BENIGN FIBROUS TUMOR ("FIBROMA")
Based on the preoperative MRI findings, the extra-axial lesion was considered to most likely represent a hemorrhagic metastasis. T2 weighted MR-signal intensity loss is mainly observed in hemosiderin or melanin containing lesions (3, 8) as well as calcified masses, cartilaginous or fibrous tissue. Thus, the differential diagnosis of entities with this radiographic finding encompasses calcified meningiomas, fibrous tumors, hemorrhagic lesions and melanoma. In our case, the unique total loss of T2-signal within a well-demarcated extra-axial mass, the intraoperative appearance and histopathological examination were most consistent with the diagnosis of a dural-based fibrous tumor.
When differentiating fibrous dural-based space occupying lesions, hemangiopericytomas (HPC), solitary fibrous tumors (SFTs), and meningiomas, especially the fibroblastic variant, (FM) should be considered. Hemangiopericytomas (HPC) typically show isointensity on T1- and isointensity to mild hyperintensity on T2-weighted images. The histomorphology is characterized by a moderate to highly cellular spindle cell tumor with patternless growth interrupted by staghornlike branching vessels and often accompanied by increased mitotic activity, and a focally dense reticulin network (5-6). The neoplastic cells stain positive for CD-34, CD99, and BCL-2, but are negative for EMA and cytokeratins (5-7, 9). Thus, imaging and histologic features exclude HPC in the present case.
Solitary fibrous tumor (SFT) is a mesenchymal tumor that shares many overlapping features with HPC, including the staghorn vascular pattern. In fact, whether or not SFT and HPC should be considered distinct lesions or part of the same spectrum of spindle cell tumors is currently under debate. Similar to HPC, the tumor cells are CD34-positive; however, the staining is more diffuse and robust (2, 6, 9). Hyper- and hypocellular areas are usually separated by characteristic thick bands of eosinophilic collagen (6, 9). Although showing many similarities with the present case, the lack of CD-34 staining was not compatible with this tumor entity.
Fibrous meningioma (FM) is another differential consideration. In this entity, the fibroblast-like tumor cells form intersecting fascicles and are embedded in a variably collagen-rich matrix (10). Nuclei exhibit meningothelial features such as nuclear pseudoinclusions and psammoma bodies, which were missing in our case (2, 7). Although fibrous meningiomas may not present all the classic histological hallmarks of meningioma mentioned above, they commonly stain positive for EMA, cytokeratins and S-100 (1). Also, the typical radiological findings of meningiomas, mostly characterized by homogenous contrast-enhancement, dural tail and hyperostosis were not be observed.
The unusual histopathological designation of "benign fibrous tumor" was confirmed by a German brain tumor reference center. Similar lesions have been reported before, for example, an infratentorial undetermined fibrous tumor (2) and an intrasylvian "fibroma" in a child with cystic fibrosis (4). The main difference from our case is that both cases were distinguished by the presence of extensive calcifications, resembling the soft tissue entity known as calcifying fibroma. Our patient has enjoyed a full neurologic recovery and was seizure-free at the last follow-up, 16 months after initial presentation. MR imaging showed no signs of recurrence of the benign fibrous tumor, nor did it show progression of the frontal falcine meningioma.
Contributed by Marian Christoph Neidert, Henning Leske, Katja Matoscevic, Günter Eisele, Elisabeth Rushing, Oguzkan Sürücü