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Background The documentation of clinically significant incidental findings (S modifiers) in low-dose CT lung cancer screening varies among radiologists. Although the Korean National Lung Cancer Screening Program adopted structured reporting for seven standardized S modifiers, the prognostic value of standardized S modifiers has not been evaluated comprehensively. Purpose To evaluate the implementation of structured reporting for prespecified S modifiers by analyzing their prevalence, mortality associations, and co-occurrence patterns. Materials and Methods This retrospective study included baseline screening participants from the Korean National Lung Cancer Screening Program between August 2019 and December 2020. The prevalence of seven S modifiers was calculated, and their prognostic value for all-cause mortality was assessed using multivariable Cox regression. Latent class analysis (LCA) was performed to identify co-occurrence patterns, which were analyzed for mortality risk stratification. Results Among 125 600 participants (mean age ± SD, 62 years ± 5.3; 123 331 men), 2.69% (n = 3380) died over a median follow-up of 3.7 years. Coronary artery calcification was most prevalent (15.07% [18 892 of 125 366 participants]), followed by emphysema (13.77% [17 300 of 125 600 participants]), interstitial lung abnormalities (ILAs) (2.65% [3324 of 125 600 participants]), and pulmonary infection (0.90% [1123 of 124 477 participants]). Extrapulmonary malignancy (74 of 125 257 participants), aortic aneurysm (78 of 125 256 participants), and pleural and/or pericardial effusion (75 of 125 253 participants) were each observed in less than 0.1% of participants. All S modifiers were associated with increased all-cause mortality, with adjusted hazard ratios (HRs) of 8.28 (95% CI: 5.48, 12.51) for pleural and/or pericardial effusion, 3.58 (95% CI: 1.97, 6.49) for extrapulmonary malignancy, 3.28 (95% CI: 1.71, 6.32) for aortic aneurysm, 2.16 (95% CI: 1.89, 2.47) for ILAs, 1.41 (95% CI: 1.30, 1.53) for coronary artery calcification, and 1.15 (95% CI: 1.05, 1.25) for emphysema (P < .001 for all except for emphysema, with P = .002). LCA helped identify four distinct classes with a stepwise increase in mortality from isolated emphysema (adjusted HR, 1.22; 95% CI: 1.10, 1.36; P < .001) to high-risk modifiers (adjusted HR, 5.35; 95% CI: 3.40, 8.41; P < .001). Conclusion In a nationwide lung cancer screening group, structured reporting using seven standardized S modifiers revealed both their prevalence and associations with all-cause mortality, validating their clinical utility in identifying clinically significant abnormalities. © RSNA, 2026 Supplemental material is available for this article. See also the editorial by White and Gierada in this issue.