The immunohistochemical profiles of 16 cases of phyllodes tumor of the breast (9 benign and 7 malignant) from 15 patients were examined by the labeled streptavidin biotin method. The expression of Ki-67, p53, bcl-2, alpha-smooth muscle actin (alpha-SMA), desmin, S-100 protein, estrogen receptor (ER), and progesterone receptor (PgR) was analyzed. The number of Ki-67-positive stromal cell nuclei of malignant phyllodes tumor were significantly more prominent than the benign tumors. The p53 expression on the stromal cell nuclei showed a significant difference between malignant and benign cases (86% vs 22%; p<0.05). bcl-2 was regularly seen on the luminal cell cytoplasm and stromal cell labeling showed no significant difference between malignant and benign cases (29% vs 33%). Stromal cells were alpha-SMA positive but refractory among cases, and desmin and S-100 protein were negative. PgR was expressed in all 16 cases and ER in most cases (12/16) the expression of which was restricted to luminal epithelial cell nuclei. These findings indicate that the Ki-67 labeling index and p53 expression in the stroma would be a good diagnostic parameter distinguishing benign tumors from malignant tumors. However, the absence of steroid receptor expression in stromal cells suggests the ineffectiveness of hormonal therapy for phyllodes tumor of the breast.

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