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Benign fibrous histiocytoma: A rare case involving jaw bone

Benign fibrous histiocytoma (BFH) is a soft tissue neoplasm which occurs mostly on the skin of extremities. BFH rarely occurs in bone and may affect femur, tibia, and pelvic bone. Jaw bone involvement is very unusual with only 11 cases reported till date. This report describes a case of BFH occurring in a 30‑year‑old female patient affecting left mandibular posterior region. Computed tomography revealed a well‑defined expansile lytic lesion in the posterior mandible. Gross examination of the tumor revealed an admixture of fibroblasts and histiocytes in a fascicular and storiform pattern. Immunohistochemical staining was positive for CD68.


Introduction
Benign fibrous histiocytoma (BFH) is a soft tissue tumor originating from mesenchyme which occurs primarily on the skin of extremities. [1] The etiology of this entity is still obscure. BFH occurs rarely in bone and most of the cases were reported in the femur, tibia, and pelvic bone. Jawbone involvement is even rare. We report one such case of BFH occurring in left mandibular region in a 30-year-old female patient which was asymptomatic since 1-year.

Case Report
A 30-year-old female reported to Department of Oral Medicine and Radiology with a chief complaint of swelling in left lower back tooth region since 1-year. History revealed that patient had undergone extraction of mobile teeth in the same region, 1½ years back. After 5-6 months of extraction, she noticed a swelling in the same region. The swelling progressively increased in size and was associated with mild intermittent pain in left ear. There was no history of pain or any pus discharge in the associated area. Her past medical and family history was noncontributory. Extra-orally, mild facial asymmetry was noticed on the left side of face near the angle and ramus of the mandible. On palpation, a hard nontender swelling measuring approximately 2 cm × 2 cm was evident on left lower third of face extending from mid body of the mandible to 1 cm anterior to angle of the mandible. On intraoral examination, lower left molars were missing. Mucosa over the alveolar ridge was normal in color and texture. On palpation, 3 cm × 2 cm hard, nontender expansion of buccal and lingual cortices was evident extending from region distal to lower left second premolar up to the anterior border of ramus of mandible with obliteration of the lower buccal sulcus [ Figure 1].
Panoramic radiograph presented a multilocular radiolucency involving the left side of the mandible [ Figure 2]. Computed tomography (CT) revealed a well-defined expansile lytic lesion involving left angle and ramus of mandible with associated thinning of the cortices and breach in the lingual cortex of left ramus of the mandible [ Figures 3 and 4]. Contrast magnetic resonance imaging (MRI) showed that lesion was heterogeneously hyperintense on T2-weighted images, isointense on T1-weighted images with heterogeneous enhancement on contrast. Minimal soft tissue extension was noted along the floor of the mouth abutting the left mylohyoid muscle. The left inferior alveolar nerve (IAN) in the mandibular canal was also seen to be displaced by the lesion [ Figure 5].
Based on the painless and slow growing nature of the swelling, located in posterior mandible and existing radiographic appearance, the following conditions were     : Contrast magnetic resonance imaging showed that lesion was heterogeneously hyperintense on T2-weighted images, with heterogeneous enhancement on contrast considered in our differential diagnosis such as benign tumors of odontogenic origin such as ameloblastoma, ameloblastic fibroma, odontogenic myxoma, and keratocystic odontogenic tumor. Nonodontogenic tumors such as ossifying fibroma, osteoblastoma, desmoplastic fibroma, intraosseous mucoepidermoid carcinoma, and BFH were considered because of the predilection for occurrence in the mandible with a similar appearance.
The patient was operated under general anesthesia, and segmental mandibulectomy was performed using intraoral approach. Gross examination of the tumor revealed well encapsulated tumor mass with areas of firm but unmineralized yellow tan tissue and partially hemorrhagic red brown tissue [ Figure 6]. The microscopic examination revealed an admixture of spindle-shaped cells (fibroblasts) and epithelioid cells (histiocytes) in a fascicular and storiform pattern. Multinucleated cells and scattered lymphocytes were also seen. The stroma consisted of large areas of foamy cells interspersed with fibrovascular septations. No mitotic activity, cellular atypia or pleomorphism of tumor cells was evident. Vascularity of tumor varied from relatively inconspicuous vessels to exceedingly prominent vascularity with striking hyalinization [ Figure 7a-c]. The histopathological features were suggestive of BFH. Immunohistochemical staining showed positive CD68 cells in the lesional tissue [ Figure 7d]. The patient has been followed up periodically, but no recurrence or other changes were noted for 2 years.

Discussion
BFH affecting the oral and maxillofacial region is very rare. To the best of our knowledge, only three cases of BFH affecting maxilla and eight cases affecting mandible have been reported in the literature. Among the eight mandibular BFH, five involved posterior region, [2][3][4][5][6] two involved condylar region [7,8] and one extended from mandibular right incisor to the left premolar region. [9] In all the cases, age ranged from 30 to 50 years, with no significant gender predilection. The most common complaints were pain or a long-standing swelling indicating a slow growing nature of this entity. Our patient reported of mild ear pain which may be due to pressure on the auriculotemporal nerve, branch of IAN.
Buccolingual expansion with multilocular radiolucency and a sclerotic rim around the osteolytic defect were typical features in cases reported in the posterior region of the mandible. A similar kind of appearance was evident in our case. There are very few case reports explaining the CT and MRI features of this entity. In CT scan, an area of bony destruction with sclerosis at periphery and breach in the lingual cortex was evident in our case. MRI depicted heterogeneously hyperintense mass on T2-weighted images, isointense on T1-weighted images with heterogeneous enhancement on contrast.
Histological characteristics of BFH in bones are similar to those of BFH occurring in soft tissues. Proliferating fibrohistiocytic cells arranged in a storiform pattern interspersed with foam cells and giant cells is the hallmark of this tumor. Histological findings of BFH and nonossifying fibroma (NOF) are generally overlapping, so clinical and radiographic features may be helpful to differentiate between the two. NOF is seen during the early growth periods and it usually affects the metaphysis of long bones. On the contrary, BFH has been reported in middle age individuals in nonlong bones. Immunohistochemistry can be used to identify the presence of CD68 which is found in the cytoplasmic granules of a range of different blood cells and myocytes. It is particularly useful as a marker for the various cells of the macrophage lineage including monocytes, histiocytes, giant cells, Kupffer cells, and osteoclasts. Recently, KP-1 (CD68) has been described as a monoclonal antibody to a cytoplasmic epitope present on tissue histiocytes and macrophages. [10] BFHs are generally locally aggressive and tend to recur after curettage, hence phenol or liquid nitrogen should be applied to bony walls before bone grafting to clear the residual microscopic defect. Local excision followed by bone grafting is the treatment of choice. Wagner et al. had used piezoelectric-assisted cutting device for removal of BFH involving mandible to prevent dentoalveolar nerve injuries in critical cases. [6] Recurrence of this entity has been reported in several cases with a mean period of 1-2 years after treatment.

Conclusion
BFH of the jaw bone is an exceptionally rare entity with only 11 cases reported till date; the exact nature of the lesion is still not certain. Clinically and radiographically, it mimics the commonly occurring benign tumors of the jaw bone and should be considered in differential diagnosis. Small biopsy samples may be misleading in some cases. A long-term follow-up is required because of its local aggressive nature, high frequency of recurrence and likelihood of transformation to malignant form.

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