A 3-year-old girl presents with a palpable abdominal mass and hypertension. Imaging reveals a left-sided renal tumor with involvement of the renal vein. What is the most common site of metastasis in Wilms tumor at presentation?

Explanation

## Sites of Metastasis in Wilms Tumor **Key Point:** The lungs are the most common site of metastasis in Wilms tumor, occurring in approximately 10–15% of patients at presentation and in up to 20% during the course of disease. ### Frequency of Metastatic Sites | Metastatic Site | Frequency at Presentation | Frequency Overall | Clinical Significance | |---|---|---|---| | Lungs | 10–15% | 15–20% | Most common; detected by chest CT | | Liver | 5–10% | 10–15% | Regional spread via hepatic veins | | Bone | <5% | 5–10% | Less common; indicates advanced disease | | Brain | <2% | 2–5% | Rare; usually late manifestation | | Contralateral kidney | 5–10% | 5–10% | Synchronous or metachronous | **High-Yield:** Pulmonary metastases are the most frequent distant spread pattern because: 1. The renal vein drains directly into the inferior vena cava (IVC), which leads to the right heart and pulmonary circulation. 2. Hematogenous dissemination via the venous system preferentially lodges tumor emboli in the lungs first. 3. Chest imaging (CT or plain radiography) is mandatory in all Wilms tumor patients for staging and prognostication. **Clinical Pearl:** The presence of pulmonary metastases upstages the patient to Stage IV (metastatic disease) and influences chemotherapy intensity. However, pulmonary-only metastases have a more favorable prognosis than metastases to other sites (liver, bone, brain). **Mnemonic: WILMS Metastasis Pattern** — **W**ell-vascularized tumor → **I**nferior vena cava involvement → **L**ungs (pulmonary circulation) → **M**ore common than **S**keletal spread. ### Anatomical Basis ```mermaid flowchart TD A[Wilms Tumor in Kidney]:::outcome --> B[Invasion into Renal Vein]:::action B --> C[Extension into IVC]:::action C --> D{Route of Dissemination}:::decision D -->|Hematogenous via IVC| E[Right Heart → Lungs]:::action D -->|Regional lymphatic| F[Abdominal/Pelvic Nodes]:::action D -->|Direct extension| G[Liver, Peritoneum]:::action E --> H[Pulmonary Metastases<br/>Most Common]:::outcome F --> I[Regional Disease<br/>Stage III]:::outcome G --> J[Advanced Local Disease]:::outcome ``` **Warning:** Do not assume all Wilms tumors have metastases. Approximately 85–90% of patients present with localized disease (Stages I–III). Metastatic disease (Stage IV) is present in only 10–15% at diagnosis, with lungs being the predominant site.