Epidemiology and Outcomes of Retroperitoneal Sarcoma: An Analysis of the Population‐Based Cancer Registry in Japan 2016–2019

Background Retroperitoneal sarcoma (RPS) is a rare cancer, so few reports have previously characterized its national profiles. The capture rate of RPS in the Bone and Soft Tissue Tumor Registry was only 20%. The present study aimed to clarify the characteristics and clinical outcomes of RPS using the National Cancer Registry (NCR), which contains nationwide population‐based data from Japan. Methods We analyzed data from 20,079 patients with soft‐tissue sarcomas (STS), entered into the NCR in 2016–2019 using the International Classification of Diseases‐Oncology, Third Edition cancer topography and morphology codes. We extracted demographics (sex, age), tumor details (tumor location, histology, extent of disease), treatment, and prognosis for each patient. Results Of the 20,079 patients, 3311 patients were diagnosed with RPS. RPS accounts for 16.5% of all STS, with an adjusted incidence of 0.41/100,000/year. Of patients with RPS, 70.9% were aged 60 years or older and 52.7% were at the ‘regional’ stage at the time of diagnosis. Surgery was performed in 70.0% of cases, while chemotherapy and radiotherapy were performed in only 21.5% and 8.3% of cases, respectively. The 3‐year overall survival (OAS) of the RPS patients was 57.3%. The multivariate analyses showed that of the 3,311 patients with RPS, worse OAS was associated with being male, older age, a histology of undifferentiated pleomorphic sarcoma/malignant fibrous histiocytoma, cancer discovery other than by cancer/health screening, being treated at low‐volume hospitals, advanced disease, and not having surgery. Conclusions The present study is the first to have clarified the epidemiology, clinical features, treatment, prognosis, and significant factors affecting prognosis of patients with RPS in Japan. Level of Evidence Level Ⅲ, prognostic studies.