Positron Emission Tomography (PET) is a medical imaging technique that provides a functional view of the body’s tissues. The scan uses a small amount of a radioactive tracer to detect areas of high metabolic activity, which often corresponds to the presence of cancer cells. Lymphoma, a cancer originating in the immune system’s lymphocytes, is characterized by cells that divide rapidly and have an elevated rate of metabolism. This high cellular activity makes PET scanning an effective tool for visualizing and monitoring the disease throughout patient care.
How the Technology Works
The PET scan relies on the Warburg effect: the tendency of cancer cells to consume glucose at a much higher rate than healthy cells. To exploit this, a radioactive compound called Fluorodeoxyglucose (FDG) is injected. FDG is a glucose analog, meaning it is chemically similar to sugar and is transported into the highly active lymphoma cells through their abundant glucose receptors.
Once inside the cell, the FDG is phosphorylated, the first step of glucose metabolism. Because the FDG molecule contains a fluorine isotope, it cannot proceed further down the metabolic pathway and becomes trapped within the cell. The fluorine isotope then emits positrons, which are detected by the PET scanner to create a detailed three-dimensional map. This map highlights areas of intense glucose uptake as bright spots, revealing the exact locations of metabolically active lymphoma tissue.
Combining the PET scan with a Computed Tomography (CT) scan creates a PET/CT image, offering both functional and structural information. While the CT portion shows the size and structure of organs and lymph nodes, the PET component reveals the metabolic activity within those structures. This combination allows physicians to distinguish between inactive scar tissue and viable, active disease, a distinction traditional structural imaging like CT or MRI cannot reliably make.
Clinical Uses in Lymphoma Management
The PET scan is used at multiple junctures in the management of lymphoma, starting with the initial diagnosis. For initial staging, the scan accurately determines the full extent and location of the cancer, including sites outside the main lymph nodes. By revealing all metabolically active lesions, the PET scan informs the Ann Arbor staging system, which is used to classify the disease and guide treatment decisions.
Mid-treatment monitoring, often called an interim scan, is performed after a few cycles of chemotherapy to assess how well the treatment is working. A decrease in the metabolic activity of the lesions indicates that the lymphoma is responding to the therapy, providing important prognostic information. If the interim scan shows a poor response, the oncologist may adapt the treatment plan by switching to a more intensive or different chemotherapy regimen.
Following the completion of all planned therapy, an end-of-treatment scan is performed to confirm the outcome. This scan is useful for differentiating between a residual mass (which may just be scar tissue) and residual active disease. A scan that is clear of all metabolically active lesions indicates a complete metabolic response, which is the goal of treatment.
The scan is also used in surveillance for certain types of lymphoma, though this is not routine for all patients. In these cases, the PET scan helps monitor for potential recurrence by detecting new areas of abnormal metabolic activity. Visualizing the metabolic state of the tumor rather than just its size makes the PET scan a highly sensitive method for assessing the patient’s status.
Preparing for the Scan and What to Expect
Patient preparation is required to ensure the PET scan accurately reflects the tumor’s glucose uptake without interference from normal body activity. Since the tracer is a sugar analog, patients are instructed to fast for several hours before the procedure (typically six hours) to lower the natural blood sugar level. Elevated blood glucose can competitively inhibit the uptake of the FDG tracer by the lymphoma cells, potentially leading to a suboptimal or false-negative result.
Patients must also refrain from strenuous physical activity for at least 24 hours prior to the scan. Exercise causes the skeletal muscles to absorb significant amounts of glucose, which could obscure the image or create false-positive areas of uptake. Patients with diabetes will receive specific instructions from the nuclear medicine department regarding the timing of their insulin or oral medications.
The procedure begins with the intravenous injection of the FDG tracer, which is generally painless. After the injection, the patient is asked to rest quietly for 60 to 90 minutes to allow the tracer to circulate and accumulate in the metabolically active tissues. The scan itself takes place while the patient lies still on a table that slides into the scanner, usually lasting between 30 and 60 minutes. Total time spent in the department is typically two to three hours.
Interpreting the Scan Results
Interpreting a PET scan for lymphoma involves more than just a visual assessment of bright spots. One quantitative measure used is the Standardized Uptake Value (SUV), which is a numerical representation of the concentration of the FDG tracer in a specific area. A high SUV in a lesion correlates with intense metabolic activity and is indicative of active lymphoma, though this value is best used as an aid to visual interpretation rather than a standalone diagnostic number.
To standardize the assessment of treatment response, particularly in Hodgkin and certain non-Hodgkin lymphomas, oncologists use the Deauville Score, a five-point scale. This visual grading system compares the FDG uptake in the lymphoma lesion to two internal reference points: the mediastinal blood pool and the liver.
The Deauville scores are defined as follows:
- Score 1 indicates no uptake.
- Score 2 signifies uptake less than or equal to the mediastinal blood pool.
- Score 3 means the uptake is greater than the mediastinum but less than or equal to the liver.
- Scores 4 or 5 denote uptake moderately or markedly higher than the liver.
Scores of 1, 2, or 3 are frequently considered to represent a complete metabolic response following treatment. Inflammation or infection can also cause cells to become metabolically active, sometimes resulting in a false-positive reading that requires correlation with the patient’s clinical history and other test results.