Clinical History
A 25-year-old female patient with the known history of tuberous sclerosis was referred for screening of her kidneys. Initial renal ultrasound was inconclusive due to markedly enlarged kidneys, and CT abdomen was recommended for further assessment.
Imaging Findings
CT abdomen showed extensive replacement and distortion of both kidneys by multiple, predominantly fat (-20 HU), and soft tissue attenuation masses. There was no perirenal infiltration, and no encasement of renal vessels, inferior vena cava (IVC) or aorta was seen. Appearances were consistent with bilateral, benign renal mesenchymal tumours associated with tuberous sclerosis.
Discussion
Renal angiomyolipoma is a benign neoplasm composed of fat, vascular and smooth muscle elements. Angiomyolipomas are uncommon lesions with prevalence of 0.3-3% [1], seen in two distinct settings: sporadic (isolated) or in association with tuberous sclerosis. Sporadic form accounts for 80% of the angiomyolipiomas [2] with female to male ratio of 4:1 [3].
Tuberous sclerosis is a phacomatosis, described as the triad of adenoma sebaceum, seizures, and mental retardation. Other typical features include periungual fibroma, shagreen patch, retinal hamartomas, cortical tuber, subependymal nodule, giant cell astrocytoma, cardiac rhabdomyoma, lymphangiomyomatosis, and renal angiomyolipoma. Renal angiomyolipomas occur in 80% of patients with tuberous sclerosis [4]. They manifest at a younger age, typically larger than sporadic form, and often they are bilateral and multiple.
Histologically, angiomyolipomas are composed of varying amount of blood vessels, smooth muscle and fat.
Small angiomyolipomas are asymptomatic and often found incidentally. Up to 40% lesions may manifest as a palpable abdominal mass [4], and cause haematuria or flank pain. Solitary tumours may cause acute abdomen and shock as a result of spontaneous haemorrhage.
Angiomyolipoma is usually echogenic on ultrasound due to its fat content and multiple tissue interfaces within it. It appears as a single, or multiple well-circumscribed noncalcified tumours containing fat attenuation (-20 HU) tissues on CT. Angiomyolipoma may have varying degree of soft tissue attenuation, depending on the relative proportion of vascular or smooth muscle composition and on the presence of intratumoral haemorrhage. MRI shows variable areas of high signal intensity within the tumour on both T1W and T2W images. High signal intensity is present on T1W images because of the fat content, and signal remains isointense relative to that of perinephric fat on T2W images. However, high signal intensity areas on T1W images are not pathognomonic of fat, and blood and pockets of fluid of high protein content may have similar appearance. Intratumoral fat is best demonstrated with fat-suppression techniques. In- and out-of-phase T1W imaging technique is extremely sensitive to small quantities of fat [5].
Rapid growth or symptoms related to retroperitoneal haemorrhage would guide the treatment including selective arterial embolisation, renal sparing surgery and nephrectomy.
In conclusion, renal angiomyolipomata is a manifestation of tuberous sclerosis. Biannual follow-up of asymptomatic patients is recommended with US, CT or MRI, and particular attention to uncharacteristic lesions is recommended due to rare association between angiomyolipoma and synchronous malignant neoplasms, such as renal cell carcinoma and malignant epitheloid angiomyolipma [6].
Differential Diagnosis List
Renal angiomyolipomata
Renal liposarcoma
Renal cell carcinoma
