Estimating Thyroid Doses From Modified Barium Swallow Studies

This study sought to obtain factors to convert entrance air kerma into thyroid doses for patients undergoing modified barium swallow studies. A commercial software package (PCXMC 2.0.1) was used to calculate patient thyroid doses from modified barium swallow studies, which were divided by the entrance air kerma to yield f(thyroid) ratios. Exposure in the lateral and posterior-anterior projections were considered where the thyroid was directly irradiated. Calculations were obtained for adult patients as well as children ranging from birth to 10 y old. The average value of f(thyroid) in a normal-sized adult was 0.63 +/- 0.11 in the lateral projection and 0.18 +/- 0.06 for an upper gastrointestinal posterior-anterior projection. Increasing the beam quality from the lowest (60 kV + 3 mm aluminum) to the highest (110 kV + 3 mm aluminum + 0.2 mm copper) values investigated nearly doubled the value of f(thyroid) from 0.42 to 0.79 in the lateral projection and quadrupled the value from 0.07 to 0.29 in the upper gastrointestinal posterior-anterior projection. Values of f(thyroid) decreased with increasing body mass index. Average values of f(thyroid) in 10 y olds were similar to those of adults but always increased as the age of the exposed child was reduced. The average f(thyroid) for newborns was 0.84, nearly one-third higher than the corresponding ratio for normal-sized adults.