De novo malignant solitary fibrous tumor of the kidney

Abstract

The kidney is a relatively infrequent site for solitary fibrous tumor (SFT). Among the previously reported cases, only two cases of malignant renal SFT developing via dedifferentiation from a pre-existing benign SFT have been reported. Here we reported a case of de novo malignant renal SFT clinically diagnosed as renal cell carcinoma in a 50-year-old woman. The tumor was circumscribed but unencapsulated and showed obvious hemorrhagic necrosis. Microscopically, the tumor was composed of patternless sheets of alternating hypercellular and hypocellular areas of spindle cells displaying mild to moderate nuclear atypia, frequent mitoses up to 8 per 10 high power fields, and a 20% Ki-67 proliferative index. Immunohistochemical studies revealed reactivity for CD34, CD99 and vimentin, with no staining for all other markers, confirming the diagnosis of SFT. No areas of dedifferentiation were seen after extensive sampling. Based on the pathologic and immunohistochemical features, a diagnosis of de novo malignant renal SFT was warranted. Our report expands the spectrum of malignant progression in renal SFTs. Even though this patient has been disease-free for 30 months, long-term follow-up is still mandatory.

Figure 1

CT of the abdomen. Arterial phase images of dynamic computed topography scan showed a highly necrotic tumor compressing the renal parenchyma without either invasion to surrounding tissues or local lymphadenopathy.

Figure 2

Gross morphology. The tumor was firm and showed a yellowish white to tan-gray, myxoid and lobulated cut surface with prominent hemorrhage and necrosis in the center.

Figure 3

Photomicrographs. A, Proliferation of spindle cells arranging in a patternless architecture (×200 original magnification). B, Alternating hypercellular and hypocellular areas of spindle cells separated from each other by bands of collagen fiber (×200 original magnification). C, Spindle cells forming haphazard, storiform, or short fascicular arrangements in a loose myxoid to fibrous stroma containing dense collagen fibers (×200 original magnification). D, Hemangiopericytoma-like staghorn-like vessels (×200 original magnification). E, Tumor cells displaying mild to moderate atypia and 3 mitoses in this high power field (arrow and arrowhead) (×400 original magnification). Abnormal mitoses were occasionally seen (inset, ×400 original magnification). F, Prominent tumor necrosis (× 400 original magnification).

Figure 4

Immunohistochemical photomicrographs. Immunohistochemicaly, the tumor cells showing A, Weak to moderate CD34 immunostaining (×200 original magnification). B, Strong and diffuse CD99 immunostaining (×200 original magnification). C, Ki-67 immunostaining, analyzed by ImmuoRatio quantitative image software, showing a 20% proliferative index (×400 original magnification).