Objectives: To show the demographics, type of treatment and clinical outcomes of patients with retroperitoneal tumors in Japan. Methods: We carried out a retrospective analysis of patients with retroperitoneal tumors treated between 2000 and 2012 at 12 university hospitals in Japan. Histology was re-evaluated using the 2013 World Health Organization classification. Results: A total of 167 patients were included in the analysis. The number of diagnosed patients increased over the 12-year study period. Liposarcoma and schwannoma were the most common histological types among intermediate/malignant and benign tumors, respectively. The intermediate/malignant tumors were larger and were more frequently found in older people. Surgical resection was the primary treatment for 151 patients. The median survival duration for patients with malignant tumors was 91 months, and was significantly shorter than that for patients with benign and intermediate tumors (P < 0.01). R2 resection was associated with significantly shorter survival than R0/R1 resection for malignant tumors (P < 0.01), but not for intermediate. Grossly complete resection of the recurrent tumors improved survival. Conclusion: The number of patients diagnosed with retroperitoneal tumors increased over time. R2 resection of primary tumors was found to be associated with poor prognosis in malignant tumors, but not in intermediate tumors. Complete surgical resection of recurrent tumors was associated with a better oncological outcome.
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