Intracerebral benign fibrous histiocytoma

Discussion

Intracranial BFH is a remarkably rare neoplasm, with less than 20 reported cases, in contrast to approximately 40 cases of the more common intracranial malignant fibrous histiocytoma (MFH).^1-7 These 2 pathologies exhibit distinct demographic profiles. Intracranial MFH predominantly affects individuals at an average age of 38.7 years, with a higher incidence in males, and the male-to-female ratio is about 2:1.⁶ Conversely, intracranial BFH is more commonly diagnosed in females, and the female-to-male ratio is 3:1, and tends to manifest in younger individuals, with an average age of 25.6 years, ranging from as young as 11 months to 56 years.^1-5 The significant demographic variations between these conditions warrant further exploration to elucidate potential underlying factors contributing to such differences.

The etiology of MFH is multifactorial and encompasses factors such as prior irradiation, surgical interventions, trauma, bone infarctions, Paget’s disease, and fibrous dysplasia.⁵ In sharp contrast, the causal factors for BFH remain elusive, although there are occasional suggestions of an association with past cranial trauma.⁵ Elucidating the precise etiology of BFH represents a direction for future research endeavors.

The histogenesis of intracranial fibrous histiocytomas (FHs) remains debated among researchers. Some hypotheses suggest that FHs connected to the dura mater may originate from mesenchymal stem cells, whereas exclusively intracerebral FHs may stem from cells within the perivascular pial sheath.² Remarkably, in both cases presented in this report, BFHs were situated entirely within the brain parenchyma, suggesting a potential origin in the perivascular pial sheath.

The BFH typically exhibits non-specific imaging characteristics. On T1-weighted MRI, the lesions often depict hypo- or isointensity, while their appearance on T2-weighted images may vary from hypo- to hyperintensity. Post-contrast T1-weighted images may display patterns of enhancement that are either heterogeneous or homogeneous.^3,5,8-10 In the presented cases, MRI revealed relatively well-defined intra-axial masses with heterogeneous enhancement and significant peritumoral edema. MRS results demonstrated maximal CHO/NAA ratios of 2.4 and 6.74, respectively, the differential diagnosis from inflammatory lesions. However, it is critical to conduct further investigations to elucidate the relevance of distinguishing BFH from MFH. So intracranial extra-axial BFH should be differentiated from meningioma, solitary fibrous tumor and osteoma, intra-axial BFH should be differentiated from malignant glioma, metastatic tumor, lymphoma, cavernous hemangioma and inflammatory lesions. In this case, both tumors were located in the brain and were initially misdiagnosed as gliomas before operation, likely due to the extremely low incidence of intracranial BFH. Therefore, the tumors exhibiting homogeneous or heterogeneous enhancement on MRI, relatively clear boundaries, peripheral edema, and a high CHO/NAA ratio on MRS should be considered the rare intracranial BFH as a potential differential diagnosis.^3,5,8-10

The diagnosis of BFH predominantly relies on histopathological evaluation and immunohistochemical (IHC) analysis. Histologically, BFH is marked by the proliferation of spindle-shaped fibrohistiocytic cells, with occasional occurrences of multinucleated giant cells and foam cells, typically organized in a storiform or swirling pattern. Importantly, malignant attributes such as nuclear atypia, pleomorphism, atypical mitosis, and necrosis are rarely found.⁵ The IHC represents a valuable tool for distinguishing BFH from other tumor types. For instance, GFAP is generally absent, facilitating the exclusion of pleomorphic xanthoastrocytoma. Conversely, EMA assists in differentiating BFH from meningiomas, although approximately 20% of BFH cases may exhibit EMA positivity.¹ Ki-67 expression aids in discerning benign from malignant forms of FH.³ In certain instances, electron microscopy may provide additional diagnostic insights.¹

The primary treatment modality for intracranial BFH centers around complete surgical resection, typically resulting in a favorable prognosis. However, it is essential to recognize that certain tumors may recur or malignantly transform, particularly those originating from the dura mater. Several cases have been documented in the literature, including instances of an intracranial BFH emerging from the pterygopalatine fossa recurring after near-total resection, a tumor in the occipital/suboccipital region that recurred and metastasized post-subtotal resection, and an initially benign intraventricular tumor that transitioned into MFH five years post-surgery.^1,4 Consequently, close postoperative monitoring with regular imaging and biopsy is highly recommended for intracranial BFH patients. Radiotherapy plays a limited benefit in the treatment of FH due to its unsatisfactory outcomes. The potential efficacy of chemotherapy in intracranial FH remains a subject of investigation, although promising results have been observed in MFH cases involving extracranial soft tissues.¹

One noteworthy aspect of the cases reported in this study is the absence of dural attachment. Both preoperative imaging and surgical findings conclusively established that the tumors were entirely intraparenchymal, devoid of any connection to the dura mater or the cerebral falx. Notably, most intracranial BFH cases reported in the literature have described extra-axial tumors originating from the scalp, cranial bone, or dura mater. In contrast, only four cases have been reported as intra-axial (Table 1). Intriguingly, these intra-axial BFHs, including our cases, shared a favorable prognosis following tumor resection, with no evidence of recurrence. This intriguing observation could suggest that intra-axial BFHs may have distinct biological characteristics compared to their extra-axial counterparts originating from the dura mater. Nevertheless, given the limited number of reported cases for both intracranial MFH and BFH, a comprehensive understanding of their cellular and molecular characteristics is imperative to elucidate whether dural and intraparenchymal BFHs are consistent in their molecular profiles.¹