BENIGN RENAL MASSES

Benign renal masses
The most common (70%) are simple cysts, present in >50% of >50-year-olds. Rarely symptomatic, treatment by aspiration or laparoscopic de-roofing is seldom considered.
Most benign renal tumours are rare; the two most clinically important are oncocytoma and angiomyolipoma.

Oncocytoma
This is uncommon, accounting for 3 - 7% of renal tumours. Males are twice as commonly affected as females. They occur simultaneously with renal cell carcinoma in 7 - 32% of cases.

Pathology
Oncocytomas are spherical, capsulated, brown/tan colour, mean size 4 - 6cm. Half contain a central scar. They may be multifocal and bilateral (4 - 13%) and 10 - 20% extend into perinephric fat. Histologically, they comprise aggregates of eosinophillic cells, packed with mitochondria. Mitoses are rare and they are considered benign, not known to metastasize. There is often loss of the Y chromosome.

Presentation
Oncocytomas often (83%) present as an incidental finding, or with loin pain or haematuria.

Investigations
Oncocytoma cannot often be distinguished radiologically from RCCs; may coexist with RCC. Rarely, they exhibit a spoke-wheel  pattern on CT scanning, caused by stellate central scar. Percutaneous biopsy is not recommended since it often leads to continuing uncertainty about the diagnosis.

Treatment
Radical or partial nephrectomy is indicated, as for renal carcinoma. No follow-up is necessary.

Angiomyolipoma (AML)
80% of these benign clonal neoplasms (hamartomas) occur sporadically; mostly middle-aged females. 20% are in association with tuberous sclerosis (TS) an autosomal dominant syndrome characterized by mental retardation, epilepsy, adenoma sebaceum, and other hamartomas. 50% of TS patients develop AMLs; mean age 30 years; 66% female; frequently multifocal and bilateral.

Pathology
AML is composed of blood vessels, smooth muscle, and fat. They are always considered benign, although extrarenal AMLs have been reported in venous system and hilar lymph nodes. Macroscopically, it looks like a well-circumscribed lump of fat. Solitary AMLs are more frequently found in the right kidney.

Presentation
AMLs frequently present as incidental findings (>50%) on ultrasound or CT scans. They may present with flank pain, palpable mass, or painless haematuria. Massive and life-threatening retroperitoneal bleeding occurs in up to 10% of cases (Wunderlich's syndrome).

Investigations
Ultrasound reflects from fat, hence a characteristic bright echo pattern. This does not cast an acoustic shadow  beyond, helping to distinguish an AML from a calculus. CT shows fatty tumour as low-density (Hounsfield units <10) in 86% of AMLs. If the proportion of fat is low, a definite diagnosis cannot be made. Measurement of the diameter is relevant to treatment.

Treatment
In studies, 52 - 82% of patients with AML >4cm are symptomatic compared with only 23% with smaller tumours. Therefore, asymptomatic AMLs can be followed with serial ultrasound if <4cm, while those bleeding or >4cm should be treated surgically or by embolization. Emergency nephrectomy or selective renal artery embolization may be life-saving. In patients with TS, in whom multiple bilateral lesions are present, conservative treatment should be attempted.