Background and Purpose: The strategy used to treat pediatric renal tumors in Japan is based on the Japanese Wilms' Tumor Study (JWiTS) protocol, which was based on the National Wilms' Tumor Study (NWTS)-5 regimen. The regimen is characterized by an initial radical operation, followed by adjuvant chemotherapy and radiotherapy. Concerning the histological classification, a new classification based on the International Society of Pediatric Oncology (SIOP) classification was used beginning in 2008. The main points of revision are that the blastemal predominant type was classified as an independent category in the Wilms' tumor subtypes. The purpose of this study was to analyze the biological characteristics from the standpoint of the newly established histological classification. Materials and Methods: From 1971 to 2005, 174 cases of Wilms' tumors treated with an initial operation followed by adjuvant therapy were re-evaluated by the new histological classification. Histologically, all these materials showed no secondary changes associated with adjuvant therapy. Results: According to the new classification, Wilms' tumors were classified into four subtypes, including the mixed type (n = 112), epithelial type (n = 17), mesenchymal type (n = 15), and blastemal predominant type (n = 26). The 5 year overall survival rates were as follows; mixed type (90.1%), epithelial type (100%), mesenchymal type (93.3%), and blastemal predominant type (65.4%). Conclusion: The patients with blastemal predominant tumors demonstrated a significantly worse prognosis compared with those of other subtypes. The treatment strategy of blastemal predominant category should be distinguished from the other favorable subtypes.
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