Diagnosing Mesothelioma: PET Scans

The positron emission tomography (PET) scan is an imaging test doctors commonly use to diagnose, stage and track the progression of mesothelioma cancer. With the help of gamma rays and radioactive materials, PET scans create three-dimensional internal images of the body.

Spotting Mesothelioma on a PET Scan

On PET scan images, cells with the metabolic traits of malignant mesothelioma show up as bright white spots. Healthy tissues appear gray and faded.

Compared to CT and MRI scans, PET scans produce less detailed images. However, PET-generated images provide useful representations of the biological processes occurring within cancerous cells. These images clearly display areas with abnormally high metabolic rates, which show up as bright white hot spots on a PET scan. The presence of these spots often indicates an area of cancerous growth.
By comparing PET scan images taken during and after mesothelioma treatment, doctors can monitor how well a patient responds to specific therapies. Tracking the disease this way provides doctors with crucial information about the cancer’s stage and progression, which strongly influences the patient’s treatment options and expected outcome.
PET scans are particularly useful diagnostic and prognostic tools because they show the location of cancer cells within the body in three dimensions, with more accuracy and finer detail than other tests.

The PET Scan Process

Definition: Fluorodeoxyglucose

Fluorodeoxyglucose (FDG) is the most common radiotracer used in mesothelioma PET scans. FDG is a chemically altered sugar molecule (hence the term glucose in the name). Because mesothelioma tumors metabolize glucose much faster than healthy cells, lesions that quickly absorb large quantities of FDG may be malignant mesothelioma.

A full PET scan appointment typically ranges from two to three hours. Before the scan, patients receive an injection of a radioactive chemical, known as a radiotracer. It takes about an hour before the radiotracer is fully distributed throughout the body. Once this uptake period is over, the patient lies down on a table that slides into a circular opening in the scanning machine.
During the scan, patients must remain as still as possible to ensure a clear image. The table passes through the scanner as it takes images of the patient's internal body structures. Throughout the scan, which typically takes one hour, the patient can communicate with the technician through a set of microphones.
Once the scan is complete, patients are free to leave. A nuclear medicine scientist will interpret the results and send them to the patient’s primary doctor within two or three days.

PET Scans for Diagnosing Mesothelioma

The PET scan is rarely the only imaging technique used to diagnose mesothelioma. Instead, doctors will often prescribe them after a patient's CT scan comes back with abnormal results.
PET scans are most effective at differentiating between cancerous and noncancerous lesions. Doctors often use PET scan results to determine whether a suspicious area is malignant mesothelioma or a benign condition such as pleural scarring. Because PET scans can immediately differentiate between cancerous and non-cancerous cells, they may eliminate the need for additional diagnostic biopsies. In cases where tissue sampling is still necessary, a PET scan can help identify the best area for an accurate biopsy.

In one study of 25 patients with various pleural diseases, FDG-PET imaging correctly identified all 16 patients with malignant mesothelioma or malignant metastatic lesions.

When nuclear technicians evaluate PET scans for malignant mesothelioma, they look for hot spots on the pleura, peritoneum and pericardium. These hot spots are cellular clusters that absorbed abnormally high concentrations of glucose-heavy radiotracers such as FDG. Since cancerous cells quickly metabolize large quantities of sugar as they grow, doctors can infer that lesions with hypermetabolic rates are malignant.
The doctors who interpret PET scans assign each hot spot a standardized uptake value (SUV). This number reflects the extent of the lesion's metabolic activity. Healthy tissues have an SUV of 1, while malignant mesothelioma tumors receive an SUV of 2 or higher. However, some benign inflammatory processes also have a high radiotracer uptake. As a result, some studies suggest that a lesion must have an SUV of 2.5 or higher to merit an official mesothelioma diagnosis.
Because PET scans can detect cellular changes that occur at the earliest stages of cancer development, they are especially useful for diagnosing early-stage mesothelioma. Unlike other imaging techniques, PET scans can help doctors diagnose the disease before tumors have begun to cause symptoms or spread throughout the body. Some studies suggest that new technologies combining PET scans with CT scans or MRIs may be even more effective at diagnosing mesothelioma than any of these scans on their own.

PET Scans for Staging Mesothelioma Tumors

After diagnosing a patient with mesothelioma, nuclear physicians look for clues about the current stage of the cancer. Mesothelioma staging is a crucial diagnostic step that influences the course of treatment and the patient’s prognosis.

According to one 2012 study, adding PET scans to conventional mesothelioma staging methods significantly increased the detection of malignant mesothelioma.

Some studies suggest that PET scans are more useful than CT and MRI scans for mesothelioma staging. PET scans are highly effective at revealing cancerous activity in the lymph nodes, which implies a later stage of cancer in the traditional TNM staging system. They are also effective for highlighting mesothelioma metastases that may not appear on other conventional imaging scans.
In one comparative study, doctors staged 15 mesothelioma patients based on results from PET and CT scan results. In nine of these patients, both test results indicated the same stage of disease. In two patients, the PET scans suggested a later stage of mesothelioma, while four patients received an earlier stage diagnosis. Although CT scans can pinpoint tumor location more accurately than PET scans, this study suggests that PET scan images are preferable in terms of accurate staging.

One small study found that mesothelioma patients whose FDG-PET scans displayed extreme metabolic activity generally experienced a poorer prognosis than patients with less metabolically active mesothelioma tumors.

By revealing the stage of the cancer, PET scans can also project a mesothelioma patient's survival. Because these scans help visualize a mesothelioma tumor’s volume and level of metabolic activity, doctors can use the results to predict how quickly the cancer will spread and how positive the patient’s prognosis will likely be. For patients with non-sarcomatoid mesothelioma, an FDG-PET scan may be an even more powerful predictor of survival than TNM staging.
Since anatomic stage is a major consideration for doctors while developing an appropriate mesothelioma treatment plan, PET scans can also help identify which patients are the best candidates for specific therapies.

PET Scans for Evaluating Response to Treatment

Oncologists may also use PET scans to evaluate how well mesothelioma patients are responding to treatment. Because CT scans are less effective at evaluating tumor response, some doctors prefer PET scans for mid- or post-treatment scanning.
PET scan images can show whether a tumor’s metabolic activity is increasing or decreasing, as well as whether or not the cancer has spread to other parts of the body. In one mesothelioma study, patients showing a positive metabolic response to treatment experienced a more delayed progression of disease than patients with metabolically unresponsive tumors.
Doctors may order follow-up PET scans every few weeks during and after treatment. By comparing the results of these scans, doctors gain a better understanding of the patient's response to treatment. When a follow up PET scan shows that a mesothelioma tumor's metabolic activity decreased by more than 25 percent, this is called a partial response. This represents a positive metabolic response to treatment.
Conversely, if the tumor's metabolic activity increases by more than 25 percent, doctors consider the disease to be progressive. When any change in the tumor's metabolic activity is less than 25 percent, the disease is considered stable. If a formerly malignant lesion displays the same metabolic rates as healthy tissues, doctors consider this a complete response to treatment.