In Wilm&;s tumor the following leads to emergence of resistance to chemotherapy -

Approximately 5% of tumors contain foci of anaplasia (cells with large, hyperchromatic, pleomorphic nuclei and abnormal mitoses). The presence of anaplasia correlates with the presence of acquired TP53 mutations and the emergence of resistance to chemotherapy. ROBBINS BASIC PATHOLOGY. 10th edition. Chapter Genetic and Pediatric Diseases. Page no. 290.