The Lung CT Screening Reporting and Data System (Lung-RADS) is a standardized quality assurance tool developed by the American College of Radiology (ACR). Its primary purpose is to structure the interpretation of findings from lung cancer screening computed tomography (CT) scans. Lung-RADS assigns a numerical score to the most suspicious lung finding, categorizing the level of cancer suspicion and providing consistent, evidence-based recommendations for follow-up care. The current version is Lung-RADS v2022, released in November 2022.
Understanding Low-Dose CT Lung Screening
The Lung-RADS system applies exclusively to findings from Low-Dose Computed Tomography (LDCT) scans, which are the only recommended screening method for lung cancer. LDCT screening is designed for high-risk individuals who show no current symptoms, defined by U.S. Preventive Services Task Force (USPSTF) criteria based on age and smoking history.
Current guidelines recommend annual screening for adults between 50 and 80 years old who have a smoking history of at least 20 pack-years. Eligibility requires the individual to be a current smoker or someone who has quit within the last 15 years. Screening is discontinued once a person has not smoked for 15 years or develops a health condition that limits their life expectancy or ability to undergo curative surgery.
The goal of LDCT is to detect malignant nodules at an early, treatable stage. Applying Lung-RADS moves the process beyond simple detection to a structured risk assessment. The scoring system provides the framework for determining whether a detected nodule is likely benign or requires further investigation, which is necessary due to the high number of non-cancerous nodules appearing on screening CTs.
Interpreting the Lung-RADS Categories
The Lung-RADS score ranges from 0 to 4, with subcategories 4A, 4B, and 4X, reflecting an increasing probability of malignancy. Category 1 signifies a negative result, meaning no nodules are present or any nodules found are clearly benign, such as those with complete calcification. The probability of cancer is less than one percent. Category 2 also represents a finding that is almost certainly benign, with a cancer risk still under one percent, typically including small solid nodules less than six millimeters at baseline. Juxtapleural nodules are also commonly classified as Category 2 in the v2022 update.
Category 3 indicates a probably benign finding, where the chance of malignancy is estimated to be between one and two percent. This score is assigned to solid nodules measuring between six and eight millimeters in mean diameter at baseline or new solid nodules between four and six millimeters. Part-solid nodules, which contain both solid and ground-glass components, may also fall into this category if the total diameter is six millimeters or greater but the solid portion is less than six millimeters. The score of 0 is reserved for an incomplete study, often when previous CTs are unavailable for comparison or when findings suggest acute infection or inflammation.
A score of 4A is considered suspicious and carries a malignancy risk of five to fifteen percent. This category includes baseline solid nodules greater than or equal to eight millimeters but less than 15 millimeters, or new solid nodules between six and eight millimeters. Category 4B is highly suspicious, with a cancer probability exceeding fifteen percent, and is assigned to the largest nodules, such as solid nodules 15 millimeters or larger.
The v2022 system emphasizes nodule growth, defining it as an increase of more than 1.5 millimeters in mean diameter over a 12-month interval. Any nodule exhibiting such growth, if not already in a higher category, is typically elevated to a 4A or 4B classification. Category 4X is reserved for findings that are particularly concerning but do not strictly fit the 4B size criteria, such as a stable or growing nodule within a segmental or more proximal airway. This 4X designation signifies a need for the highest level of diagnostic workup.
Recommended Management Based on Score
The Lung-RADS score directly dictates the next steps in patient care. For Category 1 and 2 results, the standard recommendation is to return to the routine annual screening schedule. These findings are considered stable or unequivocally benign and require no short-term follow-up imaging.
A Category 3 finding triggers a short-interval surveillance CT scan, typically scheduled for six months after the initial exam. This short-term follow-up is part of a “stepped management” approach, allowing for reassessment to confirm stability. If the nodule remains unchanged at the six-month mark, the finding is downgraded to a Category 2, and the patient returns to yearly screening. This avoids unnecessary invasive procedures while maintaining close surveillance of potentially significant lesions.
Findings classified as Category 4A require a more urgent three-month follow-up LDCT to assess for rapid growth. If the nodule is stable or decreasing at three months, the score is downgraded to Category 3, which then follows the six-month surveillance pathway. However, if the nodule grows or shows concerning characteristics, the patient is referred for a diagnostic workup.
The most suspicious categories, 4B and 4X, mandate a referral for diagnostic evaluation, bypassing short-term surveillance. This evaluation may involve advanced imaging techniques like a Positron Emission Tomography (PET) scan, especially for solid components eight millimeters or larger, or a referral for tissue sampling, such as a biopsy. While Lung-RADS provides these standardized guidelines, the ultimate management decision remains a collaboration between the interpreting radiologist, the screening team, and the patient’s clinical care team.