Radiation dose in chest CT: Assessment of size-specific dose estimates based on water-equivalent correction

In the setting of thorax Computed Tomography (CT), the main purpose of this work is to quantify differences in Size-Specific Dose Estimate (SSDE), calculated on the basis of both effective diameter and water-equivalent diameter, accounting for patient size and tissue attenuation, respectively.

An in-house software was developed to measure water-equivalent diameter and effective diameter on each CT slice, for 133 CT examinations. SSDE was calculated according to the Report of American Association of Physicists in Medicine Task Group (AAPM TG) 204 and 220. The ratio between effective and water-equivalent diameters was studied as function of cross-sectional air-to-body area ratio, for the slice located in the middle of the scan range.

When AAPM TG 220 prescriptions were applied, SSDE was mostly found larger than that obtained with AAPM TG 204 recommendations. On average, a difference of about 12% was observed, in spite of a considerable variability (from -18% to 53%). The ratio between effective and water-equivalent diameters ranged from 0.97 to 1.31, with a mean value of 1.15. Moreover, it was found considerably correlated with cross-sectional air-to-body ratio (Pearson's coefficient was 0.78 for women, 0.90 for men).

The discrepancy obtained by calculating SSDE on the basis of AAPM TG 204, instead of AAPM TG 220 Report, may vary substantially. Fluctuations were justified by the variability observed in the relative amount of low attenuating tissues in body, which was proved to be considerably correlated to the ratio between effective and water-equivalent diameters.