Assessment of thin-layer breast aspirates for immunocytochemical evaluation of HER2 status

OBJECTIVE: To determine whether immunocytochemistry (ICC) for HER2 on ThinPrep(R) (TP)-processed breast fine needle aspiration biopsies (Cytyc Corp., Boxborough, Massachusetts, U.S.A.) is comparable to the findings of immunohistochemistry on corresponding surgically removed tissue. STUDY DESIGN: Immunostaining was performed on 63 malignant breast fine needle aspirates and compared to immunostaining on paraffin sections (PSs)from the subsequent biopsies. The HercepTest(TM) (Dako, Carpinteria, California, U.S.A.) and TAB250 antibodies were utilized. Cases in which the TP and paraffin HER2 results did not correlate were further assessed for gene amplification by differential polymerase chain reaction (dPCR). RESULTS: HER2 overexpression was found in 9 of the 63 cases (14%). TAB250 versus TP (P=.008), and TAB250 had higher specificity on PS versus the HercepTest(TM) on PS and TP (P=.004 and .0001, respectively). CONCLUSION: HER2 immunostaining with both the HercepTest(TM) and TAB250 on TP is unreliable due to low specificity (72% and 83% for HercepTest(TM) and TAB250, respectively). However, both antibodies have high sensitivity (89% and 100%, respectively), suggesting that this method may have some utility as a preliminary screening test for HER2 status. Negative HER2 staining by ICC is highly predictive of the absence of HER2 overexpression, whereas positive HER2 staining on TP would require further validation by either dPCR or fluorescence in situ hybridization.