“I lit up like a Christmas tree,” says Heather McLean of the PET (positron emission tomography) scan that accompanied her 2008 diagnosis of stage 4 follicular lymphoma, a slow-growing type of non-Hodgkin lymphoma (NHL). PET scans, during which a radioactive sugar-like molecule called fluorodeoxyglucose is injected and taken up preferentially by active cancer cells, are used to determine how much cancer is present and where it’s located.
The technique has proved its worth at the front and back ends of treatment. At diagnosis, PET scans provide a more accurate view of the extent of disease compared with CT (computed tomography) scans and detect a higher stage of disease in 20 to 40 percent of patients. In some cases, this warrants a more aggressive treatment approach, possibly offering a better shot at a cure. As an immediate follow-up to treatment, a PET scan can help to root out lingering disease and identify patients who might benefit from follow-up (consolidation) chemotherapy or radiation to kill the remaining cancer cells. A negative post-treatment PET scan predicts longer progression-free survival.
Despite the lack of data to justify them, periodic surveillance PET scans during remission are performed frequently, says Bruce Cheson, head of hematology at Georgetown University’s Lombardi Comprehensive Cancer Center in Washington, D.C. Although surveillance scans are reasonably good at spotting relapses, they are not cost-effective, given that most relapses are heralded by a return of symptoms. One study showed that interim PET scans detected relapse before symptoms appeared in less than 2 percent of patients with diffuse large B-cell lymphoma.
What’s also unclear is whether PET scan results can form the basis for modifying the course of treatment midway through. “So far, studies indicate that PET scans obtained partway through NHL treatment are not informative enough to allow you to customize your treatment for that individual patient,” says Brad Kahl, chairman of lymphoma research at the University of Wisconsin in Madison. There is some evidence that patients with early-stage Hodgkin lymphoma can benefit from treatment tweaks based on interim PET scan results, but so far this has not been the case for NHL. Thus, at this stage, interim PET scans are not recommended as part of routine treatment for NHL, although this is a topic of ongoing research.
The use of Rituxan (rituximab) and chemotherapy combinations to treat B-cell lymphomas has also muddied the waters, with patients having more false-positive interim PET scan results. Why Rituxan treatment results in fluorodeoxyglucose uptake in the absence of active cancer cells is not entirely clear but seems to be associated with increased inflammation. A new approach to looking at PET scan results could help, as studies have suggested that measuring the difference in fluorodeoxyglucose uptake from one scan to the next decreased falsepositive rates compared with a positive versus negative assessment. Whether the benefits of interim PET scans in NHL will ever outweigh the risks remains to be seen.
[Read more about B-cell lymphoma in "A Long and Winding Road"]