Molecular analysis of phyllodes tumors reveals distinct changes in the epithelial and stromal components

Phyllodes tumors are fibroepithelial mammary lesions that tend to behave in a benign fashion but may undergo sarcomatous transformation. ii study of c lonality in these tumors has suggested that the epithelial component is pol yclonal, but the stroma is monoclonal, and thus forms the neoplastic compon ent of the lesion. In this study microsatellites on chromosome 1q and chrom osome 3p were assessed for allelic imbalance (AI) in 47 phyllodes tumors; i n all cases stroma and epithelium were analyzed separately. Ten of 42 (24%) phyllodes tumors showed AI at one or more markers on 3p, and 14 of 46 (30% ) showed AI on chromosome 1. Five tumors had changes in both the epithelium and stroma, Eight tumors had changes only detectable in the stroma and eig ht, changes in the epithelium only. Three tumors exhibited low-level micros atellite instability in the epithelium but not in the stroma. The results s how that AI on 3p and 1q does occur in phyllodes tumors and that it can occ ur in both the stroma and epithelium, sometimes as independent genetic even ts. These unexpected findings throw into doubt the classical view that phyl lodes tumors are simply stromal neoplasms and raise questions about the nat ure of stromal and epithelial interactions in these tumors.