A research report submitted to the Faculty of Health Sciences, University of the Witwatersrand, Johannesburg; in partial fulfillment for the degree of Master of Medicine (Anatomical Pathology) 2015. / Cytological criteria distinguishing phyllodes tumour of the breast from fibroadenoma. Fibroepithelial lesions of the breast include fibroadenomas and phyllodes tumours. Fibroadenomas are benign tumours, whereas phyllodes tumours range from benign, indolent neoplasms to malignant tumours capable of distant metastasis and occasionally resulting in death. The aim of this study was to determine whether there are statistically significant differences between fibroadenomas and phyllodes tumours with regard to selected cytological features. A ten year retrospective review was performed of patients who had an excision of a fibroadenoma or phyllodes tumour, and on whom a pre-operative fine needle aspirate was performed. The following cytological criteria were assessed: number of stromal and epithelial fragments, stromal to epithelial ratio, stromal cellularity, Stromal borders, stromal atypia and proportion of background wavy spindled cells. The patient age, tumour laterality and tumour size were recorded. Fifty fibroadenomas and 17 phyllodes tumours were included. When compared with phyllodes tumours, fibroadenomas had a larger number of epithelial fragments, a smaller number of stromal fragments and a lower stromal to epithelial ratio. The stroma tended to be less cellular and less atypical compared with phyllodes tumours and the background cellular population contained less spindled cells. Fibroadenomas and phyllodes tumours differ with regard to various cytological features, possibly aiding in their distinction on fine needle aspiration biopsy. / AC2016