When considering a cortically-based enhancing renal mass, the main consideration is renal cell carcinoma (RCC). RCC typically enhances less than renal cortex, although the degree of enhancement is variable.  Enhancement of papillary subtype RCC, in particular, is often minimal ¹. Thus, the lesion is unquestionably suspicious for carcinoma by CT.

MRI provides additional additional anatomic and tissue characterization. India ink artifact on the gradient echo sequence nicely outlines the fat-plane between the mass and most of the renal cortex, suggesting that the lesion arises directly from the capsule. The markedly T2 hypointense quality of the mass is suggestive of a fibrous composition, although papillary RCC may also by T2 hypointense.

The patient underwent two separate biopsies:

Histology

Kidney, left, mass, core biopsy ( #1)

• microscopic examination shows fibrovascular tissue. No renal parenchyma identified
• the possibility that the biopsy sampled a fragment of an angiomyolipoma was entertained and immunohistochemical stains performed which showed no staining for HMB45 or Melan-A. A smooth muscle actin immunostain highlighted vascular smooth muscle
• no evidence of malignancy.  Re-biopsy may be indicated if clinical concern persists

Kidney, left, mass, core biopsy ( #2)

• sections show dense fibrous tissue, and some additional tissue resembling benign smooth muscle. As with prior biopsy, we do not see other elements of angiomyolipoma (AML), such as fat or thick-walled vessels, and we note that the prior biopsy was negative for HMB-45, which should stain AML
• differential would include a capsular fibroma or leiomyoma; we do not see overt features of atypia or malignancy. no evidence of a renal epithelial neoplasm

Due to persistent concern about biopsy sampling error, the patient later underwent partial nephrectomy with excision of the entire mass. Final surgical pathology was as follows:

Histology

Kidney, left, 6 cm capsular mass

• sections of left renal capsular mass show a well-circumscribed, paucicellular lesion, composed of islands of bland spindle cells organized in vague fascicles embedded in a background of dense hyalinized stroma. In some areas, the spindle cells appear to blend imperceptibly into the walls of small vessels. No adipocytic component is noted. There is no appreciable mitotic activity (<1/50 hpf)
• a small panel of immunohistochemical stains, including HMB-45, Melan-A, SMA, and desmin were performed (single antibody staining procedures with appropriately staining controls). The spindle cells stain positive for SMA and desmin, and negative for HMB-45 and Melan-A
• findings are those of leiomyoma

DIAGNOSIS: renal leiomyoma