A lung nodule, often described as a small “spot” on the lung, is a common finding on chest X-rays or computed tomography (CT) scans. These growths are usually defined as tissue less than three centimeters in diameter, and the vast majority are not cancerous, often resulting from old infections or scar tissue. To assess the risk of malignancy, a Positron Emission Tomography (PET) scan is frequently used as a non-invasive diagnostic tool. The PET scan provides functional information about the nodule’s biological activity that complements the structural details provided by a standard CT scan.
Understanding the PET Scan Principle
The PET scan is a type of nuclear medicine imaging that observes the metabolic activity within the body’s tissues. This technique relies on the injection of a radiotracer, most commonly Fluorodeoxyglucose (FDG). FDG is a glucose analog, meaning it is chemically similar to sugar, and it is administered intravenously before the scan begins. This compound is taken up by cells throughout the body that use glucose for energy. Malignant cells are typically highly aggressive and rapidly dividing, requiring significantly more energy than normal, healthy cells. Consequently, these hypermetabolic cancer cells consume glucose at a much faster rate, causing the FDG to accumulate densely within the cancerous tissue. The PET scanner detects the radiation emitted by the concentrated FDG, creating an image where areas of high metabolic activity appear bright, or “hot.”
Interpreting the Scan Results
The PET scan provides a quantitative measurement of radiotracer uptake within the lung nodule. The primary metric used to evaluate this uptake is the Standardized Uptake Value (SUV). The SUV is calculated by comparing the concentration of FDG in the nodule to the average amount of radiotracer injected. A nodule showing intense FDG accumulation is described as “hot,” indicating high metabolic activity and a higher probability of malignancy. Conversely, a nodule with little to no uptake is considered “cold” and is associated with a benign finding. An SUV maximum value (SUVmax) greater than 2.5 is often used as a threshold that raises suspicion for cancer. However, this value is not absolute; some studies suggest a higher cutoff, such as 3.6, may be more specific due to the prevalence of inflammatory processes in the chest. While a high SUV is strongly suggestive of cancer, a definitive diagnosis always requires tissue sampling.
When PET Scans Cannot Give a Clear Answer
Despite its utility, the PET scan has limitations that can lead to misinterpretation, resulting in both false-positive and false-negative results.
False-Positive Results
A false-positive result occurs when a nodule shows high FDG uptake but is not cancerous. This is often caused by non-malignant conditions that also have high metabolic activity, such as active inflammation or infection. Specific examples include fungal infections, tuberculosis, sarcoidosis, and rheumatoid nodules, which can all appear “hot” on the scan.
False-Negative Results
False-negative results are also possible, meaning a cancerous nodule does not show significant FDG uptake. One common cause is the nodule’s size; very small lesions, typically less than eight to ten millimeters, may not accumulate enough radiotracer to be clearly visualized due to the scanner’s resolution limits, a phenomenon known as the partial volume effect. Another cause is the tumor type, as some slow-growing malignancies, like certain well-differentiated adenocarcinomas or carcinoid tumors, are not highly metabolic and therefore do not absorb much FDG. In these scenarios, the PET scan may be negative despite the presence of cancer.
Next Steps After the PET Scan
The PET scan result is just one piece of data, which is combined with the nodule’s appearance on CT, the patient’s medical history, and risk factors like smoking status to determine the subsequent course of action. If the scan is highly suspicious, showing a high SUVmax, the physician will recommend a tissue biopsy for a definitive diagnosis. This may involve a needle biopsy guided by a CT scan or a bronchoscopy, where a sample is taken for pathological examination.
If the PET scan is negative and the nodule is small with low uptake, the doctor may opt for active surveillance. This strategy involves carefully monitoring the nodule with scheduled follow-up CT scans, often at intervals like three, six, or twelve months, to ensure it remains stable.
If the scan is indeterminate due to moderate uptake or a small size that limits clear interpretation, the physician must weigh the potential risks of a procedure against the risk of delaying a cancer diagnosis. In these indeterminate cases, a more aggressive surveillance schedule or a biopsy may still be pursued to achieve a definitive answer.