Teaching Case

A 46-year-old female patient presented with left flank pain for 2 months. She was referred for transabdominal sonography to rule out ureteric colic. No other investigations were done at the time of ultrasonography.

On ultrasound scan, the left kidney was displaced anteromedially by a heterogeneous mass isoechoic to retroperitoneal fat. An eccentric anechoic area was noted within the mass, showing “swirling” colour flow. Right kidney was normal. Further evaluation was done with contrast-enhanced CT scan of abdomen for better lesion characterisation. The scan revealed a predominantly fat density mass measuring 15 × 10 × 8.6 cm (ccxapxlr) arising from posterior aspect of left kidney showing areas of enhancing soft tissue and few thick septations. A well-defined area measuring 1.6 x 2.3 x 1.9 cm (ccxapxlr) having enhancement similar to abdominal aorta was noted within the mass. Few blood vessels were seen coursing through the mass. Some fat stranding was noted in the adjacent planes suggesting haemorrhage. The rest of the abdominal structures was normal. Radical nephrectomy was performed using transperitoneal approach. Histopathology revealed a predominantly fat-containing mass.

Angiomyolipomas (AMLs) are the most common mesenchymal renal tumours, most are found incidentally, observed in 0.1-0.22% of the general population with female predilection (4:1) [1]. Histologically these show varied proportions of blood vessels, smooth muscle, adipose tissue. Two types are recognised: sporadic and those associated with tuberous sclerosis. Approximately 80–90% of renal AMLs occur sporadically [2].
Tuberous sclerosis complex (TSC) is a common phakomatosis inherited as autosomal dominant pattern. It commonly affects the brain, skin, eyes, heart, kidneys and lungs. Renal manifestations include angiomyolipomas and cysts [3]. AMLs are found in 70–80% of patients with TSC and 20% of individuals with AML have TSC.
On imaging they have varied appearances depending on the histologic compositions. Most AMLs classically show predominant fat content and appear hyperechoic on ultrasonography, hypodense on CT similar to renal sinus fat. The admixture of soft tissue imparts variable appearances. MRI is used as problem solving tool in doubtful cases showing T1 hyperintense signal which is suppressed with fat saturation techniques. Calcification is very rare which prompts consideration of other diagnoses. Soft tissue component enhances on contrast study. Vessels can be seen coursing through AMLs, the sign which is helpful to differentiate from liposarcoma. All these findings were seen in the present case and clinched the diagnosis of AML. AMLs are predisposed to develop small aneurysms and even spontaneous haemorrhage as blood vessels characteristically lack elastic tissue in their walls [4, 5].
In TSC, AMLs are multiple, bilateral, larger, therefore have more tendency for aneurysm formation and spontaneous hemorrhage [6]. Incidence of aneurysm formation in AML is 76% [7]. Using lesion size of 4 cm predicts the risk of rupture with a sensitivity of 100% and specificity of 38%, while an aneurysm size of 5 mm predicts the risk of rupture with a sensitivity of 100% and specificity of 86% [7].
Patients may be followed conservatively with yearly CT scans for those with isolated AML < 4 cm in diameter and half-yearly scans for those with lesions > 4 cm for growth assessment. Patients with TSC and AML < 4 cm in diameter should be followed with 6-monthly CT scan [8]. Management protocol includes partial/radical nephrectomy for tumours > 4 cm showing progression in size even without symptoms and those < 4 cm but with features of hemorrhage. Selected patients may benefit from embolisation.
Bleeding mass lesions should be kept as differential while evaluating patients with flank pain. Identifying fat-containing lesion clinches diagnosis of AML. Other associated stigmata of TSC must be promptly searched for. Frank identification of the risk factors for haemorrhage helps to avoid life-threatening intraabdominal bleeding.