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PET Scans Can Uncover Difficult-to-Find Thyroid Cancer and
Predict Survival Once Metastases Are Found

(PALM BEACH, FLA., Sept. 18, 2003) - A noninvasive imaging test, known as a PET scan, can find metastatic thyroid cancer cells -thyroid cancer that has spread to other parts of the body -that are invisible by radioactive iodine scans, and it can also predict the patient's prognosis, according to a new study being presented today at the 75th Annual Meeting of the American Thyroid Association.

There are approximately 23,000 new cases of thyroid cancer each year in the United States and 1,500 deaths, which amounts to nearly 300,000 thyroid cancer survivors who should be under observation. Some patients with lung metastases, for example, can live for 20-30 years without an obvious progression of the disease while in others the cancer will steadily progress over just two to three years.

Radioactive iodine scans are the most commonly used method of treating and monitoring thyroid cancer. Thyroid cancer cells absorb radioactive iodine (I-131) similar to how regular thyroid cells naturally absorb potassium iodide. Cancer cell deposits that absorb I-131 are called RAI positive. An I-131 scan can reveal areas of uptake outside the region of the neck that may indicate metastases of thyroid cancer. This may prompt further investigation with other radiographic tests or further treatment with I-131, which kills cancer cells.

Differentiated thyroid cancer cells may transform over time and lose some or all of the ability to take up and retain I-131. However, these cells may still be able to absorb a different radioactive tracer called 18-fluorodeoxyglucose (FDG), a form of sugar that can be detected by the positron emission tomography (PET) scan, which produces three-dimensional images of the body at work. After a patient receives a dose of FDG, the images produced by the PET scan can reveal abnormal areas of increased uptake, called FDG positive, suggestive of thyroid cancer cells.

Due to the difficult nature of predicting the outcome of patients who develop metastases from thyroid cancer, researchers at Memorial Sloan-Kettering Cancer Center (MSKCC) in New York City reviewed the records of thyroid cancer patients who underwent comprehensive diagnostic testing during 1996-2002 to find out the prognostic value of the PET scan.

Patients were followed for one to seven years. Those with medullary thyroid cancer and other active cancers were excluded. Of the 403 patients included in the analysis, 57 percent were women, 61.5 percent were RAI positive, and 55 percent were FDG positive. Their average age was 54 years old.

The researchers found that age, uptake, and the number of FDG deposits, or lesions, were significantly associated with survival. Each 10-year increase in age at PET increased the risk of death by 28 percent. Each additional FDG lesion increased the risk of death by 9 percent. A positive FDG scan, by itself, was associated with an eight-fold increased risk of death.

The researchers found that patients who had a negative PET scan have excellent short- and medium-term prognoses, regardless of any other clinical characteristic. Only three deaths, two of which were from other causes, occurred in the 179 patients who had a negative FDG scan, making it unlikely that a person would die in the next five years of thyroid cancer if they had a negative FDG-PET scan.

Patients who have a positive PET scan may need more aggressive therapy. "The positivity of the scan, however, does not directly tell the clinician how to treat the patient," said Richard Robbins, MD, lead author of the study and chief of the Endocrine Service at MSKCC and professor of medicine at Weill Medical College of Cornell University. "However, we have found that FDG positive lesions are somewhat resistant to therapy with radioactive iodine."

"There is no perfect test to find residual thyroid cancer," added Dr. Robbins. "A negative FDG-PET scan result does not mean that the patient is cured. It only means that there is no rapidly growing, aggressive, site of metastatic thyroid cancer. Small slow growing or dormant lesions may be missed by a PET scan."

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