Patterns of intra-abdominal relapse (IAR) in patients with Wilms' tumor who received radiation: analysis by histopathology. A report of National Wilms' Tumor Studies 1 and 2 (NWTS-1 & 2)

336 non-metastatic patients who received radiation in National Wilms' Tumor Study-1 and 2 have been reviewed. Follow-up was minimum of two years. 15/273 (6%) of the favorable histology patients had intra-abdominal relapse as compared to $\text{8}\text{44}$ unfavorable histology (18%) (P = 0.003). Developing intra-abdominal relapse does not seem related to low radiation therapy doses or small volumes. However, a delay of greater than ten days to start radiation therapy from nephrectomy was related to the occurrence of intra-abdominal relapse in unfavorable histology patients (P < 0.001).