Thyroid Doses and Risks to Adult Patients Undergoing Neck CT Examinations

The purpose of this study was to estimate absorbed thyroid dose and consequent cancer risks in adult patients undergoing neck CT examinations.

We used data from neck CT examinations of 68 consecutive adult patients to calculate the thyroid dose and estimate the corresponding cancer risk. Age and sex were recorded along with the volume CT dose index (CTDIvol) that was used to perform the examination. CTDIvol values were used to estimate thyroid doses in the mathematic phantom used in the ImPACT patient CT dosimetry calculator. Corresponding doses in patients were estimated by modeling each patient's neck as an equivalent cylinder of water and applying correction factors for varying neck size and scanning length and the variation of radiation intensity due to automatic exposure control.

The mean (± SD) adult patient age was 59 ± 16 years, and the mean equivalent water cylinder diameter used for modeling the patient neck was 19.4 ± 4.2 cm. The average adult patient neck size was about 3 cm larger than the mathematic anthropomorphic phantom (16.5 cm), decreasing the estimated thyroid doses by 15%. Thyroid doses were independent of age and sex, with an average of 50 ± 23 mGy. The average cancer risk for a 20-year-old woman was six times higher than the corresponding risk for a 20-year-old man. Increasing patient age of either sex from 40 to 60 years reduced the cancer risk by approximately an order of magnitude.

Patient sex and age are the most important factors in determining thyroid cancer risk, with the thyroid dose being secondary.