Spindle Cell Lipoma in an End-Stage Renal Allograft: Case Report.

BACKGROUND: Spindle cell lipoma is an uncommon variant of lipoma and usually occurs as a solitary, subcutaneous, and well circumscribed lesion in the posterior neck, shoulders, and back of older men. Primary renal lipomas are rarely reported. Spindle cell lipoma in the kidney has not been previously described in the literature.

CASE REPORT: A 60-year-old Chinese man suffered graft failure 10 years after living related donor kidney transplantation. During cancer surveillance, he was found to have a mass in the renal allograft, which increased in size and was suspicious for renal cell carcinoma on computerized tomographic scan. The patient underwent renal graft explantation. Grossly, the kidney was atrophic, containing a 6.5 cm yellowish solid lesion without hemorrhage and necrosis in the renal sinus fat. Microscopically, the lesion was composed of variably sized adipocytes and cellular areas of bland spindle cells with no cytologic atypia. There were prominent slender blood vessels within the lesion, along with focal myxoid change as well as scattered mast cells and inflammatory cells. Lipoblasts were not identified. The spindle cells were positive for CD34 and negative for Melan-A, HMB45, S100, and SMA. Pax-2 stain was nonspecific. MDM2 amplification by means of fluorescence in situ hybridization (FISH) and overexpression by immunohistochemistry were negative. The Ki-67 proliferation index was <1%. Interphase FISH revealed loss of 13q and 16q in the tumor.

CONCLUSIONS: Renal spindle cell lipoma is a rare benign tumor. Angiomyolipoma and well differentiated liposarcoma are the main differential diagnoses. Immunohistochemistry and cytogenetic techniques are helpful in differentiating it from malignant entities.