[A CASE OF CARCINOSARCOMA OF THE RETROPERITONEUM: A CASE REPORT].

We present a case of retroperitoneal carcinosarcoma. A 48-year-old woman with left-sided abdominal pain and a 15 cm tumor in the lower left renal retroperitoneal region on computed tomography (CT) was referred to our hospital. Contrast-enhanced CT showed a well-defined tumor with contrast effect on the retroperitoneum. Magnetic resonance imaging (MRI) demonstrated a heterogeneous cystic mass and well-enhanced mural nodules in the tumor on T2-weighted images. Her levels of serum CA-125 and CA-19-9 were high at 44.7 and 143.0 U/mL, respectively. Although she was diagnosed with primary retroperitoneal mucinous cystadenocarcinoma and underwent surgical resection, the pathological diagnosis was a carcinosarcoma on the retroperitoneum. No adjuvant therapy was given. CT findings at 2 months revealed local recurrence and multiple pulmonary metastases. She received paclitaxel (175 mg/m² on day 1) and carboplatin (area under the curve of 6 on day 1) (TC) every 3 weeks. After two courses of TC therapy, some pulmonary metastases disappeared and shrank, but the progression of local recurrence was observed. However, after seven total courses of TC therapy, local recurrence further progressed, with the infiltration of the abdominal wall and lumbar pain, which was determined to be progressive disease (PD). A comprehensive genomic profiling test revealed no actionable genetic mutations. She died of cancer five months after the disease recurred. Carcinosarcoma is a rare tumor with poor prognosis, for which no established treatment exists beyond surgical resection. In this case, the therapeutic agent could not be determined; however, genomic analysis should be performed to guide the treatment of carcinosarcoma in advanced cases.