Phyllodes tumors are usually benign, but may also show malignant transformation. Although malignant transformation often occurs in the stromal component, the epithelial component may rarely be malignant. In the literature, rare cases of phyllodes tumors have been reported, which have developed in situ lobular carcinoma, in situ ductal carcinoma, invasive lobular carcinoma, invasive ductal carcinoma, infiltrative carcinoma and squamous cell carcinoma in their epithelial component.

Case: A 39-year-old female patient was admitted to our general surgery clinic with a complaint of left-sided swelling ongoing for 15 days. On the basis of physical and radiological examinations, the patient who underwent trucut biopsy with pre-diagnosis of fibroadenoma was scheduled for operation due to a fibroepithelial lesion reported in the histopathological evaluation of the patient.

On the histopathological examination of the excision material; bifasic, infiltrative limited tumoral lesion was observed. In the tumor with stromal hyperplasia, moderate cellular stroma and moderate nuclear pleomorphism in the stromal cells were observed. In the epithelial component, cribriformity, increased cell proliferation and mitotic activity was observed in a focal area. The case was diagnosed as in situ ductal carcinoma arising in a malignant phyllodes tumor.

Conclusion: The epithelial component of phyllodes tumor is usually benign, but rarely can transform into a malignancy, too. Our case was presented with its clinical and histopathologic features due to a rare occurance of in situ ductal carcinoma within malignant phyllodes tumor. Anahtar Kelimeler : Phyllodes tumor, In situ ductal carcinoma, Malignant phyllodes tumor