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Renal leiomyoma is a rare benign tumor that is found in approximately 5%of the specimens from autopsy.This tumor may arise from smooth muscle cells of the renal capsule,the muscularis of the renal pelvis,or cortical vascular smooth muscles,with the renal capsule being the most common site.Renal leiomyomas are most commonly found in white women.Most leiomyomas are small and are asymptomatic.Large leiomyomas may manifest with a palpable flank mass and pain.At macroscopic examination,renal leiomyoma is a solid well-circumscribed encapsulated mass with a whorled cut surface,focal areas of hemorrhage,and irregular calcifiation.At histologic examination,interlacing bundles of spindle cells without nuclear pleomorphism or mitotic fiures may be identifid(Fig 7a).Tumor cells show immunoreactivity to smooth muscle markers such as actin and desmin.Figure 7.Renal leiomyoma in a 39-year-old woman.(a)Photomicrograph(original magnifiation,40;hematoxylin-eosin stain)of a leiomyoma of the kidney shows fascicles of smooth muscle(arrows)adjacent to renal tubules(arrowheads).Renal leiomyoma appears as a well-circumscribed peripherally located hyperattenuating solid mass on CT images obtained without contrast material(Fig 7b).The tumor typically demonstrates relatively homogeneous enhancement on CT images obtained after the administration of contrast material(Fig 7c).Large tumors may show areas of hemorrhage and cystic or myxoid degeneration.In addition to demonstrating hyperattenuation on non-enhanced CT images,renal leiomyoma typically has a peripheral location,well-defied margins,and associated buckling of the renal cortex.Although not pathognomonic for a renal leiomyoma,the combination of these findings should prompt the inclusion of leiomyoma in the differential diagnosis.At MR imaging,leiomyoma typically has homogeneously low signal intensity on T1-and T2-weighted images.Larger tumors are indistinguishable from renal cell carcinoma and leiomyosarcoma with imaging studies.The main differential diagnosis is usually made with angiomyolipoma of the kidney(AML).Most AMLs are composed of a variable mixture of mature fat,thickwalled blood vessels and smooth muscle,but there are times when only a smooth component is the most represented.AMLs are characterized by a co-expression of melanocytic marker(HMB45)and smooth muscle markers.Currently,the differential diagnosis between leiomyoma and leiomyosarcoma is only histopathological after nephrectomy because the radiological aspect is not diriment in the diagnosis.Ultrasonographic evaluation detects leiomyoma as an hypoechoic lesion that could appear solid or cystic.CT scan features are helpful for the differential diagnosis.The first feature is density.All leiomyomas examined before contrast were hyperdense compared to the kidney,with density similar to muscles.After contrast medium injection,the lesions had a lower enhancement than surrounding renal parenchyma.The second and final feature is localization and margins.Usually,these lesions have a peripheral location with well-defined margins,with no signs of infiltration into surrounding tissues.Renal leiomyomas are benign and their behaviour is not aggressive.They do not metastasize.The prognosis,after surgery,is excellent without recurrence.The differential diagnosis is still possible by histological examination.
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