p27 Kip1 Expression and grading of breast cancer diagnosed on cytological samples

The progressive reduction in p27 Kip1 (p27) protein immunohistochemical staining with increasing histological grading is a well‐established finding occurring in breast cancer, and its role as diagnostic complement and prognostic marker has been thoroughly evaluated. To clarify whether this test may be applied to breast cytopathology, we performed p27 immunostaining on fresh fine‐needle cytology (FNC) samples from 10 benign and 40 malignant breast lesions. On average, p27 immunostaining was significantly lower in carcinomas than in benign lesions ( P < 0.005). In particular, among carcinomas, p27 immunostaining progressively reduced from well‐to poorly differentiated lesions (G1 vs. G2, P < 0.05; G1 vs. G3, P < 0.001; G2 vs. G3; P < 0.001). A similar trend was noted in a subgroup of 20 matched FNCs and histological samples of breast carcinomas, when p27 immunostaining on FNCs was stratified according to the histological grading (G1 vs. G2, P = 0.18; G1 vs. G3, P < 0.05; G2 vs. G3, P < 0.05). In addition, p27 immunostaining on FNCs showed a good positive correlation with that on histology (Spearman R = 0.58; P < 0.01), with a diagnostic concordance between samples of 85%, by using the standard 50% positive cell cutoff. Taken in concert, our data suggest that p27 immunostaining is a reliable marker of tumor cell differentiation in breast cytopathology as well as in histopathology. Accordingly, staining FNCs for p27 may be an useful complement in addition to cytological grading in the preoperative assessment of breast cancer. Diagn. Cytopathol. 2004;30:375–380. © 2004 Wiley‐Liss, Inc.