Pet scan vs CT scan

Oncology Applications

Early Detection: Since PET images biochemical activity, it can accurately predict whether a tumor is benign or malignant, thereby avoiding surgical biopsy when the PET scan is negative. Conversely, since a PET scan images the entire body, confirmation of non-local metastasis can alter treatment plans. In some cases this may mean a change from surgical intervention to chemotherapy. Some patients may avoid unwarranted surgery and a delay in the most appropriate treatment in these cases.

Staging of CA: PET is extremely sensitive in determining the full extent of disease, especially in lymphoma, melanoma, breast, lung, colon and ovarian cancers. Confirmation of advanced metastases allows the physician and patient to more accurately decide on how to proceed, especially if the patient is against extraordinary measures or decides not to undergo the rigors of chemotherapy in the time they have remaining. PET improves the quality of a patient's remaining time in these cases.

Checking for Recurrences: PET is the most accurate diagnostic procedure to differentiate tumor recurrences from radiation necrosis or post-surgical changes. Patients can often avoid additional surgery in these cases.

Tracking the Effectiveness of Cancer Treatment
Research indicates that PET is playing a growing role in tracking the effectiveness of chemotherapy and evaluating early response to a selected drug. The level of tumor metabolism is compared on PET scans taken before and after chemotherapy. A successful response seen on a PET scan frequently precedes alterations in anatomy and would therefore be an earlier indicator of tumor shrinkage than would be seen with other diagnostic modalities. The same assessment applies to tracking the effectiveness of radiation therapy.

Although PET has set its sights beyond the realm of cancer imaging, oncology remains the cornerstone application for the modality.

"PET will replace some studies that are considered conventional nuclear medicine now because of its tremendous capability to image the biochemistry of tumors noninvasively," said Dr. Steven Larson, chief of nuclear medicine at Memorial Sloan-Kettering Medical Center in New York City.

PET has been approved by the FDA for characterization of solitary pulmonary nodules, lung cancer staging, evaluation of colorectal cancer in patients with rising levels of cardioembryonic antigen, and imaging of patients who have lymphoma and melanoma. Researchers at several institutions are focused on establishing PET's comparative accuracy in other cancers.

"One of our primary research goals is to improve the noninvasive staging of tumors for better surgical management, and we're looking at specific circumstances, such as esophageal cancer, to validate PET as a diagnostic tool," Larson said.

Other sites are focusing on advancing PET's applications in brain tumor, which was the first clinical indication for fluorodeoxyglucose (FDG) in tumor imaging. Researchers have found the technique useful in patients with both metastatic and primary brain tumor who have been treated with chemotherapy and whose MRI scans show an increase in the residual abnormality (like necrotizing leukoencephalopathy).

"Morphologically, it's impossible to distinguish radiation change (like radiation necrosis) from recurrent tumor," Segall said. "PET in this situation is very accurate in distinguishing between those two diagnoses. And it is often used to base treatment decisions without further biopsies, since brain biopsy isn't easy and is very invasive."

In staging cancers of the head and neck, comparative studies report a slight statistical advantage for PET over CT or MRI, with some published reports documenting high sensitivity and specificity for PET. For example, FDG PET is more sensitive in detecting primary tumors (PET sensitivity, 89% to 100%; CT or MRI sensitivity, 68% to 92%), and lymph node involvement (PET sensitivity, 74% to 100%; CT or MRI sensitivity, 36% to 94%).

Other studies, however, document few such differences. But in no study has PET proved inferior to either CT or MRI, according to Dr. Val Lowe, a nuclear medicine specialist at the Mayo Clinic and president-elect of the Institute for Clinical PET.

Detecting the recurrence of disease in head and neck cancer is an area where PET clearly excels. In a prospective study of patients post-therapy, researchers found that PET can detect recurrence when it may be unidentifiable by other clinical methods.1

"PET is very useful in looking for residual disease after primary treatment, usually chemo and radiation, to determine whether salvage surgery is necessary," Segall said. "We also find it's useful in detecting mediastinal disease, which doesn't happen very often, but occasionally can occur with non-enlarged lymph nodes, so it's missed by CT."

Another PET application seeing widespread use is in differentiating recurrent tumor from scar after therapy for colorectal cancer. The technique has also made inroads in assessing thyroid tumor recurrence in postsurgical patients.

"The diagnostic evaluation usually includes another radioiodine scan, but that scan is negative about one-third of the time," Segall said. "So most physicians then order an ultrasound or MRI of the neck, but those studies are negative more than half the time. We find that about two-thirds of these patients can have their recurrent thyroid cancers localized by PET, permitting either follow-up surgery or external beam radiation therapy."