Patient Size-Specific Analysis of Dose Indexes From CT Lung Cancer Screening

The U.S. Centers for Medicare & Medicaid Services (CMS) recently approved the use of low-dose CT for lung cancer screening and described volumetric CT dose index (CTDI_vol) requirements. These were based on the National Lung Screening Trial, which used only fixed-tube-current techniques. The aim of this study was to evaluate dose index data from a lung cancer screening program using automatic exposure control (AEC) techniques to ensure compliance with requirements and to correlate dose index values with patient size.

CTDI_vol, dose-length product (DLP), and body mass index (BMI) data were collected for 563 lung cancer screening examinations performed with AEC between January 1, 2014, through August 31, 2015. CTDI_vol and DLP were analyzed according to the patient's BMI classification. Results were compared with the CMS requirement that the CTDI_vol for a standard-sized patient (height, 170 cm; weight, 70 kg) be 3.0 mGy or less, with adjustments for patients of different sizes. For a subset of patients, the average water-equivalent diameter and size-specific dose estimate were estimated.

The average CTDI_vol for a standard-sized patient was 1.8 mGy, which meets CMS requirements. CTDI_vol values were lower for smaller patients and higher for larger patients. Overall, the mean CTDI_vol and DLP were 2.1 mGy and 74 mGy⋅cm, respectively. The size-specific dose estimate for the average water-equivalent diameter (27.5 cm) of the patient subset was 2.6 mGy.

The screening protocols using AEC resulted in CTDI_vol values that were compliant with CMS requirements. CTDI_vol values greater than 3.0 mGy were only observed for overweight or obese patients.