CONTEXTO: Cisto de ducto tireoglosso é a anormalidade cervical congênita mais comum da infância. As lesões malignas de cisto de ducto tireoglosso são raras (cerca de 1%). OBJETIVO: Apresentar um relato de caso de um carcinoma papilífero originado em um cisto do ducto tireoglosso. TIPO DE ESTUDO: Relato de caso. RELATO DE CASO: Relatamos o caso de uma paciente de 21 anos com cisto de ducto tireoglosso, operada pelo método de Sistrunk, com punção aspirativa por agulha negativa para neoplasia. A histologia firmou o diagnóstico de carcinoma papilífero, com margens livres. Optou-se por não realizar tireoidectomia total ou parcial complementar, devido a não invasão da cápsula do cisto pelo tumor e pela ausência de lesões no parênquima tireoideano. No seguimento de dois anos a paciente não mostrou evidência de doença. São discutidas opiniões da literatura quanto à conduta referente à glândula tireóide.

CONTEXT: Thyroglossal duct cysts are the most common congenital cervical abnormality in childhood. Malignant lesions are rare in thyroglossal duct cysts (about 1%). OBJECTIVE: To report a case of papillary carcinoma in thyroglossal duct cysts. DESIGN: Case report. CASE REPORT: The patient was a 21-year-old female with a four-month history of an anterior midline neck mass but without other symptoms. The physical examination revealed a 4.0 cm diameter, smooth, painless, cystic nodule at the level of the hyoid bone. The thyroid gland was normal by palpation and no neck lymph nodes were found. Indirect laryngoscopy, fine-needle biopsy aspiration and cervical ultrasound were normal and compatible with the physical findings of a thyroglossal duct cyst. The patient underwent surgery with this diagnosis, under general anesthesia, and the mass was resected by the usual Sistrunk procedure. There were no local signs of invasion of the tissue surrounding the cyst or duct at surgery. The patient was discharged within 24 hours. Histopathological examination of the specimen showed a 3.5 x 3.0 x 3.0 cm thyroglossal cyst, partially filled by a solid 1.0 x 0.5 cm brownish tissue. Histological sections showed a papillary carcinoma in the thyroid tissue of a thyroglossal cyst, with normal thyroid tissue at the boundary of the carcinoma. There was no capsule invasion and the margins were negative. The follow-up of the patient consisted of head and neck examinations, ultrasonography of the surgical region and thyroid, and total body scintigraphy. The patient has been followed up for two years with no further evidence of disease.