Purpose: To estimate the increase in effective radiation dose from diagnostic x-rays for overweight and obese adult patients, as compared with the effective dose for lean reference phantoms.
Materials and methods: Relative effective radiation doses (E/E(0)) for the acquisition of chest and abdominal radiographs were calculated by using Monte Carlo computer simulations of effective doses delivered to adult phantoms with (E) and without (E(0)) subcutaneous adipose tissue added to the torso for five fat distributions. Total (anterior plus posterior) fat thicknesses ranged from 0 to 38 cm.
Results: For 30 cm of additional fat, E/E(0) values for 120-kVp chest and 80-kVp abdomen radiographs ranged from approximately 2 to 31 and 2 to 83 for male patients, respectively, and from 2 to 45 and 2 to 76 for female patients, respectively, depending on the type of fat distribution and patient orientation in the x-ray beam (anteroposterior or posteroanterior). Orienting the patient such that the thinnest fat layer was facing away from the x-ray tube minimized E/E(0), which was well approximated by using the formula E/E(0) = [B(t)/B(0)] x exp(kt(DF)), where B(t) and B(0) are the antiscatter grid Bucky factors for patient thicknesses of t and t = 20 cm, respectively; k, a constant; and t(DF), the distal (beam exit) fat layer thickness. Reductions in E/E(0) reached 14% and 20% for the thickest phantoms when x-ray tube voltages were increased by 10 and 20 kVp, respectively, for abdominal radiography in the male phantom.
Conclusion: Effective doses from radiographic examinations in the extremely obese can exceed 100 mSv from only a small number of abdominal examinations and should be minimized to the extent possible and monitored. Exponential dose increases for increased subcutaneous fat thicknesses can be reduced substantially by positioning the patient so that the thinnest fat layer (anterior or posterior) is closest to the image receptor. Increasing the tube voltage also reduces the dose-but to a much smaller extent.
(c) RSNA, 2009.