Papillary thyroid carcinoma with anaplastic transformation showing a rhabdoid phenotype solely in the cervical lymph node metastasis

We describe a rare case of anaplastically transformed papillary thyroid carcinoma with a rhabdoid phenotype appearing solely in a metastatic focus. A 77-year-old man presented with a rapidly enlarging, painful right lateral cervical mass. CT scan revealed a tumor in the right upper pole of the thyroid gland and a right lateral cervical mass. Examination of surgically resected specimens disclosed that the thyroid tumor was a well-differentiated papillary carcinoma (2.0 cm in diameter), and the right lateral cervical mass was an anaplastic carcinoma (2.4 cm in diameter) showing a rhabdoid phenotype with scant amounts of a papillary carcinoma component in the periphery, considered to be transformed through the metastasis of the papillary thyroid carcinoma in a cervical lymph node. The rhabdoid cells had eccentric nuclei with conspicuous nucleoli and spherical hyaline cytoplasmic inclusions, which are immunoreactive for vimentin and sarcomeric actin. Ultrastructurally, these had globular aggregation of thin and intermediate filaments. Nuclear immunoreactivity for INI1 indicated that the tumor had no INI1 abnormalities, suggesting a secondary rhabdoid tumor. Recurrence developed in the right cervical and mediastinal lymph nodes, and the patient died of disease 6 months after surgery. A rhabdoid phenotype is a pathological hallmark indicating the aggressive nature not only in the neck region, but also in other organs.