IMAGING

Fat Poor Renal Angiomyolipoma: Patient, Computerized Tomography and Histological Findings
Milner J, McNeil B, Alioto J, Proud K, Rubinas T, Picken M, Demos T, Turk T, Perry KT Jr.
Loyola University, Chicago, Illinois, USA
J Urol. 2006; 176: 905-9

• Purpose: We reviewed our experience with fat poor cases of angiomyolipoma.
• Materials and methods: The records of patients with angiomyolipoma, as determined by pathological study, from 1998 to 2004 were reviewed by recording patient demographics and outcomes. Fat poor cases were defined as the failure of imaging to demonstrate fat in a lesion. Computerized tomography and histological characteristics were assessed.
• Results: Histologically confirmed angiomyolipoma was found in 15 patients. Multiple lesions were found in 3 of 15 cases (20%). Of these 15 patients who underwent surgery 11 (73%) had unsuspected angiomyolipoma due to absent fat on computerized tomography and they underwent intervention for presumed renal cell carcinoma. Mean age +/- SD in this group was 54 +/- 15 years and 8 of 11 patients (73%) were female, of whom 4 (50%) had uterine fibroids. These lesions were found incidentally in 7 of 11 cases (64%). Operative complications developed in 2 of 11 patients (18%). Average maximal diameter on pathological evaluation was 3.2 +/- 1.3 cm (range 1.5 to 6). Nonenhanced computerized tomography was available in 7 of 11 cases, of which 3 of 7 (42%) showed hyperdense lesions and 4 of 7 (57%) showed isodense lesions. The percent of fat identified per high power field was less than 25% in 12 of 13 fat poor angiomyolipoma lesions (92%) compared to 2 of 4 classic lesions (50%) known to be angiomyolipoma before surgery (p = 0.04).
• Conclusions: We suggest that a general definition of fat poor angiomyolipoma should be the failure of imaging to reveal fat within a lesion, thus, making it unsuspected at surgery. A pathological definition should be less than 25% fat per high power field, which to our knowledge is a formerly undefined quantity. Not all cases are hyperdense on nonenhanced computerized tomography. These lesions cannot be reliably identified by imaging and they should be managed like all enhancing renal masses.
  
  
• Editorial Comment
  CT is the method of choice for identification of angiomyolipomas (AMLs), even those with small amounts of fat. However, 5-14% of these tumors do not present detectable fat by CT examination .Classically the finding of a homogeneously hyperdense renal mass on pre-contrast scans with homogeneously and prolonged enhancement on contrast-enhanced scans, has been considered suspicious for AML without radiological evidence of fat.
  The authors present an original contribution to this subject by showing that fat poor AMLs tended to have less than 25% fat per high power field when compared with AMLs with radiological evidence of fat. We agree with the authors regarding the unreliable criteria for specific imaging diagnosis of AMLs without radiological evidence of fat. When there is no detectable fat within a single or multiple renal mass by CT, two main differential diagnoses should be considered: renal cell carcinoma and oncocytoma. Thus, CT or US-guided percutaneous biopsy of the renal mass should be performed in order to establish the correct diagnosis before surgery.

Dr. Adilson Prando
Chief, Department of Radiology
Vera Cruz Hospital
Campinas, São Paulo, Brazil