The radiological diagnosis and treatment of renal angiomyolipoma—current status

Angiomyolipomas (AMLs) are the most common benign renal neoplasm and are often discovered incidentally. Due to both an increase in the use of imaging, as well as advances in imaging technology, they are being increasing identified in the general population. As these lesions are benign, there is good evidence that the majority of them can be safely followed up without treatment. However, there is an increasing wealth of information available suggesting there are individuals with AMLs where prophylactic treatment is indicated to prevent complications such as haemorrhage. In such cases, treatment with radiological interventional techniques with subselective particle embolization has superseded surgical techniques in most cases. Even in emergency cases with catastrophic rupture, prompt embolization may save the patient with the additional benefit of renal salvage. Confident identification of a lesion as an AML is important as its benign nature obviates the need for surgery in most cases. The presence of fat is paramount in the confirmatory identification and characterization of these lesions. Although fat-rich AMLs are easy to diagnose, some lesions are fat poor and it is these cases where newer imaging techniques, such as in-phase and out-of-phase magnetic resonance imaging (MRI) may aid in making a confident diagnosis of AML. In this paper, we comprehensively review the imaging techniques in making a diagnosis of AML, including features of both characteristic lesions as well as atypical lesions. In addition, we discuss current guidelines for follow-up and prophylactic treatment of these lesions, as well as the increasing role that the interventional radiologist has to play in these cases.

Introduction

Angiomyolipoma (AML) is the most common benign renal neoplasm. The incidence has been found to be 0.1% of males and 0.2% of females in a population without tuberous sclerosis (TS).¹ They consist of variable amounts of muscle, fat, and vascular tissue (Fig. 1). AMLs are pathologically characterized as hamartomas, or more accurately, choristomas. The reason for the distinction is that hamartomas are composed of abnormal amounts of a tissue that is usually present in an organ, but neither fat nor muscle are normally found in the kidney.² AMLs are not limited to the kidney and may be found in several other organs, most commonly in the liver. In 80% of cases, AMLs are sporadic and in such cases they are likely to be solitary lesions. In the remainder of cases they are associated with the tuberous sclerosis complex and are likely to be multiple within the kidney and widespread ³ (Fig. 2)

The relative amount of each tissue varies, with a minority (4.5%) of these tumours being classified as minimal fat AMLs, as they contain only microscopically detectable fat.⁴ Arteries within AMLs characteristically are numerous, have no internal elastic membrane and a disordered smooth muscle pattern. The blood supply is frequently bizarre and torturous. If large, a tumour may recruit from a variety of surrounding vessels including renal, adrenal, ureteral, gonadal, phrenic, and lumbar arteries. These features account for the high frequency of aneurysms detected within AMLs and, consequently, for the high incidence of haemorrhage from these lesions.⁵

The increased availability of radiological imaging has lead to the most common presentation of AMLs being as an incidental finding. However, once a size of 4 cm is reached, symptoms may develop in up to 68–80% of patients, with 50–60% presenting as haemorrhage. Haemorrhage leads to shock in one-third of patients presenting in this manner.6, 7 A rarer presentation would be as a palpable mass or as a cause of mass effect. In addition to their potential for life-threatening haemorrhage, AMLs provide a significant diagnostic and management challenge due to their potential to mimic malignant renal processes. Recently 5.7% of presumed renal cell carcinomas (RCC) removed at surgery were found to be AMLs pathologically.⁸

Although the majority of AMLs behave in a benign manner, there is a small subgroup of AMLs which may behave in a more aggressive fashion and become locally invasive. If they do spread in this manner, the vast majority affect either the renal vein or the inferior vena cava. Some authors have described local lymph node involvement.⁹ These tumours tend to be large, averaging 9.5 cm in diameter and approximately 80% occur in females. Although the majority of non-invasive AMLs are asymptomatic, more than 70% of aggressive AMLs may present with symptoms.¹⁰ Epithelioid AMLs are a rare subgroup of aggressive AMLs, which are characterized by the presence of epithelial cells staining strongly for melanoma-associated markers when assessed pathologically.¹¹ In addition to their locally aggressive behaviour, cases of metastatic disease have been documented.12, 13 Approximately one-third may demonstrate local extension or distant metastasis at presentation; the larger the tumour, the more likely it is to spread.¹⁴ They are composed of atypical epithelioid cells, are highly cellular, and most contain little or no fat. Their similarity on computed tomography (CT) to RCC can lead to diagnostic difficulty and thus these tumours are often managed surgically (Fig. 3).

Thus the most important issues when imaging a potential AML are the differentiation of an AML from malignant renal disease and the assessment of the potential for complications. The management of AML is often conservative in the asymptomatic patient, whereas a diagnosis of malignancy will necessitate surgical intervention. For patients with complications or those meeting the other criteria for intervention (see below), there are both established and emerging radiologically and surgically based treatment options available. The role of the radiologist in managing this disease is thus more important than ever.