Diagnosis of renal angiomyolipoma with hounsfield unit thresholds: effect of size of region of interest and nephrographic phase imaging

PURPOSE

To retrospectively determine the optimal Hounsfield unit threshold and region of interest (ROI) size required to accurately diagnose renal angiomyolipoma (AML) and differentiate it from renal cell carcinoma (RCC).

MATERIALS AND METHODS

This retrospective study was institutional review board approved and HIPAA compliant, and the requirement for written informed patient consent was waived. The radiologic reports on 4502 dual-phase abdominal computed tomography (CT) examinations (nonenhanced and nephrographic phases, 5-mm collimation, 120-140 kVp, variable milliampere-second settings) performed in 2872 patients from June 2002 through October 2007 were reviewed. Solid-component masses reported as suspicious for RCC or AML were correlated with histologic and/or follow-up imaging findings. ROIs of three different sizes-tiny (8-13 mm(2)), small (19-24 mm(2)), and medium (30-35 mm(2))-were drawn in the lowest-attenuation focus on images obtained during both phases. The test characteristics (sensitivity, specificity, positive predictive value, negative predictive value, false-positive rate, false-negative rate) of multiple attenuation thresholds at each combination of ROI size and contrast enhancement phase were calculated, and receiver operating characteristic (ROC) curves were derived. Areas under the ROC curve were calculated.

RESULTS

There were 217 RCCs and 65 AMLs. With an attenuation threshold of -10 HU or lower at nonenhanced CT, RCC would be misdiagnosed as AML in 11 (5.1%) cases, one (0.5%) case, and one (0.5%) case with use of the tiny, small, and medium ROIs, respectively. With use of the tiny, small, and medium ROIs, misdiagnosis rates would be 2.3%, 0.5%, and 0.5%, respectively, at a threshold of -15 HU or lower and 1.8%, 0%, and 0%, respectively, at a threshold of -20 HU or lower. Areas under the ROC curve for the nonenhanced phase images (range, 0.874-0.889) were superior to those for the nephrographic phase images (range, 0.790-0.826).

CONCLUSION

Nonenhanced CT images were superior to nephrographic phase CT images for the diagnosis of AML. An attenuation threshold of -10 HU or lower with an ROI of at least 19-24 mm(2) is optimal for the diagnosis of AML. This threshold is not accurate with use of smaller (8-13-mm(2)) ROIs.

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.