Wilms tumor is the most common malignant renal tumor in children. Dramatic improvements in survival have occurred as the result of advances in anesthetic and surgical management, irradiation and chemotherapy. Today treatments are based on several multicenter trials and studies conducted by the SIOP (International Society of Paediatric Oncology) in Europe and COG (childrens Oncology Group, former: NWTS: National Wilms Tumour Study Group)in the USA. Main objectives of these trials and studies are to treat patients according to well-defined risk groups in order to achieve highest cure rates, to decrease the frequency and intensity of acute and late toxicity and to minimize the cost of therapy. In that way the SIOP trials and studies largely focus on the issue of preoperative therapy. The concept of neoadjuvant chemotherapy plays an important role in the treatment for most pediatric solid tumors today. The complete surgical removal of a shrunken tumor is facilitated, mutilation caused by surgical procedures is minimized or avoided and micrometastases, not visible at diagnosis, are treated as early as possible. Besides that, response to treatment can be measured individually by tumor volume reduction and / or percentage of therapy induced necrosis in the histological specimen. This might give an early individual prognostic parameter and can be used for further stratifying and more individualizing postoperative treatment as in other pediatric cancers. The International Society of Paediatric Oncology (SIOP) did enroll children with Wilms tumor into 7 studies up to now.
