Computation of thyroid doses and carcinogenic radiation risks to patients undergoing neck CT examinations

Abstract

The aim of the study was to investigate how differences in patient anatomy and CT technical factors in neck CT impact on thyroid doses and the corresponding carcinogenic risks. The CTDIvol and dose-length product used in 11 consecutive neck CT studies, as well as data on automatic exposure control (AEC) tube current variation(s) from the image DICOM header, were recorded. For each CT image that included the thyroid, the mass equivalent water cylinder was estimated based on the patient cross-sectional area and average relative attenuation coefficient (Hounsfield unit, HU). Patient thyroid doses were estimated by accounting for radiation intensity at the location of the patient's thyroid, patient size and the scan length. Thyroid doses were used to estimate thyroid cancer risks as a function of patient demographics using risk factors in BEIR VII. The length of the thyroid glands ranged from 21 to 54 mm with an average length of 42 ± 12 mm. Water cylinder diameters corresponding to the central slice through the patient thyroid ranged from 18 to 32 cm with a mean of 25 ± 5 cm. The average CTDIvol (32-cm phantom) used to perform these scans was 26 ± 6 mGy, but the use of an AEC increased the tube current by an average of 44 % at the thyroid mid-point. Thyroid doses ranged from 29 to 80 mGy, with an average of 55 ± 19 mGy. A 20-y-old female receiving the highest thyroid dose of 80 mGy would have a thyroid cancer risk of nearly 0.1 %, but radiation risks decreased very rapidly with increasing patient age. The key factors that affect thyroid doses in neck CT examinations are the radiation intensity at the thyroid location and the size of the patient. The corresponding patient thyroid cancer risk is markedly influenced by patient sex and age.

Thyroid doses and risk to paediatric patients undergoing neck CT examinations

“…3 Water cylinder diameter as a function of age Medicine (AAPM) report 204 [18]. The average CTDI vol was 12 mGy in paediatric patients, less than half compared to the average CTDI vol previously reported in adult subjects [13]. We also found substantial CTDI vol variability for necks of equivalent size.…”

“…3). Furthermore, the thyroid is usually at the interface between the neck and the thoracic inlet, although the precise location of the thyroid gland in the neck is variable among individuals [13]. The thyroid is positioned in a relatively more attenuating anatomical area compared to the rest of the neck.…”

“…The thyroid dose for each subject was estimated using previously described methodology [13,14]. The average radiation intensity used to perform each patient CT examination is given by the scan CTDI vol (mGy).…”

“…Correction factors (R L ) for various scan lengths were estimated based on previous work by Huda et al [13]; (3) Radiation intensity -Since the CT scans were performed using the automatic exposure control (AEC), it is necessary to take into account the variation of the tube current along the long axis of the patient. The tube current, i.e., (mA) at each slice, was obtained from the DICOM header that permitted quantitative determination of the average tube current value (mA ave ) used to perform the CT examination, as well as the specific value that was used at the middle of the thyroid gland (mA center ).…”

“…The thyroid gland is known to have a significantly greater sensitivity to radiation exposure in young children than adults [12]. A systematic analysis of the relationship between patient characteristics (size of the cervical region), CT parameters (kVp, mAs), scanner-related factors, and the resulting thyroid dose is necessary to provide an estimate of thyroid cancer risk stratified by patient age and gender from a routine cervical CT. We previously developed a method to calculate the absorbed radiation dose to the thyroid gland in a patient undergoing a CT examination [13].…”