Enlargement and transformation of thyroglossal duct cysts in response to radiotherapy: imaging findings.

BACKGROUND AND PURPOSE Thyroglossal duct cyst remnants (TGDC) are a common midline neck mass that is mostly encountered in childhood. Enlargement after an upper respiratory tract infection or, rarely, cancer within TGDC, is a well-documented phenomenon. In the pediatric population, it rarely presents a diagnostic quandary. However, in an adult population, especially one treated for head and neck cancer with radiation therapy (RT), any enlarging cystic mass would be concerning and would raise the specter of metastatic disease. We propose that inflammation associated with RT results in secretory stimulation and/or obstruction of a persistent thyroglossal duct, resulting in changes within a pre-existing TGDC. MATERIALS AND METHODS We present a case series of 8 patients with subclinical TGDC treated with RT for various head and neck cancers, in which we radiographically observed enlargement or cystic transformation (more cystlike appearance) of the cyst. RESULTS In all patients there was in retrospect a small thyroglossal remnant, though not necessarily a discernible cyst, that was observed after radiotherapy to enlarge and become more cyst-like. CONCLUSIONS In patients with pre-existing TGDC undergoing RT to the neck, enlargement and cystic transformation may be encountered radiographically and should not be misinterpreted as a tumor. Our series bolsters a recently reported description of the same phenomenon.