Renal Angiomyolipoma in Tuberous Sclerosis

Tuberous sclerosis complex is an autosomal dominant neuorcutaneous disorder characterized by the presence of hamartomatous lesions in multiple organs [1]. Approximately two-thirds of the cases are sporadic. Tuberous sclerosis classically demonstrates the clinical triad of mental retardation, epilepsy and facial angiofibromas (Vogt triad) [1, 2]. Tuberous sclerosis is considered to be caused by mutation of two genes kwon as TSC1 and TSC2. TSC1 is located on long arm of chromosome 9 and encodes for protein hamartin, TSC2 is located on short arm chromosome 16 and encodes for protein tuberin. TSC1 and TSC2 are tumor suppressor genes are responsible for regulate the cell function and growth. When they are altered by mutation it disturbs the regulation of cell growth which results in formation of tumors involving multiple organs. TSC2 gene is found continuous with PKD gene and thought to be why sometimes multiple renal cysts are found in tuberous sclerosis [2]. The recent criteria for diagnosing tuberous sclerosis consist of both major and minor features. Renal involvements of tuberous sclerosis are angiomyolipoma (AML), renal cysts and renal cell carcinoma. AML is the commonest renal manifestation with a frequency of 55% to 75% in tuberous sclerosis patients and conversely 20% of patients with renal AML have tuberous sclerosis [2]. An angiomyolipoma is defined as a benign lesion that demonstrated imaging characteristics of fat. Angiomyolipomas are seen in two separate clinical entities. The sporadic (isolated) form which accounts for 80% and AML associated with tuberous sclerosis [3]. AML less than 4cm in diameter are usually asymptomatic and they are and found incidentally, or they may cause symptoms such as abdominal pain, nausea, vomiting, palpable mass, hematuria, anemia and hypertension[2]. Most significant complication of renal AML is hemorrhage due to rupture of aneurism. Patient with ruptured aneurism present with pain and shock of acute onset [4]. The main predictor of hemorrhage reported to be the tumor size of more than 4cm. some studies have shown that aneurism size as a predictor of hemorrhage has higher specificity than tumor size. As the tumor grows in size blood flow entering the tumor increases and the abundant, elastin poor abnormal vessels in these tumors are prone for aneurism formation and rupture [4]. On sonography the echo pattern depends on percentage of fat, smooth muscle, vascular element and hemorrhage. Classically the AML are Hyperechoic relative to renal parenchyma and Isoechoic to the renal sinus fat [5,6]. Common pit falls are a junctional parenchymal defect, a renal infarct and a fat filled postoperative renal cortical defect [5]. AML may be hypoechoic if it predominantly consists of muscle, vascular and hemorrhagic components. Smaller renal cell carcinomas (< 3cm) are also hyperechoic on Ultrasonography and may simulate AML in 33% of the time. Some features that help differentiate AML from RCC are the presence of shadowing in RCC and tumor infiltration to IVC and the presence of lymphadenopathy [5]. AML are seen on CT as well circumscribed renal masses and the presence of intratumoral fat is almost diagnostic of AML. Presence of fat is best shown by CT and fatty tissue is considered to be present if the attenuation of the lesion measures -10 HU or lower. Some rare cases such as Wilms’ tumor, renal oncocytoma and rarely some renal cell carcinomas are reported to contain some fat within. Approximately 5% of AML do not show fat on CT and hemorrhage may obscure the presence of fat within the tumor. CT cannot be used to differentiate AML from RCC in these patients. AML are always benign although tumor may show extrarenal extension to perinephric space, renal veins or IVC [3]. MRI also depicts the presence of fat. Presence of macroscopic fat within the mass is very specific for AML. AML is identified in opposed phased chemical shift MRI by the presence of India ink artifact (loss of signal at the interface of fat and non fat containing areas) at the interface of mass and the renal parenchyma or within the mass. Non fat containing exophytic masses would demonstrate India ink artifact only at the interface of extra renal component and the perinephric fat and not within the mass. If AML is suspected on chemical shift imaging it can be further confirmed by acquiring fat saturated T1W and water saturated T1W gradient echo images [3]. Renal cysts are associated with tuberous sclerosis in 18% of the time and they are simple cortical cysts. They may be few in number or multiple and macroscopically simulating adult polycystic kidney disease. Ultrasound scan demonstrate an anechoic lesion with imperceptible walls that has enhanced through transmission. CT criteria identifying a cyst are a round well defined lesion which has water attenuation and sharp circumscribed wall and lack enhancement or fat attenuation. MRI imaging demonstrates fluid intensity in T1W, T2W and fat saturated sequences [7].