What to Know about iPET

Doctor explaining results of PET/CT scan.

The two main categories of lymphoma are Hodgkin’s lymphoma and non-Hodgkin’s lymphoma, and PET/CT scans are used in various ways for imaging both, in addition to many other malignancies. Lymphomas are cancers that arise from the lymphoid tissue, which includes lymphocytes, a kind of white blood cell. This lymphoid tissue is found throughout the body, and especially in the lymphoid organs: the lymph nodes, spleen, thymus and bone marrow.


Both PET and CT are imaging techniques used to identify cancer. PET uses a radioactive form of sugar, which is taken up, in general, by lymphoma cells quite well, or as clinicians say, with high avidity. The exposure to radiation from combined PET/CT is about 10-20 times higher than from a regular x-ray, so the risks and benefits of each PET/CT scan need to be weighed carefully to help ensure that the benefits outweigh the risks.

Usefulness of PET/CT

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 that might be hard to see using CT alone. That said, CT is often the go-to scan when sharp, detailed images are needed, so PET is often combined with CT to allow comparison of areas of higher radioactivity on the PET scan with the more detailed appearance of that area on the CT.

PET/CT is the method of choice for the initial, or baseline, evaluation of many different types of lymphoma – Hodgkin’s lymphoma as well as common non-Hodgkin’s lymphoma types, including diffuse large B cell lymphoma, or DLBCL, and follicular lymphoma.

Limitations of PET

There may be false positives -- for instance, a tuberculosis infection may look similar to cancer on a PET scan.

False negatives are also possible since not all cancer types use glucose for fuel: certain cancers use glutamine or amino acids; additionally, if the metabolism of a cancer is slow it may not show up on the PET. This is one of the reasons a PET scan is done rarely in some types cancers such as prostate cancer.

Staging and Response Assessment

More accurate imaging is one of the many factors that have lead to advances in both initial staging and assessing the response to therapy.  Staging refers to seeing where in the body the cancer is growing, and how big the cancer has become. Response assessment traditionally refers to measuring how much the cancer has shrunk, receded or disappeared after the initially planned therapy is complete, or to confirm there has not been progression.

iPET, Interim Assessment and Response-Adapted Therapy

iPET is interim PET/CT imaging to get a look at how the tumor is responding to the chosen therapy, or whether or not there has been progression. iPET is done after staging, but before the end of treatment. Interim PET/CT imaging is increasingly used today, especially with so many patients participating in clinical trials.

Many different studies support a central role for PET/CT in staging and response assessment in Hodgkin and non-Hodgkin lymphomas, but its role in different types of lymphoma -- and its usefulness in scenarios involving changing or adapting therapy based on interim results -- is still being evaluated, and more research is needed.

In reporting on expert recommendations, Barrington and colleagues suggest that iPET may be very useful, but many questions still need to be answered. On the one hand, some international guidelines recommend interim imaging. And studies have shown that iPET is a strong indicator of prognosis in Hodgkin's lymphoma and aggressive non-Hodgkin's lymphoma types. On the other hand, experts point out that, for many malignancies, it is not yet known whether interim imaging actually improves outcomes.

Many different clinical trials are attempting to answer these questions for different cancers. The hope is that early detection of response to therapy -- or non-response -- can help fine tune the treatment needed and improve outcomes.

For now, according to the August 2014 article published in “Journal of Clinical Oncology,” the expert group makes the following recommendation:

“If mid-therapy imaging is performed, PET-CT is superior to CT alone to assess early response. Trials are evaluating the role of PET response–adapted therapy. Currently, changing treatment solely on the basis of iPET-CT is not recommended, unless there is clear evidence of progression.”


Cheson BD, Fisher RI, Barrington SF et al. Recommendations for initial evaluation, staging and response assessment of Hodgkin and non-Hodgkin lymphoma: the lugano classification. J Clin Oncol. 2014;32(27)3059-3068.

Barrington SF, Mikhaeel NG, Kostakoglu L, et al. Role of imaging in the staging and response assessment of lymphoma: consensus of the international conference on malignant lymphomas imaging working group. J Clin Oncol. 2014;32(27):3048-358.

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