The role of PET scanning in lymphoma

Peeyush Bhargava MD
Last Modified: August 4, 2002

Dear OncoLink "Ask The Experts,"
I would like to have some idea about the role of PET scan in lymphomas. Should we use PET even if CT scan is normal? Does a negative PET Scan exclude and positive confirm residual disease?

Peeyush Bhargava MD, Chief Fellow in the Department of Nuclear Medicine at the University of Pennsylvania, responds:

PET scans are playing an ever-increasing role in the management of patients with lymphoma. They are useful at the initial diagnosis and work-up to help in disease staging. They can also be very helpful in evaluation of response to therapy. They have been proven to be superior to the conventional imaging modalities in some circumstances. Various studies have reported very high sensitivity and specificity of PET in the diagnosis of lymphoma and the identification of metabolically active lesions.

The images below (Figure 1) are from a patient who was diagnosed with intermediate grade lymphoma in a mesenteric abdominal mass. The patient was treated with chemotherapy and presented with a mass in the left lower neck at a follow-up examination. A CT Scan of the neck was performed but did not show any abnormality. A PET scan was then performed that showed active lesions in the lower neck and in the abdomen.

In figure 1, images 1 and 2 are the images from the CT scan of the patient. These do not demonstrate any abnormality in the region of the swelling in the neck. Image 3 is the PET scan of the neck region, which shows foci of intense activity in the lower neck in the region of the swelling. Image 4 is the image from the CT scan showing a mass in the abdomen and this area shows intense activity on the PET scan as seen on image 5. Image 6 shows multiple lesions in the spine and image 7 is the whole body projection image. In this patient with known lymphoma in the abdomen, the PET scan identified more lesions in the neck and in the bones.

PET scans can be superior because of the excellent contrast resolution and the useful metabolic information they provide. The fact that the whole body can be imaged in one scan and both soft tissue and bone lesions can be detected have made PET scans very popular.

The rare false negative and false positive scans with PET can be due to some specific circumstances. Very rarely, some low-grade lymphomas may not show increased FDG activity, and not look "hot" on the scan and be missed with PET. Also, PET has limited role in evaluating small lesions that are less than one centimeter in size as there may not be enough activity in these lesions to show up on a scan. Sometimes inflammation from and infection or recent surgery can light up on a PET scan and be misinterpreted as cancer.