Concordant HER2 status between metastatic breast cancer cells in CSF and primary breast cancer tissue

It is not known whether the HER2 status of malignant CSF cells coincides with that of the original breast carcinoma cells. We investigated whether CSF cytology specimens were suitable to evaluate HER2 status by fluorescence in situ hybridization (FISH) in patient with leptomeningeal metastasis (LM). Both formalin-fixed paraffin-embedded (FFPE) breast cancer tissue and liquid based CSF cytology specimens were evaluated for HER2 status in 16 patients with LM. We evaluated HER2 gene amplification using FISH on destained CSF cytology slides containing a minimum of 20 malignant cells per slide, and compared these with the HER2 status by immunohistochemistry (IHC) or FISH in FFPE tissues. HER2 was considered positive when the HER2:CEP17 ratio was a parts per thousand yen2.0 or IHC 3+. Of 16 cases, four were HER2 positive and 12 were HER2 negative by FISH analysis in CSF cytology. All CSF-positive cases were HER2 positive by IHC in FFPE tissue. Of 12 HER2 FISH-negative cases in CSF cytology, 10 were HER2 negative (IHC 0 or 1+) and two were IHC 2+ in FFPE tissue. Two IHC 2+ cases had HER2:CEP17 ratios of 1.27 and 2.1, respectively, by FISH in FFPE tissue. As a result, the HER2 status concordance rate between metastatic breast cancer cells in CSF and FFPE primary tissue by IHC and FISH was very high. When CSF cytology specimens were appropriately prepared and had adequate cellularity without dry artifacts, the CSF cytology was suitable to evaluate HER2 status by FISH analysis in patients with LM.