Coexistence of Oncocytic (Hürthle cell) carcinoma with foci of anaplastic carcinoma and a metastasizing papillary carcinoma in one thyroid gland: Rare case report.

[INTRODUCTION AND IMPORTANCE] Anaplastic thyroid cancer is considered one of the most fatal aggressive malignancies with a survival duration estimated in months. When compared to anaplastic thyroid cancer, a well-differentiated thyroid tumor has a better prognosis and a longer survival duration even if it metastasized. Left untreated, the transformation of well-differentiated thyroid carcinoma to aggressive anaplastic malignancy has been considered one of the most devastating complications.

[CASE PRESENTATION] A 60-year-old male presented with a complaint of anterior neck swelling and hoarseness examination revealed a huge left thyroid swelling that was mobile, not tender, not attached to the underlying structures. Ultrasonographic examination of the thyroid gland showed a massively enlarged left thyroid lobe. Fine needle aspiration revealed undifferentiated (anaplastic) thyroid carcinoma. Preoperative CT excluded invasion or metastasis, and patient underwent total thyroidectomy and level 6 lymph node dissection. Histopathology showed anaplastic carcinoma foci within a background of Oncocytic (Hürthle cell) carcinoma and an incidental papillary thyroid carcinoma metastasizing into one lymph node.

[CLINICAL DISCUSSION] The predominance of an anaplastic thyroid tumor with a few foci of well-differentiated thyroid malignancy, although rare, is the known commonly observed histopathological finding. However, it is extremely rare to find Oncocytic (Hürthle cell) thyroid carcinoma within the anaplastic component. It's presumed that patients who have coexisting well-differentiated thyroid cancer with the anaplastic component are at an overall survival advantage when compared to those who have pure anaplastic thyroid cancer. Our patients had a predominantly well-differentiated component, with a ratio of 80/20, the lesser being anaplastic, which might explain his 10 months cancer free outcome.

[CONCLUSION] It's extremely rare to encounter a predominant Oncocytic (Hürthle cell) carcinoma with foci of anaplastic tumor and a separate papillary carcinoma that metastasized to one lymph node. This rare histopathological finding supports that theory of anaplastic transformation from a pre-existing well differentiated thyroid tumor.