Angiomyolipoma

Angiomyolipomas are the most common benign tumour of the kidney. Although regarded as benign, angiomyolipomas may grow such that kidney function is impaired or the blood vessels may dilate and burst, leading to bleeding. Angiomyolipomas are the most common benign tumour of the kidney. Although regarded as benign, angiomyolipomas may grow such that kidney function is impaired or the blood vessels may dilate and burst, leading to bleeding. Angiomyolipomas are strongly associated with the genetic disease tuberous sclerosis, in which most individuals have several angiomyolipomas affecting both kidneys. They are also commonly found in women with the rare lung disease lymphangioleiomyomatosis. Angiomyolipomas are less commonly found in the liver and rarely in other organs. Whether associated with these diseases or sporadic, angiomyolipomas are caused by mutations in either the TSC1 or TSC2 genes, which govern cell growth and proliferation. They are composed of blood vessels, smooth muscle cells, and fat cells. Large angiomyolipoma can be treated with embolisation. Drug therapy for angiomyolipoma is at the research stage. The Tuberous Sclerosis Alliance has published guidelines on diagnosis, surveillance, and management. If the dilated blood vessels in an angiomyolipoma rupture, the resulting retroperitoneal haemorrhage causes sudden pain, accompanied with nausea and vomiting. When the patient presents in the emergency department, up to 20% are in shock. Angiomyolipomas are tumours consisting of perivascular epithelioid cells (cells which are found surrounding blood vessels and which resemble epithelial cells). A tumour of this kind is known as a PEComa, from the initials of perivascular epithelioid cell. Older literature may classify them as hamartomas (benign tumours consisting of cells in their correct location, but forming a disorganised mass) or choristoma (benign tumours consisting of normal cells in the wrong location). PEComas are themselves a kind of mesenchymal tumour which involves cells that form the connective tissue, cardiovascular, and lymphatic systems. An angiomyolipoma is composed of varying proportions of vascular cells, immature smooth muscle cells, and fat cells. These three components respectively give rise to the components of the name: angio-, myo-, and lip-. The -oma suffix indicates a tumour. Angiomyolipomas are typically found in the kidney, but have also been commonly found in the liver and less commonly the ovary, fallopian tube, spermatic cord, palate, and colon. The Maclean imaging classification system for renal angiomyolipomas is based on the location of the angiomyolipoma within the kidney. Since all three components of an angiomyolipoma (vascular cells, immature smooth muscle cells, and fat cells) contain a 'second-hit' mutation, they are believed to have derived from a common progenitor cell that suffered the common second-hit mutation. Three methods of scanning can detect angiomyolipoma: ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI). Ultrasound is standard and is particularly sensitive to the fat in angiomyolipoma, but less so to the solid components. However, accurate measurements are hard to make with ultrasound, particularly if the angiomyolipoma is near the surface of the kidney (grade III). CT is very detailed and fast, and allows accurate measurement. However, it exposes the patient to radiation and the dangers that a contrast dye used to aid the scanning may itself harm the kidneys. MRI is safer than CT, but many patients (particularly those with the learning difficulties or behavioural problems found in tuberous sclerosis) require sedation or general anaesthesia, and the scan cannot be performed quickly. Some other kidney tumours contain fat, so the presence of fat is not diagnostic. Distinguishing a fat-poor angiomyolipoma from a renal cell carcinoma (RCC) can be difficult. Both minimal fat AMLs and 80% of the clear-cell type of RCCs display signal drop on an out-of-phase MRI sequence compared to in-phase. Thus, a lesion growing at greater than 5 mm per year may warrant a biopsy for diagnosis.