Anaplasia: Loss of structural differentiation and useful function of neoplastic cells.Wilms Tumor: A malignant kidney tumor, caused by the uncontrolled multiplication of renal stem (blastemal), stromal (STROMAL CELLS), and epithelial (EPITHELIAL CELLS) elements. However, not all three are present in every case. Several genes or chromosomal areas have been associated with Wilms tumor which is usually found in childhood as a firm lump in a child's side or ABDOMEN.Rhabdomyosarcoma, Embryonal: A form of RHABDOMYOSARCOMA arising primarily in the head and neck, especially the orbit, of children below the age of 10. The cells are smaller than those of other rhabdomyosarcomas and are of two basic cell types: spindle cells and round cells. This cancer is highly sensitive to chemotherapy and has a high cure rate with multi-modality therapy. (From Holland et al., Cancer Medicine, 3d ed, p2188)Cerebellar Neoplasms: Primary or metastatic neoplasms of the CEREBELLUM. Tumors in this location frequently present with ATAXIA or signs of INTRACRANIAL HYPERTENSION due to obstruction of the fourth ventricle. Common primary cerebellar tumors include fibrillary ASTROCYTOMA and cerebellar HEMANGIOBLASTOMA. The cerebellum is a relatively common site for tumor metastases from the lung, breast, and other distant organs. (From Okazaki & Scheithauer, Atlas of Neuropathology, 1988, p86 and p141)Medulloblastoma: A malignant neoplasm that may be classified either as a glioma or as a primitive neuroectodermal tumor of childhood (see NEUROECTODERMAL TUMOR, PRIMITIVE). The tumor occurs most frequently in the first decade of life with the most typical location being the cerebellar vermis. Histologic features include a high degree of cellularity, frequent mitotic figures, and a tendency for the cells to organize into sheets or form rosettes. Medulloblastoma have a high propensity to spread throughout the craniospinal intradural axis. (From DeVita et al., Cancer: Principles and Practice of Oncology, 5th ed, pp2060-1)Kidney Neoplasms: Tumors or cancers of the KIDNEY.