Abstract
We read the article by Lin et al. (1) with a great interest. The effect of radiation on the thyroid gland increases when computed tomography (CT) scans are used frequently in the close follow-up period for the evaluation of the thyroid gland volume changes. Ultrasonography is sufficient to assess the volumetric changes of the thyroid gland after radiotherapy. When thyroid volume measurements are required, a method of thyroid volume calculation using ultrasonography described by Shabana et al. can be used, which is based on the use of an automatically calculated continuous trace transverse surface area measurement. By means of that significant inter-observer variation in calculation of thyroid lobe volumes are avoided (2). The only exception for not using ultrasonography (US) for thyroid volume calculation is for goiter with substernal extension. CT should be preferred for only this kind of goiter when thyroid gland volume measurement is necessary (3). We recommend US rather than CT in the volumetric follow-up of the thyroid gland to avoid radiation exposure. US is less expensive, easier to use, and more cost effective. Radiation doses from CT are a cause for concern with respect to cancer induction, even when used as a diagnostic tool (4). Therefore, we recommend US for the follow-up of thyroid volume and endoscopic examination or magnetic resonance imaging for most studies of locoregional recurrence of nasopharyngeal carcinoma.