Fluorescence In-Situ Hybridization (FISH)
This study is primarily used to determine which women with HER2/neu metastatic breast cancer are most likely to benefit from the breast cancer therapeutic drug Herceptin. The FISH test, which can be done on new or old tumor tissue samples, is considered the most accurate test for HER2/neu gene over expression.
Description of Test Results
FISH test results are reported as either positive or negative:
"FISH positive" means that excessive amounts of the HER2/neu cancer gene are present. "FISH positive" tumors tend to behave more aggressively, but respond best to Herceptin.
"FISH negative" means that normal amounts of the HER2/neu cancer gene are present.
Most laboratories still use IHC to measure the amount of HER2/protein in the cells. IHC test results are reported in numerical categories:
- "0 to 1+ HER2/neu by IHC" means that a normal amount of the HER2/neu protein is present.
- "2+ HER2/neu by IHC" means that a moderately high amount of the HER2/neu protein is present.
- "3+ HER2/neu by IHC" means that an excessive amount of the HER2/neu protein is present.
The 0 to 1+ and the 3+ categories are considered reliable, but the 2+ category has a tendency to underestimate (called a "false negative") or overestimate ("false positive") the HER2/neu cancer gene's activity. As a result, many facilities restrict FISH testing to +2 IHC tumors to make sure the final result is as accurate as possible.
FISH testing is better than IHC testing at predicting the response to Herceptin. Patients with advanced breast cancer and identified as 3+ HER2/neu positive by IHC testing are likely to derive the most benefit from Herceptin. Patients with IHC result of 2+ should consult their physician about FISH testing to determine the true status of HER2/neu.
