They are relatively uncommon, being found in ~5% of subjects in autopsy series, more commonly in white women.
Most renal leiomyomas are small and asymptomatic, but larger lesions can cause pain or even be palpable.
The imaging appearance of renal leiomyoma is nonspecific. They tend to be well-circumscribed and peripherally located lesions that cause buckling of the renal cortex. Non-contrast CT reveals a high-attenuation mass that can have areas of hemorrhage and cystic degeneration (larger lesions). Calcification is uncommon. Contrast enhanced CT images reveal relatively homogeneous enhancement.
MRI typically reveals a lesion with homogeneously low signal intensity on T1- and T2-weighted images. As on CT, larger lesions can have a complex appearance due to calcifications, hemorrhage, and cystic or myxoid degeneration. The appearance of these large tumors can be identical to that of renal cell carcinoma.
The images above are from a renal tumor protocol CT in a patient with an incidentally detected left renal lesion. The lesion is peripheral, relatively small, and has foci of calcification (uncommon in leiomyomas) as well as an area of low-attenuation/non-entrancement. The lesion is hypervascular. The imaging findings are nonspecific and the lesion should be thought of as renal cell carcinoma until proven otherwsie. Core biopsy prior to radiofrequency ablation revealed a diagnosis of leiomyoma.