Abstract: To investigate the effect of external foreign bodies on radiation dose during chest computed tomography (CT) scans under automatic tube current-modulation technology and to assess the necessity of re-scanning a scout image, we retrospectively categorize historical data into foreign body and non-foreign body cohorts. Propensity score matching is employed to balance five covariates: patient gender, examination age, number of image layers, scanning-bed height, and average cross-sectional area of patients. After balancing, we statistically analyze the effect of foreign bodies on the volume CT dose index (CTDI_vol) and dose-length product (DLP), and compare the dose increment with the scout-image dose. The study includes 17342 cases, with 4740 cases (27.3%) in the treatment group. Before matching, all five covariates in the treatment group are significantly lower than those in the control group. After matching, 3944 pairs of balanced samples are obtained and no significant differences are observed between the two groups in the five covariates. The treatment group shows slightly higher CTDI_vol and DLP values compared with the control group. External foreign bodies can slightly increase the radiation dose, although the incremental dose does not differ statistically from that of the scout image. Operators should flexibly determine whether to re-scan the scout image based on specific clinical circumstances.