We report on an exceedingly rare case of noninvasive ductal carcinoma arising in malignant phyllodes tumor of the breast. The patient was a 75-year-old woman who presented with a chief complaint of an indolent tumor mass of the left breast. Papillotubular carcinoma was diagnosed by aspiration cytology, and mastectomy with preservation of the pectoral muscle was subsequently performed (Bt+Ax+Ic, R2). Histopathological examination showed proliferation of monotonous, uniform tumor cells in a cribriform pattern amid atypical and spindle-shaped cells. Neither stromal invasion of the epithelial tumor cells nor clear transition between epithelial tumor cells and non-epithelial tumor cells was seen. Immunohistochemical staining revealed that the epithelial component was positive for antibodies such as CEA, EMA and keratin, while the non-epithelial component was negative for the same antibodies. Malignant phyllodes tumor with a noninvasive ductal carcinoma was diagnosed rather than true carcinosarcoma of the breast. No metastasis was detected in the axillary lymph nodes, and the patient was classified as stage II A (T2N0M0). Although neither chemoendocrine therapy nor irradiation was employed postoperatively, no recurrence was observed two years and two months after the surgery. There is little consensus on the treatment or prognosis of the disease. Careful observation of the present case is therefore important.

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