Recent studies by Green et al. (12) and Paradis et al. (13) demonstrated the clonal nature of this neoplasm, which is associated with non-random X chromosome inactivation. In some cases of AML, except abnormalities in the TSC2 region of chromosome band 16p13 have been identified (14, 15). Perivascular epithelial cells (PEC) that give rise to the 3 cellular types identified in renal AML are the progenitor cells of AML. It has been shown that these precursors may differentiate into spindle-shaped cells with features of smooth muscle, fat and eosinophilic and epithelioid cells, giving rise to a family of neoplasms (PEComas) that includes angiomyolipoma, lipoangioleiomyoma and clear cell tumours of the lung and pancreas (14).

There is also an epithelioid variant of AML characterized by epithelioid proliferation (renal PEComa), which was recently added to the 2004 WHO classification of the renal tumours (16) and as such exhibits features of malignancy. Since patients are usually asymptomatic, the diagnosis of AML is often incidental; this happens with lesions with a diameter of less than 4 cm. Lesions greater than 4 cm in diameter are often symptomatic and manifest with a clinical picture characterized by lumbar pain, anaemia and haematuria (7). Retroperitoneal haemorrhage and/or bleeding into the renal collecting system are the major complications of AML; both conditions may put the patient��s life at risk. The haemorrhagic tendency is related to the angiogenic component of AML, owing to the presence of aneurysmatic vessels with an irregular course (8), to tumour size and to the association with TSC (9).

The therapeutic strategy varies from case to case: selective embolization of the renal artery and surgical removal of the lesion are the pillars of AML management (11). Alternatively, it is possible to follow the clinical course, with periodic surveillance of the lesion. Nephrectomy can be opted for in more severe cases. In case the aforementioned alternatives cannot be performed, a medical approach with hormonal therapy or with agents such as sirolimus, an inhibitor of the mammalian target of rapamycin (mTOR), can be chosen (10, 11). Case report A 41-year old female patient, weighing 73 kilograms and 158 cm tall, presented to us from another hospital with a diagnosis of retroperitoneal haematoma.

The patient had presented at the aforementioned hospital with a sudden onset of pain in the lumbar region and no evidence of fever, weight loss, anorexia, urinary retention or haematuria. The patient did not report any relevant diseases. A non-contrast CT scan revealed a probable Cilengitide retroperitoneal haematoma that required angiography. Therefore, the patient was urgently transferred to our hospital. The patient was in good general conditions. Pain was elicited upon deep palpation of the right flank, mesogastrium and right inguinal area.