Hodgkin Lymphoma and the Role of PET/CT

A PET/CT scan can help detect sites of active Hodgkin lymphoma in the body.

According to the American Cancer Society, doctors may choose from a number of different methods to take a sample of the involved tissue, or a biopsy, for testing and microscopic analysis. When PET/CT is available, it is used for staging the cancer. PET/CT scans combine CT and PET, or positron emission tomography to find out how far the cancer has spread and how big it has become.

This article will define PET/CT and discuss its role in Hodgkin lymphoma.

About PET and PET/CT

PET uses a radioactive form of sugar, or FDG, which is taken up, in general, by Hodgkin lymphoma cells quite well, or as clinicians say, with high avidity. In terms of the physics, the form of radiation is similar to that of X-rays, however, combined exposure from a PET-CT is about 10-20 times higher than exposure from a regular X-ray. Such scanning is associated with potential benefits but also potential risks from a substantial radiation dose and cancer risk. Thus, the risk-benefit ratio should be carefully weighed for each individual prior to every study.

PET scanning is generally more sensitive than CT in detecting lymphoma. For instance, PET may reveal disease in normal-sized lymph nodes and help to evaluate disease that is outside of the lymph nodes, but not evident on CT. PET is often combined with CT to allow a comparison of areas of higher radioactivity on the PET scan with the more detailed appearance of that area on the CT. Compared with conventional imaging, the use of PET/CT at baseline can influence staging and lead to a different treatment in a substantial number of cases.

PET/CT for Hodgkin Lymphoma

Guidelines from the National Comprehensive Cancer Network recommend the use of PET/CT for initial staging and final response evaluation in patients with HL. Because PET/CT is good for staging and response assessment during and after therapy, it is also being used to try to optimize therapy. With more and more patients participating in clinical trials, interim scans may be used for a variety of malignancies, including HL, to try to predict the need for a different therapy but are not formally recommended at present.

PET/CT is very good at detecting lymph nodes that have become involved with Hodgkin lymphoma. It is also good at detecting disease in the bone marrow and in other organs. The stage of HL depends on the number and location of involved lymph nodes and other involved structures. Studies have shown that the extra detecting power of PET/CT leads to some 10 to 15 percent of patients with HL being shifted to a more advanced stage -- more advanced, that is, than staging by CT, alone. So, more patients may be getting the treatment they need.

With radiation therapy often planned for early-stage HL patients, PET/CT has another potential advantage. In some cases, the more precise identification of initially involved regions or nodes may help doctors planning the radiation therapy to irradiate a smaller area, potentially impacting less of the healthy tissue.

PET/CT in Treatment and Prognosis

Hodgkin lymphoma is one of several lymphoma types that are considered the most curable. Anatomical staging—viewing where in the body the lymphoma is present and growing—is especially important in HL, not only because it helps inform the treatment decision, but also because it helps predict the overall prognosis.

Patients with early-stage disease may receive radiation therapy alone, while in more advanced disease, chemotherapy is offered and radiation may be used more for residual disease, according to Hutchings and colleagues.

Adult patients with early-stage disease have an overall survival exceeding 90 percent in many trials. In advanced-stage disease, overall survival is 65–80 percent. In both early-stage and advanced-stage disease, further grouping according to risk factors is often done.

In addition to more precise staging, PET/CT can help detect any disease that remains after treatment. This can be helpful, for example, when there is a residual mass, or enlargement in the chest and it is not clear whether it reflects scarring or active disease.