A limitation of current cancer care is the difficulty of quickly assessing how well a therapy is working. However, expanding the use of existing positron emission tomography (PET) technology can provide early and accurate assessment of a tumor’s response to a particular therapy allowing physicians to better tailor a patient’s treatment, according to Malik Juweid, M.D., associate professor of radiology at the University of Iowa Roy J. and Lucille A. Carver College of Medicine.
Writing in the Feb. 2 issue of the New England Journal of Medicine, Juweid and co-author Bruce Cheson, M.D., of the Lombardi Comprehensive Cancer Center at Georgetown University Hospital, outline the advantages and limitations of PET imaging in assessing cancer therapy and suggest that increasing the role of PET imaging has the potential to further improve cancer care management.
PET is a non-invasive imaging technique that uses radioactivity emitted from injected tracer chemicals to measure and image biological activity. The most commonly used PET radiotracer is fluorodeoxyglucose (FDG), a radiolabeled form of glucose, which is consumed more avidly by tumors than by normal tissue.
In fact, Juweid’s work already has led to changes in how PET is used in cancer care.
In a study published in the Journal of Clinical Oncology in July 2005, Juweid and colleagues showed that adding FDG-PET to the standard computer tomography (CT)-based assessment criteria made the predictions of treatment outcome much more accurate for aggressive non-Hodgkin’s lymphoma. Based on this study, the new internationally accepted criteria will include PET as an essential part of the assessment and management of lymphoma.
“Basically, about half of patients were called partial or incomplete responders because CT found a residual mass after treatment despite lack of any other evidence of disease,” Juweid said. “However, PET showed that the residual mass was scar tissue rather than viable tumor in more than half of these patients. These patients were, therefore, reassigned as complete responders, and we found that they did just as well as patients who had been given the ‘all clear’ from the CT scan.”
The study showed that if PET is negative, patients can be safely observed without biopsy, and patients and physicians can be much more confidant that the patient will remain progression free. However, if a PET scan is positive and the findings are confirmed by a biopsy, patients can move more quickly to necessary salvage therapy.
Juweid, who also is a member of the UI Holden Comprehensive Cancer Center, adds that PET is currently a routine part of response assessment for patients being treated for aggressive lymphoma at UI Hospitals and Clinics.
PET is currently approved by the Centers for Medicare and Medicaid Services (CMS) for restaging in breast, colorectal, esophageal, head and neck, and non-small-cell lung cancers, as well as melanoma and lymphoma. Restaging means checking whether there is residual or recurrent tumor after completion of treatment and, if so, determining its extent.
Although PET is extremely useful and is routinely used for restaging in many cancers, Juweid sees an even more important role for PET as a tool to enable physicians to monitor tumor response during treatment and potentially to tailor the treatment based on the PET results.
Currently, the only CMS-approved clinical use for PET in monitoring response to treatment is for breast cancer. However, Juweid says that many studies are suggesting that PET could also be useful in monitoring the effect of treatment in other cancers.
“For PET to be approved for monitoring treatment, clinical trials will need to demonstrate that using PET to tailor treatment has a beneficial effect in terms of patient outcomes,” Juweid said.
Although benefit usually means improved patient outcomes, Juweid argues that other outcomes also could be viewed as beneficial to the patient. For example, stopping chemotherapy when a PET scan shows the treatment is not working would spare the patient from toxic side effects and might also prevent a tumor from becoming resistant to chemotherapy before a more potent treatment can be tried. Alternative treatment options would be pursued for these patients.
Juweid also suggests that another way to capitalize on PET’s ability to detect tumor response early will be using PET images to establish end points in trials aimed at testing cancer therapies. This approach could shorten the time required to evaluate drugs or therapies.
The UI has long-standing program in PET imaging. UI Hospitals and Clinics is equipped with a PET scanner for both clinical and research purposes, and another devoted purely for research, and a PET-CT scanner, which combines the spatial information obtained in a CT scan with the metabolic information from PET. There also are plans to purchase a second PET-CT scanner.