International Society of Pediatric Oncology-SIOP staging system is the post - chemotherapy based staging system used in Wilm'stumor. Protocol of SIOP consists of initial clinical staging by physical examination, radiologic examination and neoadjuvant chemotherapy. The tumor burden is reduced by preoperative chemotherapy, facilitating resection of the tumor and reducing the total amount of postoperative chemotherapy and radiotherapy. Ref: Pediatric Oncology (Surgical and Medical Aspects) By Devendra K. Gupta, Page 628