LP-9 Phillodes Tumor of the Breast: Histological Classification, Immunoprofile and Ultrastructure Stela Bulimbasic 1 , Danica Ljubanovic 1 , Ana Marija Bauer- Segvic 1 , Fabijan Knezevic 2 1 University Hospital Dubrava, Avenija Gojka Suska 6, Zagreb, Croatia E-mail: sbulimba@kbd.hr 2 University Hospital for Tumors, Zagreb, Croatia Phyllodes tumors (PTs) are rare biphasic neoplasms composed of benign epithelial elements and a cellular spindle cell stroma. They account for less than 1% of all breast neoplasms and approximately 2.5% of all fibroepithelial tumors [1]. Although they show many features of much commoner fibroadenomas, PTs are likely to recur and can be locally aggressive. In addition, a small percent of PTs show sarcomatoid features and have a potential to metastasize [2]. Phyllodes tumors usually occur as solitary unilateral tumors in middle aged women. On gross examination they are well circumscribed, vaguely lobulated, sometimes with leaf like fronds projecting into cystic spaces. One of the most characteristic histological features of PT is formation of variable sized cystic spaces lined with double layer of benign-looking epithelium into which projects hypercellular stroma. A cellular zonation may be noted with hypercellular areas in close vicinity to the epithelial surface, sclerotic changes at the periphery, and an admixture of fat/spindle cells in between. Mitotic activity can be brisk in the more cellular, subepithelial areas. Other features common in PTs include giant stromal cells, patchy myxoid changes, foci of pseudoangiomatous stromal hyperplasia and metaplastic changes[1-3]. Immunohistochemical analysis of stromal component showed constant and diffuse immunoreactivity for vimentin, while CD34, smooth muscle actin and desmin were detected in variable proportion of the cells. Dominant cell population in the stroma of benign PTs showed ultrastructural features of fibroblasts, while number of cells with myofibroblastic features varied from case to case. Ultrastructural features of the stroma in the most of analyzed malignant PTs were consistent with fibrosarcoma, sometimes with foci of leiomyomatous or rhabdomyoblastic differentiation [2]. World Health Organization recommends further subclassification of phyllodes tumors as benign, borderline or malignant. Grading is based on histological evaluation of stromal cellularity, pleomorphism,