Lung cancer remains a leading cause of cancer death worldwide, largely because it is often detected at advanced stages when treatment options are limited. The introduction of computed tomography (CT) scanning for high-risk individuals has provided a powerful tool for early detection, offering a significant opportunity to improve survival rates. This technology allows physicians to visualize the lungs and identify small, suspicious changes that would otherwise go unnoticed. Understanding the accuracy of these scans involves looking beyond a simple positive or negative result to examine the specific measures used to evaluate performance in a screening setting. This breakdown explains the scientific metrics and clinical workflow that define how reliable a CT scan is for finding early-stage lung cancer.
The Role of Low-Dose CT Screening
Low-dose computed tomography (LDCT) is the imaging technique used for screening, which is distinct from a standard diagnostic CT. LDCT uses a lower amount of radiation, making it suitable for annual screening of people who do not show any symptoms. The procedure is quick and non-invasive, creating detailed three-dimensional images of the entire chest. The primary goal of using LDCT is to catch small, localized tumors before they spread, significantly increasing the likelihood of a successful cure.
Screening is highly targeted toward individuals at the highest risk due to their history of smoking, and is not recommended for the general population. Major health organizations recommend yearly LDCT scans for adults between the ages of 50 and 80. Eligibility requires having a smoking history of at least 20 pack-years, meaning smoking an average of one pack of cigarettes per day for 20 years. Crucially, this screening benefit extends to people who currently smoke or those who have quit within the last 15 years.
Measuring Accuracy: Sensitivity and Specificity
The accuracy of any medical test, including an LDCT scan, is measured using two statistical concepts: sensitivity and specificity. Sensitivity refers to a test’s ability to correctly identify a person who has the disease, meaning it is the true positive rate. A highly sensitive test is effective at minimizing false negative results, ensuring that existing cancers are not missed. For example, the landmark National Lung Screening Trial (NLST) reported a sensitivity of approximately 93.1% for the detection of lung cancer.
Specificity, conversely, measures a test’s ability to correctly identify a person who does not have the disease, which is the true negative rate. A highly specific test effectively minimizes false positive results, reducing unnecessary follow-up procedures for healthy people. Specificity rates observed in major trials show a greater variation compared to sensitivity, often depending on the specific protocol used. The NLST reported a specificity around 76.5%, while the European-based NELSON trial, which used a more advanced nodule volume measurement protocol, reported a higher specificity of about 95.8% at baseline.
The difference in specificity results highlights how the method of evaluating suspicious findings directly impacts accuracy. The NELSON trial’s use of volumetric analysis, which measures the three-dimensional size and growth rate of a nodule, proved more effective at distinguishing benign nodules from true cancers than the traditional diameter-based measurement used in the NLST. The high negative predictive value of these scans, consistently reported near 99% in both trials, provides strong reassurance that a negative result is indeed correct.
Understanding Inaccuracies: False Results and Limitations
Despite high overall accuracy, LDCT screening is not perfect and can produce two types of incorrect results: false positives and false negatives. A false positive occurs when the scan identifies a suspicious finding, usually a small lung nodule, that is ultimately determined to be non-cancerous. These nodules are often caused by old infections, inflammation, or benign scarring and are present in up to 96% of high-risk patients. The cumulative probability of having at least one false-positive result over multiple annual screenings can be high, reaching over 30% after just two years of screening.
While a false positive does not mean a person has cancer, it often leads to patient anxiety and requires further testing, which can include additional CT scans or, in a small percentage of cases, invasive procedures like a biopsy. Factors that increase the chance of a false positive include having a history of emphysema or chronic obstructive pulmonary disease (COPD), being older, and undergoing the initial baseline scan.
A false negative is the less common, but more concerning, outcome where a small cancer is present but is not detected by the scan. This can happen if the tumor is very small, located near the heart or diaphragm, or obscured by surrounding scar tissue. The risk of a false negative is reflected in the fact that the test sensitivity is not 100%. Some aggressive, fast-growing tumors may develop between annual screening intervals, leading to a late-stage diagnosis. The challenge for radiologists is to differentiate between the vast majority of benign findings and the small number of true, early-stage cancers.
Next Steps Following Screening Results
After an LDCT scan is performed, the findings are categorized using a standardized system called the Lung CT Screening Reporting and Data System (Lung-RADS). This system was developed by the American College of Radiology to ensure consistent interpretation and follow-up across all screening centers. Lung-RADS assigns a category score from 1 to 4 to every scan, directly correlating the findings with the estimated probability of cancer. A Lung-RADS 1 or 2 result indicates a negative or benign finding. The recommended action for these low-risk results is to return for the next annual screening in 12 months.
If the results are uncertain, such as a Lung-RADS 3 finding, which has an estimated cancer risk of 1-2%, a short-term follow-up LDCT is typically recommended in six months. This strategy uses the scan’s ability to detect growth to determine malignancy; if the nodule is stable, it is likely benign. Findings that are more suspicious, such as Lung-RADS 4A (cancer risk 5-15%), usually prompt a repeat CT scan in three months or a positron emission tomography (PET) scan.
The most concerning categories, Lung-RADS 4B and 4X, indicate a high probability of malignancy and require the most aggressive follow-up. These results typically lead to immediate diagnostic evaluations, which may include a PET scan to check for increased metabolic activity characteristic of cancer cells. If the concern remains high, a tissue biopsy is often performed to obtain a definitive diagnosis. This structured approach ensures that patients with highly suspicious findings receive timely, definitive care while minimizing unnecessary invasive procedures for those with benign nodules.