CHICAGO--Positron emission tomography (PET) has excellent potential as a screening test for spotting axillary lymph node metastases in women with newly diagnosed, early-stage breast cancer, thereby sparing many women from unnecessary lymph node dissection, Lee Adler, MD, of Case Western Reserve University, said at the Radiological Society of North America meeting.
In his study, PET scans of the axilla that measured uptake of fluorine-18 deoxyglucose (FDG), a radioactive tracer that illuminates regional glucose utilization, identified nearly all women with positive lymph nodes.
Based on his analysis of 56 women, Dr. Adler believes that "a negative PET scan is strong evidence against the presence of axillary metastases." After categorizing all definitive, probable, and equivocal emission scans as positive, he found that FDG PET had a sensitivity and a negative predictive value of 95%.
Specificity May Be Sacrificed
Because the degree of uptake of FDG at times appeared to be the same whether or not a woman had metastases in the axillary lymph nodes, Dr. Adler acknowledged that specificity may be sacrificed in order to assure high sensitivity.
As one example, he pointed to a small focus of moderately increased tracer accumulation that was found in one women with metastatic breast cancer and a nearly identical focus of tracer accumulation seen in another woman who had a negative lymph node dissection. If foci with moderate tracer uptake are classified as positive in order to assure high sensitivity, "we're going to have to call these other foci positive as well, and we'll lose specificity," he said.
Despite a specificity and positive predictive value of only 62% and an overall accuracy rate of 75%, Dr. Adler believes that FDG PET is a promising screening tool for women with newly diagnosed breast cancer. "One might at first be disappointed by the relatively low specificity and overall accuracy values," he said. "But remember," he added, "we are talking about using PET as a screening test rather than a diagnostic test. What's most important for a screening test is a high sensitivity and an acceptable specificity, which I believe we have."
Another factor is the cost that may be saved by preventing unnecessary surgical dissection. If the lymph node dissection had not been performed in the 22 patients who had negative scans, Dr. Adler determined that FDG PET would have saved $180,000 in surgical costs. After subtracting the cost of performing FDG PET on all 56 patients, he calculated a total net savings of $2,000.
"The strategy of avoiding axillary lymph node dissection in the patients with negative PET tests would have prevented nearly two thirds of the negative lymph node dissections that were performed in our study group," he concluded.