## Histological Subtypes of Wilms Tumor **Key Point:** Mixed histology (containing epithelial, stromal, and blastemic components) is the most common subtype, accounting for approximately 50% of all Wilms tumors. ### Distribution of Wilms Tumor Histology | Histological Subtype | Frequency | Prognosis | Key Feature | |---|---|---|---| | Mixed (triphasic) | ~50% | Intermediate | All three components present | | Epithelial | ~5–10% | Favorable | Mature differentiation | | Stromal | ~10% | Favorable | Predominantly mesenchymal | | Blastemic | ~10% | Unfavorable | Undifferentiated, high mitotic rate | | Anaplastic | ~5–10% | Very unfavorable | Marked nuclear pleomorphism | **High-Yield:** The presence of all three embryonal components (epithelial, stromal, blastemic) in a single tumor is the most frequent presentation. This reflects the tumor's origin from the metanephric blastema—a multipotent tissue capable of differentiating along multiple lineages. **Clinical Pearl:** Histological subtype is one of the strongest independent prognostic factors in Wilms tumor. Favorable histology (epithelial, stromal, mixed without anaplasia) carries significantly better outcomes than unfavorable histology (anaplastic, blastemic-predominant). **Warning:** Do not confuse "mixed" with "anaplastic." Mixed histology contains all three normal embryonal components; anaplastic histology shows abnormal nuclear features and is associated with worse prognosis. ### Why Mixed Histology Is Most Common Wilms tumor arises from the metanephric blastema, which is inherently multipotent. During malignant transformation, the tumor typically retains the capacity to differentiate along multiple pathways simultaneously, resulting in the coexistence of epithelial (tubular/glomerular), stromal (connective tissue), and blastemic (primitive undifferentiated) elements within the same lesion.
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