## Wilms Tumor Histologic Classification **Key Point:** The triphasic pattern (blastema, epithelium, and stroma) represents the classic favorable histology of Wilms tumor and carries the best prognosis. ### Favorable Histology (Low Risk) - Triphasic differentiation: blastemal, epithelial, and stromal components - Represents approximately 55% of Wilms tumors - 5-year survival rate: >90% - Characterized by mature, well-differentiated elements - Absence of anaplasia ### Unfavorable Histology (High Risk) **Anaplasia** is the defining feature: - Nuclear enlargement (3× normal size) - Hyperchromasia - Abnormal mitotic figures - Present in ~5% of Wilms tumors - 5-year survival rate: 50–70% **High-Yield:** Anaplasia—not the presence of blastemal cells—determines unfavorable histology. Diffuse anaplasia carries worse prognosis than focal anaplasia. ### NWTS Risk Stratification | Risk Category | Histology | 5-Year Survival | Key Features | | --- | --- | --- | --- | | Low | Favorable (triphasic) | >90% | No anaplasia, mature differentiation | | Intermediate | Epithelial-dominant, stromal-dominant | 85–90% | Favorable histology but specific patterns | | High | Unfavorable (anaplasia) | 50–70% | Diffuse or focal anaplasia present | **Warning:** Do NOT confuse histologic grade (favorable vs. unfavorable) with TNM stage. Stage refers to extent of disease spread; histology refers to cellular differentiation. A stage 1 tumor with unfavorable histology has worse prognosis than a stage 2 tumor with favorable histology. **Clinical Pearl:** The presence of all three tissue components (triphasic pattern) is actually a favorable prognostic sign, indicating differentiation of the primitive blastemal cells. 
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