[Retroperitoneal Dedifferentiated Liposarcoma with Metaplastic Bone Formation : A Case Report and Review of the Literature].

: A 69-year-old female was introduced to our department for a retroperitoneal mass, 25 mm in diameter, in the right perirenal space indicated by computed tomography (CT). On the basis of magnetic resonance imaging (MRI), retroperitoneal soft tissue sarcoma was suspected. Because she rejected surgical treatment, we continued imaging surveillance. Subsequently, the mass grew larger, but her intention did not change. At 21 months after initial consultation, CT revealed further increase of the above mass and a new lesion with calcification. Ultimately, she underwent mass resection with concomitant resection of the right kidney. Histological examination showed dedifferentiated liposarcoma with metaplastic bone formation and positive surgical margin, but she refused adjuvant chemotherapy. She has survived 7 months since the operation with no evidence of recurrence.