Abstract
We agree that three-dimensional (3D) ultrasonography is reasonably accurate in the assessment of thyroid gland volume (1,2), and it has the advantage over computed tomography (CT) of being radiation free. Nevertheless, since CT provides high-resolution 3D anatomical information, the thyroid gland can be finely delineated in corresponding CT slices and allows highly accurate volume measurement. Besides, the volume calculation in CT is not affected by the presence of thyroid abnormalities such as cysts, nodule, or goiter, which is the limitation of ultrasonography (3).
Our study worked on patients with postradiotherapy nasopharyngeal carcinoma (NPC) (4). All patients were required to undergo a mandatory planning CT scan of the head and neck region for radiotherapy treatment planning, which already provided the baseline thyroid gland volume before radiotherapy. In order to monitor the radiation treatment results, it is a routine practice that patients with NPC receive a CT scan during their follow-up visits at 6, 12, and 18 months after radiotherapy. Therefore, the thyroid volume data was conveniently obtained from these CT data without extra intervention to the patients. As a result, the use of ultrasonography was not necessary.
Further, ultrasonography was reported to have slight discrepancies in thyroid volume measurement, in which the volume calculated was usually smaller than that of CT (3). This might affect the outcome of our study, because we expected to detect relatively small changes of volume at various intervals of the study period, which ultrasonography might not be able to achieve.