Angiomyolipoma are benign hamartomatous tumors of the kidney. They can occur as single or multiple lesions, uni- or bilaterally, as a solitary finding, or with other disease, so in 80% of patients with tuberous sclerosis. The latter group usually presents with multiple small bilateral tumors in young patients. By contrast, larger, solitary, and unilateral tumors are found predominantly in middle-aged women. The lesions are usually intraparenchymal, but may extend through the renal capsule and compress or invade the collecting system. Rarely, they are located extrarenal. Extension into the renal vein, inferior vena cava or lymph nodes is possible and no sign of malignancy. Histologically the tumor is composed of blood vessels, smooth mucle, and fat tissue which are present in various proportions. Large tumors often show evidence of necrosis and hemorrhage. In the vast majority of patients, angiomyolipomas are found incidentally. Possible symptoms include flank pain, a palpable mass, hematuria, hypertension, anemia, and fever. Hemorrhage due to rupture, severe hypertension, or progressive renal insufficiency, are significant complications of the disease. On sonography, most angiomyolipomas show increased echogenicity, but in the presence of hemorrhage, the tumor may become hypoechoic. Extension of the tumor into the extrarenal fat may escape detection. On CT or MRI lesion with fat-equivalent attenuation or intensity values are the most typical findings. To detect small amounts of fat, it is important to use thin-section and nonenhanced CT scans. The most important differential diagnoses is renal cell carcinoma, which also can sonographically be hyperechoic. Therefore even in typical cases of an incidentally detected small angiomyolipoma, the diagnoses should be verified by CT or MRI. In those patients, no further workup is necessary. Due to the increased risk of rupture, angiomyolipomas larger than 5 cm should be operated upon or angiographically embolised. Up to this size, follow-up sonography should be performed.
