Poorly differentiated thyroid carcinoma in a 9-year-old boy: case report.

Poorly differentiated thyroid carcinoma (PDTC) is a rare disease with a poor prognosis in children. We describe a 9-year-old boy with a thyroid nodule composed of cystic and solid components, which became completely solid and hypoechoic and was subsequently proved to be PDTC. The tumor consisted of small- to intermediate-size round cells in a trabecular or insular pattern with hyperchromatic nuclei and mitotic figures. The tumor cells were positive for thyroid transcription factor 1 and thyroglobulin. PDTC is morphologically and prognostically between the well-differentiated and anaplastic carcinomas. It must be distinguished from the solid variant of papillary carcinoma and well-differentiated follicular carcinoma with a predominantly solid/trabecular growth pattern. The tumor stage was T2N0M0. The patient was treated with total thyroidectomy, left-sided neck level VI lymph node dissection, recombinant human thyrotropin-stimulated 131I ablation therapy, and thyroid-stimulating hormone suppression. Malignancy should be suspected in a cystic thyroid nodule that becomes solid and hypoechoic.