Cystosarcoma phylloides of the breast is a
tumor composed of breast ducts and a cellular stromal component that can be benign or malignant. The origin of the stromal cells is controversial. We undertook an immunohistochemical and ultrastructural study of 11 cases of
cystosarcoma phylloides to assess the histogenesis of the stromal component. By light microscopy, 4 were diagnosed as benign, and 7 were diagnosed as malignant.
Antibodies to
vimentin,
desmin, actin, high- and low-molecular-weight
keratins, and
S100 protein were used for immunohistochemical staining. In the 4 benign cases of
cystosarcoma phylloides, the stromal cells stained positively only for
vimentin. In the malignant
tumors, the spindle cell component stained for
vimentin in all the cases. In addition, the malignant stromal cells coexpressed
desmin in two cases and
keratin and
S 100 protein in another case. By electron microscopy the stromal component in the benign case and in two of five malignant cases was composed of a mixture of fibroblasts and myofibroblasts. The entire neoplastic stroma in two other malignant cases showed features of smooth-muscle differentiation, whereas in another case all the stromal cells showed myoepithelial differentiation. Thus, in benign and malignant
cystosarcoma phylloides, the stromal component consists of a mixture of fibroblasts and myofibroblasts.
Leiomyosarcomas and
myoepitheliomas can mimic malignant
cystosarcoma phylloides, but immunohistochemistry and electron microscopy can differentiate these entities. This is important since their
biologic behavior is different.