Lung Cancer Screening CT: Sex-Specific Conversion Factors to Estimate Effective Radiation Dose From Dose-Length Product.

BACKGROUND: Effective dose (ED) is used to understand radiation-related cancer risk of CT scans. Currently, ED for low-dose CT (LDCT) lung cancer screening (LCS) is estimated by multiplying the CT scan-reported dose-length product (DLP) by a DLP-to-ED conversion factor (k-factor) for general chest CT imaging, which does not account for sex. The purpose of this study was to calculate sex-specific k-factors for LDCT LCS.

METHODS: This retrospective study evaluated consecutive LCS patients across a large health system from 2016 to 2017. Patient and CT scan-related data were obtained from the radiology information system, the picture archiving and communication system, and a radiation dose index-monitoring system. Each patient's ED was determined by patient-specific Monte-Carlo simulation using Cristy phantoms and divided by study DLP to determine the k-factor. The k-factors were compared vs the standard of 0.014 mSv·mGy⁻1 ·cm⁻1 for a chest CT scan by using a one-sample Student t test. Bivariate and multivariable analyses were performed for k-factors based on patient and CT scan factors.

RESULTS: A total of 1,890 patients were included in the study. The mean k-factor for all patients was 0.0179 mSv·mGy⁻1 ·cm⁻1 , which was 22% greater than the standard value of 0.014 mSv·mGy⁻1 ·cm⁻1 for a chest CT scan previously applied to LDCT imaging (P < .001). The mean k-factor in women (0.0213 mSv·mGy⁻1 ·cm⁻1 ) was 43% greater than in men (0.0149 mSv·mGy⁻1 ·cm⁻1 ) in the multivariable model (P < .001).

CONCLUSIONS: The overall k-factor for LCS is higher than the previously used value for chest CT imaging; when stratified according to sex, it was 43% greater in women than in men. Sex- and LCS-specific k-factors should be used to estimate effective radiation dose in LCS programs.