Introduction
This topic covers the imaging of the main genitourinary malignancies: solid and cystic renal tumours, urothelial tumours of the bladder, the urethra and the upper tract, and prostate adenocarcinoma. For each, the answer should explain the typical clinical presentation, the best imaging modality for detection, characterisation and staging, the key imaging signs, the main differential diagnosis, and how imaging findings guide treatment.
Renal tumours
Differentiating between benign and malignant formations is the most important challenge. The most common benign renal tumours are angiomyolipoma, adenoma and haemangioma. Renal cell carcinoma (RCC) is the commonest primary renal malignancy in adults. It is usually unilateral but bilateral in 3-5% of patients. The clear cell type predominates. Many tumours are now found incidentally on imaging, and the classic triad of haematuria, flank pain and palpable mass is seen only in advanced disease; patients can be asymptomatic, and when there are symptoms the tumour is at a late stage. The tumour can be a solid homogeneous or heterogeneous formation with areas of necrosis, cystic transformation and haemorrhage, and can be TNM staged.
- Ultrasound detects renal masses and can show whether a lesion is solid or cystic, but it cannot reliably characterise a solid mass. RCC is seen as a solid rounded formation. Regional lymph nodes can appear enlarged if big enough. Doppler US can be used to look for thrombosis or infiltration of the renal vein, and for a right kidney tumour also infiltration of the IVC. There may be calcifications, amorphous in the centre of the tumour and annular at the periphery. A rare form is cystic RCC, showing many anechoic areas.
- Multiphase contrast-enhanced CT (unenhanced, corticomedullary, nephrographic and excretory phases) is the main method for characterisation and staging. A subcapsular haematoma may mask the tumour. In three phases: native, a hypodense oval tumour; corticomedullary, heterodense; nephrogenic, heterodense. With contrast, a tumour in the right kidney appears as an oval heterodense lesion.
- A solid renal mass that enhances by 20 HU or more after contrast is considered malignant until proven otherwise.
- Angiomyolipoma is benign and contains macroscopic fat (density of about -10 HU or lower). Small fat-poor angiomyolipomas can mimic RCC and may need MRI with chemical-shift imaging.
- Oncocytoma is usually benign and may show a central stellate scar, but this sign is not fully specific because some RCCs look similar.