Contributed by Teresa LaCaria, MD and John Ozolek, MD
Published on line in May 2007
The patient is a 17-year-old female with a history of neurofibromatosis type 1 (NF1) and Arnold-Chiari type I malformation, who presented with painful enlargement of a subcutaneous left thigh mass. The mass, which had been present for several years, was previously stable at 1.5 cm and assumed to be a neurofibroma. Over several months, the lesion grew to 2.5 cm and became painful. These changes were worrisome for transformation of the assumed neurofibroma to a malignant nerve sheath tumor.
The mass was resected revealing a cystic lesion with a smooth gray-brown laminar wall and hemorrhagic center. Histologic sections demonstrated a nodular proliferation of histiocytoid cells surrounded by a densely fibrotic capsule [Figs. 1 and 2]. The cellular areas were fragmented and contained abundant hemorrhage, fibrin, and hemosiderin pigment [Fig. 3]. Unlined "pseudoangiomatoid" spaces were prominent within the cellular and capsular tissue, containing red blood cells and resembling vessels [Fig. 4]. The histiocytoid cells were oval to spindle shaped with moderate cytoplasm, even chromatin, and small central nucleoli [Figs. 5 and 6]. Mitotic activity or other cytologic features of malignancy were not identified. The capsule was peripherally infiltrated by small mature lymphocytes, in some areas forming follicles [Figs. 7 and 8].
Immunohistochemical stains demonstrated that the histiocytoid cells were focally positive for CD68 [Fig. 9], CD99, and EMA, and negative for CD163, desmin, S-100, CD31, CD34, CD21, HMB-45, HHF-35, and wide spectrum cytokeratin. The fibrous capsule also contained cells positive for CD68 and CD99 as well as CD163 [Fig. 10].