A cautionary note regarding the application of Ki-67 antibodies to paraffin-embedded breast cancers.

Immunocytochemical studies examining the Ki-67 proliferation marker in paraffin-embedded material have recently been made possible by the availability of several antibodies, notably MIB-1, which are readily applicable to microwaved sections. Using breast cancer material, the present study examines correlations shown by these new paraffin assays and also by PCNA (proliferating cell nuclear antigen), an existing marker of proliferation, with the established Ki-67 cryosection assay. Paraffin sections were microwaved prior to incubation with Ki-67 or MIB-1 antibodies. Signal detection was carried out with a biotinylated secondary antibody, peroxidase-conjugated streptavidin, and DAB/H2O2 chromogen. The results suggest that caution is required when studying proliferation in paraffin-embedded breast cancers by immunostaining using Ki-67 antibodies. Nuclear staining in wax sections (Ki-Par, MIB-1, PCNA) greatly exceeded that in cryosections (Ki-Froz) and thus correlations were notably absent between Ki-Par or PCNA immunostaining and the routine Ki-Froz assay. Immunostaining with MIB-1 or PCNA may, however, be useful to assess proliferation if cut-offs are applied to eliminate weak immunostaining associated with wax sections. Thus, an approximately linear relationship was seen between MIB-1/Ki-Froz, which was improved if only moderately or moderately/strongly MIB-1-positive cells were scored. Similarly, a significant correlation was also revealed between PCNA/Ki-Froz if such a cut-off was applied.