Boman, is an uncommon tumor of the neck displaying the same histologic features as mediastinal thymoma. Since it is commonly located in the anterolateral part of the neck or is subjacent to or inside the lower pole of the thyroid, the mass is often confused as being of thyroid origin. CASE: A 68-year-old female presented with dyspnea and an anterior neck mass found on routine chest roentgenography. The thyroid scan showed a cold nodule in the lower pole of the left part of the thyroid. Fine needle aspiration (FNA) cytology revealed large numbers of small lymphocytes with hyperchromatic nuclei and frequent clumping pattern in the pale, eosinophilic, fluid background. A few clusters of epithelial cells without atypism were interpreted as thyroid follicular cells. The overall cytologic features were misinterpreted as malignant lymphoma of the thyroid. However, the histologic diagnosis was thymoma, predominantly cortical type. CONCLUSION: The ectopic cervical thymoma is sometimes misdiagnosed as Hashimoto's thyroiditis, anaplastic carcinoma and malignant lymphoma of thyroid on FNA cytology or frozen diagnosis due to its rarity. Therefore, the differential diagnosis of a neck mass showing a variable composition of lymphocyte and epithelial component in a pale, eosinophilic, fluid background should also include ectopic cervical thymoma, especially in elderly females.