Presentation
Incidental finding on imaging. Distant history of prior malignancy.
Patient Data
There is a solid hypoechoic mass closely associated with the upper left kidney. A thin rind of peripheral hyperechogenicity, together with posterior acoustic shadowing artifact, raise the possibility of peripheral calcification. The margins are somewhat indistinct, but the mass is about 6 cm.
A solid mass is associated with posteromedial aspect of the upper pole left kidney. The mass has smooth margins and is capsular or cortically-based. No associated perinephric stranding. The dynamic characteristics are as follows:
- Non-contrast: homogenous, slightly hyperdense to renal parenchyma (40 HU)
- Late arterial phase: homogenous low level enhancement (~10 HU), hypoenhancing relative to kidney
- Late portal phase: homogenous low level enhancement (~25 HU), again hypoenhancing relative to kidney
MRI more clearly shows that the mass originates from the renal capsule, particularly on the coronal "true FISP" (balanced gradient echo) sequence. It is markedly T2 hypointense and T1 isointense to skeletal muscle. As on CT, there is relatively mild homogenous enhancement after contrast infusion. No diffusion restriction.
Case Discussion
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
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