6.2.1 Lung CT Screening Reporting and Data System (Lung RADS)

The ACR developed the “Lung-RADS” system for the specific reporting, follow-up, and management of both negative and positive LDCT screens. The first edition of the Lung-RADS classification scheme was modeled after the “BI-RADS” (Breast Imaging-Reporting and Data System), now in its fifth edition, long utilized for reporting screening mammography results. Like BI-RADS, Lung-RADS scores or categories are determined largely by the dominant lesion size and morphology. The size threshold for an actionable nodule or positive screen applying Lung-RADS is ≥ 6 mm for solid and part-solid nodules and ≥ 20 mm for nonsolid (ground-glass) nodules. On follow-up screening CT exams, the size cutoff is ≥ 4 mm for solid and part-solid nodules and/or an interval growth of ≥ 1.5 mm of preexisting nodule(s). New or growing nonsolid nodules must meet the ≥ 20-mm size threshold to be considered positive.

Five potential numerical Lung-RADS categories (0–4) may be assigned when reporting nodules on screening LDCT. Three modifiers (“X,” “C,” and “S”) can also be added to any one of these numerical categories if findings other than nodules are present. The “category number” plus the “modifier code” creates the final Lung-RADS score. The “X” modifier may be used when there are additional imaging findings such as spiculated borders, a rapidly enlarging ground-glass nodule with a doubling time less than 1 year, or enlarged lymph node(s) are seen. The “S” modifier denotes the presence of additional potentially clinically significant incidental findings (e.g., coronary artery disease, emphysema, aorta aneurysm, etc.; Chapter 7). The “C” modifier applies to those screening individuals with a prior established diagnosis of lung cancer returning for follow-up for screening. It may be added to any Lung-RADS category.

Lung-RADS 0 is rarely applied in those cases in which the screening exam could not be fully completed or one in which preexisting CT studies exist but are not available to the radiologist at the time of the initial screen interpretation. This score is then modified accordingly once the study is completed or those antecedent studies become available to the radiologist for direct correlation. Lung-RADS 1 is essentially a negative screen. That is one in which no nodules are seen on the screening exam or one that demonstrates nodules with distinctly benign patterns of calcification (e.g., completed calcified, peripheral egg shell calcifications, popcorn kernel-like central calcifications, or internal fat; ▶ Fig. 6.3). Lung-RADS 2 is also considered a negative screen, demonstrating nodules that have a very low likelihood of becoming a clinically active cancer due to their size, morphology, or lack of growth on follow-up studies. Both Lung-RADS 1 and Lung-RADS 2 screen results are followed annually with repeat LDCT as long as the individual remains eligible. Lung-RADS 3 studies are likely benign revealing nodules with a low likelihood of becoming a clinically active cancer but warrant closer surveillance with short-term follow-up imaging over 3 to 6 months for reassessment. Lung-RADS 4 is a suspicious screen with findings representing potential lung cancer until proven otherwise. These latter cases often warrant further additional diagnostic testing (contrast-enhanced chest CT, positron emission tomography [PET]/CT) and or tissue sampling. Lung-RADS categories 0 to 4 are summarized in ▶ Table 6.2. In the vast majority of screening studies, 90% of individuals will receive a score of Lung-RADS 1 or 2. About 4% of individuals will receive a score of Lung-RADS 3 and roughly 4% a Lung-RADS 4 score (▶ Table 6.3). Lung-RADS 4 is further stratified into the following: Lung-RADS 4A—solid nodules ≥ 8 to < 15 mm or part-solid nodules ≥ 8 mm with solid component ≥ 6 and < 8 mm; Lung-RADS 4B—solid nodules ≥ 15 mm or part-solid nodules with solid component ≥ 8 mm; and Lung-RADS 4X, which basically represents either category 3 or 4 nodules with additional features suspicious of malignancy (e.g., lymphadenopathy). The nodule with the highest individual Lung-RADS score ultimately defines the final assigned Lung-RADS category. “S” and “C” modifiers may also be applied to any Lung-RADS score on initial baseline and follow-up screens. A comprehensive breakdown of the Lung-RADS categories, description, imaging findings, management recommendations, probability of malignancy, and estimated prevalence in the screening population is provided in ▶ Table 6.4, and is also directly available through the ACR at their Web site (https://www.acr.org/~/media/ACR/Documents/PDF/QualitySafety/Resources/LungRADS/AssessmentCategories.pdf).