Nephrectomy Avoided in a Patient with Lipid Poor Angiomylipoma with Radiological Features Suspicious for Renal Cell Carcinoma: A Case Report

Angiomyolipoma (AML) is a benign neoplasm that is easily mistaken for renal cell carcinoma (RCC) upon radiological investigation. The management of the two pathologies is significantly different, and so accurate diagnosis is vital. We report a case in which ultrasound guided biopsy saved the patient from an unnecessary nephrectomy when an AML had radiological appearances consistent with RCC. This case report also discusses the literature surrounding radiological features and novel imaging techniques of minimal-fat AML and RCC.


Introduction
Angiomyolipoma (AML) is the most common solid, benign renal neoplasm that clinicians encounter [1]. AML is described as "triphasic" as it is formed from three constituents to varying degrees: adipose tissue, smooth muscle and dysmorphic blood vessels [1]. The presence of large amounts of adipose tissue assists in the radiological diagnosis of "classic" AML [1] [2]. Where there is minimal fat, or fat is masked by intramural haemorrhage, there can be radiological diagnostic difficulty, and AML can be mistaken for renal cell carcinoma (RCC) [1] [3].
AMLs are now understood to be a heterogenous group of tumours with vary-*The patient in this case report gave consent for her case to be published.

Presentation
A 77-year-old lady presented with generalised, colicky abdominal pain and was admitted under the general surgeons. She had not noticed any visible haematuria. She had a past medical history of type 2 diabetes, hypertension, hypothyroidism and hypercholesterolaemia. On examination of the abdomen, she had generalised tenderness but no guarding or peritonism. She had a normal urine dipstick result and blood haematology and biochemistry revealed normal renal and liver function as well as normal inflammatory markers.
She underwent a contrast-enhanced CT of the abdomen and pelvis which demonstrated adhesions, no evidence of bowel obstruction, and an incidental mass in the left renal pelvis (see Figure 1 and Figure 2). This was reported as a       She was reviewed in clinic two weeks later with an interim urology MDT discussion. During MDT, images were reviewed, and the working diagnosis changed from upper tract TCC to RCC. She continued to be asymptomatic and denied any visible haematuria. As the diagnosis was not clear at this stage, the MDT decision was that she should undergo ultrasound-guided biopsy of the left renal lesion, rather than proceeding to nephroureterectomy. This was performed two weeks later and the patient was reviewed shortly afterwards with the histology result. Surprisingly, the histology demonstrated AML, not RCC in Figure 8. A week later, her stent was removed without complication. There was no growth in the AML following six month interval surveillance and she has now been discharged. She has been advised that unless she becomes symptomatic, no further follow up or intervention is required.

Discussion
There are other cases in the literature in which AML and RCC are mistaken for each other. AMLs occur sporadically in 80% of cases, while 20% of AMLs are There have been many papers exploring imaging features of minimal fat AML.

Sung et al., compared morphology and enhancement features in sized matched
AML and RCC. It was concluded that, in CT imaging, "non-round shape" in the absence of capsule and prolonged enhancement were factors that could distinguish minimal-fat AML from RCC [8]. Prolonged enhancement was also found to be an important radiological feature in distinguishing the two pathologies by Kim et al. [9]. Zhang et al. performed a retrospective study comparing the imaging and post-operative histology of minimal fat AML to RCC. Each patient had a pre-operative helical CT, which were reported by two blinded radiologists. The study found that unenhanced attenuation characteristics, intra-tumoural vessels and the attenuation values of unenhanced and early excretory phase scans were useful features to distinguish between the two pathologies [10].
The MRI signal intensity and enhancement characteristics can also be helpful in the diagnosis of lipid-poor AMLs. Low signal intensity on T2 weighted imaging compared to the signal intensity of the renal cortex favours a diagnosis of lipid-poor AML rather than clear cell RCC [11]. However, papillary RCCs can also have low T2 signal intensity but these lesions typically have a lower level enhancement which is less prolonged than AMLs [12].

Conclusion
Despite advances in radiological practice particularly with development of multiparametric MRI techniques, there can still be uncertainty regarding the nature of renal tumours, particularly if less than 3 cm. In these cases, histology should be obtained from either ureteroscopy or percutaneous biopsy if the diagnosis remains uncertain. This can avoid unnecessary nephrectomy and the consequential associated morbidity and mortality.