A Positron Emission Tomography (PET) scan is a functional imaging technique that provides insight into how tissues and organs work on a molecular level, unlike anatomical imaging (CT or MRI). For a PET scan, a small amount of a radioactive tracer, most commonly fluorodeoxyglucose (FDG), is injected into the bloodstream. FDG is a glucose analog. Because cancer cells often consume glucose at a much higher rate than normal cells, the tracer accumulates in malignant tissue. The PET scanner detects the energy emitted by the tracer, creating detailed images of metabolic activity used in cancer diagnosis and surveillance.
The Role of PET Scans in Melanoma Management
A PET scan is not a routine screening tool for early-stage melanoma, but it is valuable for patients with advanced or high-risk disease. It is primarily used for initial staging to determine if the cancer has spread beyond the primary tumor site. Doctors often order a PET/CT scan for patients diagnosed with Stage III melanoma (spread to nearby lymph nodes) or high-risk Stage II melanoma to check for distant metastases.
The scan helps assess the total burden of disease, which is a significant factor in determining the most appropriate treatment strategy. Finding an unsuspected distant metastasis can change a patient’s disease stage from Stage III to Stage IV, altering the management plan from surgery toward systemic drug therapies. PET/CT has a high sensitivity for detecting distant spread, particularly in the lymph nodes and visceral organs.
Another application is detecting recurrence, or restaging, after a patient has completed initial treatment. For individuals with a history of advanced melanoma, surveillance scans may be performed if blood markers are elevated or if new symptoms suggest the cancer has returned. A negative PET/CT result is highly reassuring, offering a high negative predictive value that the disease has not recurred.
PET scans are also used to monitor a patient’s response to therapies like targeted drugs or immunotherapy. By measuring changes in the metabolic activity of tumors, physicians determine whether the treatment is effectively shrinking or halting tumor growth. This metabolic assessment can provide an earlier indication of treatment success or failure than standard anatomical imaging alone.
Preparing for a PET Scan and What to Expect
Preparation for an FDG-PET scan is specific because the tracer is a glucose analog, requiring controlled blood sugar levels for an accurate image. Patients are instructed to fast for at least six hours before the procedure, consuming only plain water. This fasting ensures the FDG tracer does not compete with circulating glucose from food, which could reduce the tracer’s uptake by melanoma cells and affect image quality.
Another preparatory step is avoiding strenuous physical activity for 6 to 24 hours prior to the scan. Vigorous exercise can cause the tracer to accumulate in the muscles, creating confusing areas of uptake that can obscure potential tumor sites. The patient is also advised to stay warm, as cold temperatures can activate brown fat, a specialized tissue that consumes glucose and generates false-positive hot spots on the scan.
Upon arrival, a small amount of the FDG tracer is injected intravenously, usually into an arm vein. Following the injection, there is a waiting period, lasting between 45 and 90 minutes, during which the patient must rest quietly. This “uptake period” allows the tracer sufficient time to circulate and be absorbed by metabolically active tissues, including melanoma cells.
The scan takes place in a large, donut-shaped machine that combines the PET scanner with a CT scanner. The patient lies still on a table that slides through the machine; the imaging process typically lasts 20 to 45 minutes. The procedure is non-invasive and painless, and the small amount of radioactive tracer decays quickly after the scan. Drinking plenty of water post-scan is encouraged to help flush the remaining tracer from the system.
Interpreting the Scan Results
The fundamental principle behind interpreting an FDG-PET scan for melanoma is the detection of “hot spots,” or areas of increased tracer concentration. Melanoma cells exhibit a high metabolic rate, often referred to as the Warburg effect. This high metabolism means they aggressively draw in the FDG tracer, which appears as bright areas on the final image.
The results are provided by a nuclear medicine physician or radiologist who analyzes the combined PET/CT images. The CT component provides the anatomical map, while the PET component overlays the metabolic activity, allowing correlation between function and structure. The intensity of the FDG uptake is quantified using the Standardized Uptake Value (SUV), which helps characterize the malignancy of a lesion.
Interpreting these scans is complex because increased FDG uptake is not exclusive to cancer cells, leading to potential false-positive results. Any area of high metabolic activity, such as inflammation from an infection, recent surgery, or inflammatory conditions, can also create a hot spot. This is relevant in melanoma patients receiving immunotherapy, as the treatment can cause inflammatory side effects that mimic cancer activity on the scan.
Conversely, a false-negative result can occur when the scan fails to detect existing cancer. This may happen if the tumor is too small (less than eight millimeters) due to the physical limitations of the scanner’s resolution. Additionally, certain types of melanoma may be less metabolically active and not absorb enough FDG to register as a hot spot. It is crucial that scan results are always considered in conjunction with the patient’s full medical history and other diagnostic tests.