Anaplastic thyroid carcinoma with rhabdoid phenotype: An unusual case and a comprehensive review

Anaplastic thyroid carcinoma (ATC) is a highly aggressive thyroid malignancy predominantly affecting the elderly with a fatal outcome. ATC with rhabdoid phenotype is a rare variant, with only a few cases reported in the literature to date. We herein report a case of a 44-year old female diagnosed as ATC with rhabdoid phenotype. She had a slow-growing neck mass with no gross extrathyroidal extension (ETE) or nodal/distant metastasis at presentation. Computed tomography of the neck showed a well-defined heterogeneously hypodense nodule in the right lobe of the thyroid. On cytology, a diagnosis of papillary thyroid carcinoma (PTC) with possible anaplastic transformation was made based on the presence of vague papillae with focal nuclear features of PTC and atypical pleomorphic/rhabdoid cells. The total thyroidectomy specimen showed a relatively circumscribed lesion with no gross ETE. Histopathological examination revealed sheets of rhabdoid cells with a focus of poorly differentiated thyroid carcinoma. On immunohistochemistry, rhabdoid cells were positive for AE1/AE3, focally positive for PAX8 and were negative for TTF-1, synaptophysin, desmin, myogenin, S100P, and SMA. The neck lymph nodes were non-metastatic. The patient was further treated with adjuvant radioactive iodine. Four-months post-operatively, the patient developed pulmonary metastasis which on biopsy examination revealed metastatic ATC. Apart from being a rare tumor type, this case is unusual with its presentation too; wherein, unlike described earlier in the literature the patient had a relatively mitigated clinical course with no gross ETE or nodal/distant metastatic disease. We also review the relevant literature along with this case.