title: Angiomyolipoma with minimal fat: can it be differentiated from clear cell renal cell carcinoma by using standard MR techniques?
abstract: OBJECTIVE
  To retrospectively assess whether magnetic resonance (MR) imaging with opposed-phase and in-phase gradient-echo (GRE) sequences and MR feature analysis can differentiate angiomyolipomas (AMLs) that contain minimal fat from clear cell renal cell carcinomas (RCCs), with particular emphasis on small (<3-cm) masses.
  
  
  METHODS
  Institutional review board approval and a waiver of informed consent were obtained for this HIPAA-compliant study. MR images from 108 pathologically proved renal masses (88 clear cell RCCs and 20 minimal fat AMLs from 64 men and 44 women) at two academic institutions were evaluated. The signal intensity (SI) of each renal mass and spleen on opposed-phase and in-phase GRE images was used to calculate an SI index and tumor-to-spleen SI ratio. Two radiologists who were blinded to the pathologic results independently assessed the subjective presence of intravoxel fat (ie, decreased SI on opposed-phase images compared with that on in-phase images), SI on T1-weighted and T2-weighted images, cystic degeneration, necrosis, hemorrhage, retroperitoneal collaterals, and renal vein thrombosis. Results were analyzed by using the Wilcoxon rank sum test, two-tailed Fisher exact test, and multivariate logistic regression analysis for all renal masses and for small masses. A P value of less than .05 was considered to indicate a statistically significant difference.
  
  
  RESULTS
  There were no differences between minimal fat AMLs and clear cell RCCs for the SI index (8.05%±14.46 vs 14.99%±19.9; P=.146) or tumor-to-spleen ratio (-8.96%±16.6 and -15.8%±22.4; P=.227) when all masses or small masses were analyzed. Diagnostic accuracy (area under receiver operating characteristic curve) for the SI index and tumor-to-spleen ratio was 0.59. Intratumoral necrosis and larger size were predictive of clear cell RCC (P<.001) for all lesions, whereas low SI (relative to renal parenchyma SI) on T2-weighted images, smaller size, and female sex correlated with minimal fat AML (P<.001) for all lesions.
  
  
  CONCLUSIONS
  The diagnostic accuracy of opposed-phase and in-phase GRE MR imaging for the differentiation of minimal fat AML and clear cell RCC is poor. In this cohort, low SI on T2-weighted images relative to renal parenchyma and small size suggested minimal fat AML, whereas intratumoral necrosis and large size argued against this diagnosis.
authors.affiliation: Department of Radiology, New York University Langone Medical Center, New York, NY, USA.
authors.forename: Nicole
authors.initials: N
authors.lastname: Hindman
authors.forename: Long
authors.initials: L
authors.lastname: Ngo
authors.forename: Elizabeth M
authors.initials: EM
authors.lastname: Genega
authors.forename: Jonathan
authors.initials: J
authors.lastname: Melamed
authors.forename: Jesse
authors.initials: J
authors.lastname: Wei
authors.forename: Julia M
authors.initials: JM
authors.lastname: Braza
authors.forename: Neil M
authors.initials: NM
authors.lastname: Rofsky
authors.forename: Ivan
authors.initials: I
authors.lastname: Pedrosa
mesh_terms: D018262:Adenocarcinoma, Clear Cell / Q000473:pathology*; D000273:Adipose Tissue / Q000473:pathology*; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D018207:Angiomyolipoma / Q000473:pathology*; D002292:Carcinoma, Renal Cell / Q000473:pathology*; D003937:Diagnosis, Differential; D005260:Female; D006801:Humans; D007680:Kidney Neoplasms / Q000473:pathology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008404:Massachusetts; D008875:Middle Aged; D009518:New York; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity