Beyond RCC, the kidney can be involved by sarcoma, haematologic malignancy, and — most commonly of all — metastases from other primaries. Distinguishing these from RCC matters because their management is often non-surgical.
Renal Sarcoma
Renal sarcoma is rare (1–2% of malignant renal tumours, peak in the 5th decade) but more lethal than sarcoma at any other genitourinary site; leiomyosarcoma is the commonest subtype. It presents like a large, rapidly growing RCC (mass, pain, haematuria) and is hard to distinguish from sarcomatoid RCC. Management is surgical — wide excision with negative margins (often radical nephrectomy with en-bloc resection), as the initial resection is the best chance of cure. Chemotherapy responses are disappointing, and adjuvant chemoradiation (effective for extremity sarcoma) does not help renal or retroperitoneal sarcoma. The key prognostic factors are margin status and tumour grade.
Lymphoma and Leukaemia
The kidney is commonly involved by haematologic malignancy, and lymphoma is the most common. Suspect renal lymphoma with infiltrative lesions, splenomegaly, disproportionate lymphadenopathy, or nodes outside the usual renal landing zones; B symptoms (fever, weight loss, fatigue) are common. Because treatment is systemic chemotherapy ± radiotherapy, obtain a percutaneous biopsy and avoid nephrectomy (reserved for uncontrollable haemorrhage or the rare primary renal lymphoma). Leukaemic renal involvement is commoner in children.
Metastases to the Kidney
Metastases are the most common malignant neoplasm in the kidney, outnumbering primary tumours (found at autopsy in ~12% of cancer deaths). The commonest sources are lung, breast, and GI cancers, melanoma, and haematologic malignancy; lesions are typically multifocal, infiltrative, and poorly enhancing. Suspect metastasis with multiple renal lesions plus widespread disease or a known non-renal primary, and confirm with percutaneous biopsy.
Other Tumours
Carcinoid tumours arise from neuroendocrine cells (associated with horseshoe kidney; diagnosed by urine/plasma serotonin; carcinoid syndrome is uncommon) and have a generally good prognosis with surgical excision. Renal small-cell carcinoma, Ewing/round-cell tumours, and adult Wilms tumour warrant multimodal therapy.
Self-Test
1. What is the most common renal sarcoma, and what are its key prognostic factors? Leiomyosarcoma; margin status and tumour grade.
2. How should suspected renal lymphoma be managed? Confirm with percutaneous biopsy and treat with systemic chemotherapy ± radiotherapy — avoid nephrectomy.
3. What is the most common malignant neoplasm in the kidney overall? Metastases from another primary (commonly lung, breast, GI, melanoma, or haematologic).