Immunohistochemistry Applied to Breast Cytological Material

Fine-needle aspiration biopsies (FNABs) of the breast are minimally invasive procedures enabling the diagnosis of suspicious breast lesions. Unfortunately, they are often perceived as inferior to core-needle biopsies, namely because they are supposedly unable to differentiate between high-grade ductal carcinoma in situ and invasive carcinoma or provide material for ancillary testing. Several studies have shown, however, that FNAB samples, when handled properly, are indeed capable of providing sufficient and adequate material for ancillary testing, namely immunocytochemistry (ICC). We reviewed the published literature regarding the use of ICC for both diagnostic and theranostic uses in the different types of cytological samples obtained from FNABs of the breast, including smears, liquid-based cytology samples, and cellblocks. We found that p63 and 34βE12 show promise in aiding in the differential diagnosis between in situ and invasive lesions and that most other diagnostic markers may be used as in tissue. Regarding theranostic ICC markers, results vary between publications, but with care, these can successfully be performed in cytological samples. Air-dried smears should be avoided, and cellblocks are overall more versatile than cytology slides, enabling the evaluation of not only hormonal receptors and HER2 by ICC, but also of Ki-67. Particular attention should be paid to fixation and antigen retrieval procedures in all cases. We recommend that laboratories without experience perform short validation runs before adopting these techniques into clinical practice.