DOI:10.1111/j.1365-2303.2007.00521.x Characteristic cytological features of histological grade one (G1) breast carcinomas in fine needle aspirates T. Sauer* ,  and M. Karimzadeh   *Department of Pathology, Ulleva ˚ l University Hospital, Oslo, Norway and   Faculty of Medicine, University of Oslo, Oslo, Norway Accepted for publication 8 August 2007 T. Sauer and M. Karimzadeh Characteristic cytological features of histological grade one (G1) breast carcinomas in fine needle aspirates Objective: To analyse the spectrum of nuclear features as well as dissociation pattern found in fine needle aspirates (FNAC) from histological grade 1 breast carcinomas and evaluate the critical cytological features of these lesions. Material and methods: The material consisted of FNAC smears from 494 histologically confirmed grade 1 breast carcinomas. All smears were revaluated for cell dissociation pattern, nuclear size, cell uniformity, nucleoli, nuclear margin and chromatin pattern. All features were compared with the histological subtype and cytological grading. Results: 73.9% of the cases were cytological grade 1, 24.3% were grade 2 and 1.8% were grade 3. The majority of the cases had a cell dissociation pattern showing both a population of single carcinoma cells and cell clusters (65.9%). Practically all tumours had a granular chromatin pattern (94.7%) and a slightly irregular nuclear margin with folds and grooves (94%) irrespective of histological subtype and cytological grading. Nucleoli were mostly indistinct or small (74%), whereas 24.3% were noticeable and 1.7% abnormal. Practically all cases revealed some degree of pleomorphism with 74.3% showing mild and 22.4% a distinct pleomorphism. A small subgroup of IDC was classified as monomorphic (3.3%). Almost all tumours had nuclear sizes in the range of 2–4 · RBC (96.9%). Conclusion: Not all histological grade 1 carcinomas are cytological grade 1. About 25% were grade 2, and a small subpopulation reached grade 3. The typical ⁄ average findings in FNAC from grade 1 breast carcinomas were a population of both groups and single cells showing mild pleomorphism, granular chromatin, slightly irregular nuclear margin, indistinct nucleolus and nuclear size 2–4 · RBC. Keywords: breast carcinoma, pathology grading, fine needle aspiration, FNAC, diagnostic pitfalls, cytodiagnosis, diagnosis, breast disease Introduction A diagnosis of breast carcinoma on fine needle aspiration (FNAC) is based on a number of cytological features. The Ôpattern of the smearÕ consists of growth pattern (tubular, papillary, micro papillary), cell dis- sociation pattern, arrangement of cells in clusters, sheets and aggregates. An additional component of extracellular material may give information about the tumour subtype. A general evaluation of the cellular pleomorphism is important. The nuclear criteria are critical and we carefully evaluate the size of the nuclei, irregularities of the nuclear margin, chromatin pattern and nucleoli in order to find diagnostic malignancy-associated abnormalities. Histological grade 1 breast carcinomas are charac- terized by a slight or moderate nuclear atypia and few mitoses. They often have a partly tubular growth pattern. The cytological abnormalities will often be discrete and a definite, preoperative cytological diag- nosis may occasionally be difficult even in cell rich and technically optimal specimens. 1 Correspondence: T. Sauer, Department of Pathology, Ulleva ˚ l University Hos- pital and Faculty of Medicine, University of Oslo, N-0407 Oslo, Norway Tel.: +47 22118921; Fax: +47 22118239; E-mail: torill.sauer@medisin.uio.no Cytopathology 2008, 19, 287–293 ª 2007 The Authors Journal compilation ª 2007 Blackwell Publishing Ltd 287