A PET scan of the lungs is an imaging test that detects abnormal cellular activity in lung tissue, most commonly used to evaluate whether a lung nodule or mass is cancerous. It works by tracking how cells consume sugar: cancer cells burn through glucose far faster than normal cells, so they light up on the scan. The test is almost always performed as a combined PET/CT, which layers metabolic information from the PET over the detailed anatomical images of a CT scan.
How the Scan Detects Cancer
Before the scan, you receive an injection of a glucose-based tracer that contains a small amount of radioactive fluorine. This tracer behaves like regular sugar in the body. Your cells pull it in through the same transport channels they use for glucose, and enzymes inside the cell begin processing it. But unlike real glucose, the tracer gets stuck partway through the process. It can’t be broken down further, so it accumulates inside the cell.
Cancer cells are particularly good at trapping this tracer for two reasons. First, they have far more sugar transport channels on their surface, pulling in glucose at a much higher rate than healthy tissue. Second, they lack enough of the enzyme that would normally release the tracer back out. The result is that cancerous tissue collects significantly more tracer than surrounding healthy lung tissue, creating bright spots on the scan that radiologists can identify and measure.
Why Your Doctor Ordered It
The most common reason for a lung PET scan is to evaluate a suspicious nodule or mass found on a prior CT scan or chest X-ray. PET/CT has a diagnostic accuracy of about 93.5% for identifying malignant lung nodules, making it one of the most reliable noninvasive tools for distinguishing cancer from benign growths. If the nodule shows high metabolic activity, your doctor will likely recommend a biopsy. If activity is low, monitoring with follow-up imaging may be sufficient.
Beyond that initial evaluation, PET scans of the lungs serve several other purposes:
- Staging a known lung cancer. After diagnosis, the scan helps determine how far the disease has spread. It detects involvement of nearby lymph nodes with about 90% accuracy and catches metastases that a standard CT misses in roughly 10% of patients. In up to 62% of cases, the cancer’s stage is reclassified after a PET/CT, which can significantly change the treatment plan.
- Monitoring treatment response. PET scans track whether chemotherapy or radiation is working. A drop in tracer uptake correlates with better outcomes. Patients who still show high activity after the first cycle of chemotherapy have a median survival around 12 months, compared with 34 months for those with low activity.
- Checking for recurrence. After treatment, if routine imaging picks up something suspicious, a PET scan can determine whether cancer has returned. It has a sensitivity of 98% and overall accuracy of 93% for detecting recurrence.
What the Results Mean
Radiologists measure tracer activity using a number called the SUV, or standardized uptake value. For lung nodules larger than 1 centimeter, an SUV of 2.5 or higher is generally considered suspicious for cancer. For smaller nodules (1 centimeter or less), that cutoff is less useful because small cancers may not accumulate enough tracer to reach 2.5, and the scan can miss them. There is no single SUV number that definitively separates benign from malignant in all cases, so your doctor interprets the result alongside your imaging history, risk factors, and other test findings.
It’s worth knowing that a “positive” PET scan doesn’t always mean cancer. Inflammation and infection can also cause cells to consume more glucose than normal. Tuberculosis is one of the most common culprits, especially when it forms nodules or masses that look similar to tumors on imaging. Inflammatory pseudotumors, which are benign growths caused by the body’s immune response, account for a significant portion of false positives as well. In one analysis, inflammatory pseudotumors and tuberculosis-related growths together made up nearly 80% of false positive diagnoses. If your scan lights up but your doctor suspects one of these conditions, a biopsy or additional testing will usually follow.
How to Prepare
You’ll need to fast for at least 4 to 6 hours before the scan, and some facilities require a full 12-hour fast. The goal is to keep your blood sugar low so that your cells are hungry for glucose when the tracer arrives. If your blood sugar is elevated, normal cells compete with abnormal ones for the tracer, making the scan harder to read. Diabetic patients may receive specific instructions about managing insulin and medications beforehand.
You should also avoid strenuous exercise for 24 hours before the scan. Working out causes muscles to take up more glucose, which can create bright spots on the image that have nothing to do with disease. On the day of the scan, wear comfortable clothes without metal zippers or snaps, since you’ll be lying still inside the scanner.
What Happens During the Scan
The appointment typically takes about 90 minutes total, though the actual scanning portion is much shorter. Here’s the general sequence:
A technologist places an IV line, usually in your arm, and injects the radioactive tracer. You then sit or lie quietly in a dimly lit room for about 60 minutes while the tracer circulates and gets absorbed by your cells. During this time, you’ll be asked to stay as still as possible and avoid talking, chewing gum, or using your phone, since muscle activity can pull the tracer into areas where you don’t want it.
After the uptake period, you lie on a narrow table that slides into the PET/CT scanner. The machine looks like a large donut, similar to a CT scanner. The active scanning portion takes about 30 minutes. You won’t feel anything from the scan itself, though lying still on a hard surface can be uncomfortable. The machine may make humming or clicking sounds.
Radiation Exposure
A PET/CT scan involves more radiation than a standard CT alone. The average effective dose from a whole-body PET/CT is around 17 to 18 millisieverts (mSv), with some variability depending on the equipment and protocol used. For context, a standard chest CT delivers roughly 7 mSv, and natural background radiation exposes you to about 3 mSv per year. The dose is considered safe for diagnostic purposes, but it’s one reason doctors don’t order PET scans as a screening tool for people without a known or suspected problem.
After the Scan
The radioactive tracer decays quickly. Fluorine-18, the radioactive component, has a half-life of about two hours, meaning half of the radioactivity is gone within that window. Drinking plenty of water after the scan helps your body flush the tracer through your kidneys faster. Research shows that hydrating and urinating within 30 minutes of the scan can reduce the radiation you emit by roughly 22 to 25%, on top of the natural decay. Most facilities recommend drinking several glasses of water in the hours after your appointment.
If you’ll be in close contact with young children or pregnant women, some centers suggest maintaining a short distance for a few hours after the scan as an extra precaution. Results are typically available within a few days, though urgent cases may be read sooner.