Dosimetry using average-sized phantoms leads to considerable errors

The use of non-size-dependent reference phantoms to calculate CT dose can lead to errors in calculating the radiation received by a patient, according to an article published in the American Journal of Roentgenology. Using data from the National Lung Screening Trial, researchers found using an average height and weight to calculate dose can lead to errors of up to 200 percent when compared with a more accurate estimate using a particular patient’s height and weight. 

The National Lung Screening Trial (NLST) was a large-scale randomized controlled trial that compared low-dose helical CT with standard chest x-rays, finding that participants who received CT scans had a 15 to 20 percent lower risk of dying from lung cancer than patients who received chest x-rays. However, the calculations of organ and effective doses used average-sized reference phantoms, and actual patient size wasn’t taken into account.

“Our study represents the first calculation of body-size-specific organ and effective doses for NLST CT participants,” wrote Choonsik Lee, Ph.D, with the National Institutes of Health, and colleagues. “These calculations used CT parameters, the height and weight of participants, and a CT dose calculator based on body size–dependent computational human phantoms.”

While the mean CT dose to enrollees’ lungs and thymus glands was similar, the patients on either end of the BMI bell curve were responsible for errors in dose estimates.

“Our study results indicated that dosimetry approach based on reference size phantoms tends to underestimate organ doses to underweight participants and overestimates doses to overweight participants,” the authors wrote. “For the underweight participant group, the median effective dose based on the reference phantom (2.29 mSv) is 82 percent of the value given for body size–specific phantoms (2.80 mSv). For the obese participant group, the median effective dose based on the reference size phantoms (2.71 mSv) was 1.3-fold greater than that based on the body size–specific phantoms (2.10 mSv).

While reference size phantoms correctly estimate dose to patients with a normal BMI—18.5 to 24.99—they overestimate lung dose by up to 200 percent for overweight patients. This may be a major problem in the U.S., a country with an average BMI of 26.6.

The authors acknowledged some limitations in the study, including the lack of scanning start/stop landmarks for the NLST cohort, but they are confident the results will shine a light on the need for size-dependent dosage estimation.

“This comparison highlights that potential errors up to twofold can occur with the use of single-sized reference phantoms, a method adopted in most CT calculation tools,” the authors wrote. “The body-size dependent dosimetry methods used in the current study will be useful in future studies of organ doses for participants undergoing CT examinations.”