New objective criteria improve PET scan reliability in detecting breast cancer metastasis

The spread of breast cancer cells to lymph nodes under the arm is a significant indication of the stage of the disease and the patient's prognosis, and lymph node biopsies or even surgical removal often are needed to determine whether metastasis has occurred.

In an effort to develop noninvasive options to assess lymph node involvement, PET scans (positron emission tomography) and other technologies are being refined and studied for their ability to detect breast cancer cells in lymph nodes. But while noninvasive imaging procedures offer reduced discomfort and risk, scans may be open to interpretation when analyzed visually.

PET scans for invasive breast cancers and their metastases are performed after injection of a glucose-like substance called fludeoxyglucose F 18 (FDG) and a radioactive tracer. Because malignant cells use more glucose than normal cells, FDG accumulates in them and the tracer makes them visible on the PET scan.

The key to accurate diagnosis, according to the article, is the amount of tracer uptake – a “standardized uptake value” (SUV). At Cedars-Sinai, axillary nodes with SUVs of 2.3 or greater were 15 times more likely to contain metastasis than nodes with SUVs less than 2.3. In fact, with the SUV threshold set at 2.3, 60 percent of those with axillary metastases were identified (60 percent sensitivity), there were no false positive readings (100 percent specificity) and all those with a positive result were accurately diagnosed (100 percent positive predictive value). Accuracy was 72 percent.

“The purpose of the article is not to advocate using PET scans to detect axillary node metastasis but to suggest that if physicians are going to perform a PET scan before surgery for systemic staging, an SUV should be calculated. PET should be performed and interpreted in the proper clinical context, and SUVs should be used as an adjunct to clinical judgment,” said Edward H. Phillips, MD, the paper's senior author who is executive vice chairman of Cedars-Sinai's Department of Surgery, chief of the Division of General Surgery, and director of the Saul and Joyce Brandman Breast Center.

Although Cedars-Sinai's threshold was 2.3, that number is expected to vary from one institution to another because of technical and calibration factors. Therefore, each PET center is advised to develop it own reference values.

“FDG-PET is not a perfect test, but by quantifying the cancer-cell activity in the axillary lymph nodes, it can become a more accurate test,” said Phillips. “As we continue to see advances in technologies, I believe we will be able to identify smaller and smaller tumors with greater accuracy.”

In their study, the researchers reviewed the medical records of 462 women with invasive breast cancer who underwent FDG-PET for staging at the Saul and Joyce Brandman Breast Center between Nov. 1, 2001 and Aug. 31, 2005. The study focused on the cases of 51 women (with 54 invasive cancers) who had PET scanning performed before undergoing axillary lymph node surgery or chemotherapy.

It was determined an SUV greater than 2.3 was sufficient to diagnose lymph node involvement, and that there would be no need for sentinel lymph node biopsy or needle biopsy. The study suggests that with SUV validation of metastasis, chemotherapy can be initiated or the surgeon can proceed directly to axillary lymph node dissection to control the spread of the disease.