Diagnosis: Unfavorable histology of Wilms tumor
• Each of the other three features may be seen in Wilms tumor. The predominance or even exclusivity of blastema, heterologous stromal elements, including skeletal muscle cells and nephrogenic rests, do not justify the classification in the unfavorable histologic group.
• Wilms tumor is a malignant embryonal neoplasm, with a peak incidence at 2 to 3 years of age and an excellent prognosis of more than 90% survival due to the current therapeutic regimens.
• Anaplasia in Wilms tumor is defined as nuclear pleomorphism of more than three times the size of the other tumor cells, and multipolar mitotic figures. Anaplasia is associated with an increased risk of drug resistance and p53 mutations.
• Focal anaplasia is to be distinguished from diffuse anaplasia and is not considered an unfavorable histology. It is usually focal and lacks severe nuclear pleomorphism. Anaplasia involving tumor within vascular spaces, at the margin, or in metastatic sites is considered as diffuse anaplasia, irrespective of the morphology of the remaining tumor.
