- Involvement of the kidney is commonly part of a disseminated disease.
- More multifocal and bilateral, but may present as solitary tumor mimicking primary tumor (image A) & (image B).
- Lung carcinomas are the most frequent source of metastatic carcinoma.
- Other common culprits: contralateral kidney, breast and GI tract.
- Morphology, most common is adenocarcinoma (30%) (image C) followed by squamous cell carcinoma and small cell carcinoma.
- Retroperitoneal sarcomas such as liposarcoma may involve the kidney and simulate a primary renal tumor.
- Dedifferentiated areas may invade into the kidney as a more solid mass than the retroperitoneal well-differentiated lipomatous tumor.
- The dedifferentiated liposarcoma mass can be mistaken as a renal tumor and the well-differentiated area may look like perinephric fat (image D).
- High grade dedifferentiated liposarcoma are composed of spindle cells and may resemble sarcomatoid RCC.
- Liposarcoma is MDM2+ and CDK4+ and sarcomatoid RCC is keratin+.