Peripheral odontogenic fibroma exhibiting features of benign fibrous histiocytoma: Report of an unusual case

INTRODUCTION

Odontogenic fibroma (OF) is a rare benign mesenchymal odontogenic tumor consisting of mature fibrous connective tissue interspersed with varying amounts of inactive odontogenic epithelial islands and focal areas of calcification. It may manifest either as a central (intraosseous) or peripheral (extraosseous) variant.^[1] On the basis of the amount of presence of odontogenic epithelium, the World Health Organization (WHO) in 2005 classified OF into two histopathological subtypes, namely complex or epithelium-rich type (WHO-type) and simple or epithelium-poor type. However, in 2017, the WHO excluded the simple type from the classification.^[2]

Peripheral OF (POF) is more common than central and has a female predilection. It usually manifests in the 2^nd– 4^th decades of life and has a site predilection for the anterior gingiva of mandible and maxilla.^[3] Here, we report a case of POF (WHO- type) in the mandibular gingiva of a 52-year-old male.

CASE REPORT

A 52-year-old male reported to a tertiary health care center with a complaint of painless swelling in the lower posterior gingiva for the past 6 months. The patient was apparently well 6 months ago. Then, he started noticing a small fleshy painless mass in the left lower posterior gingiva, which slowly grew to its present size. The patient’s medical, family, and personal histories were nothing significant. His general survey examination and vitals were within normal limits.

Clinical findings

On extraoral examination, there was mild facial swelling and asymmetry on the left lower third of the face. No scar mark and/or discharging sinus was present. Regional submandibular lymph nodes were palpable, firm, mobile, and non-tender. On intraoral examination, there was a firm, pedunculated, non-ulcerated, lobulated growth in the gingiva extending from 34 to 38 teeth region [Figure 1]. It was non-tender on palpation. 18, 28, 38, and 48 were missing. 26 and 27 were extracted earlier as those were decayed due to caries. 36 became mobile due to the growth and was extracted in the recent past. Based on the patient’s history and clinical evaluation, a provisional diagnosis of a reactive gingival lesion was made. Differential diagnosis included peripheral ossifying fibroma, pyogenic granuloma, peripheral giant cell granuloma, and localized gingival enlargement.

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Figure 1: Intraoral view showing a firm, pedunculated, nonulcerated, lobulated, non-pulsatile growth in the gingiva extending from 34 to 38 teeth region.

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Radiological findings

Orthopantomogram of the jaws revealed no such gross abnormality. Neither was there any severe bone loss in the 34–38 teeth region [Figure 2]. However, along with all the 3^rd molars, 26, 27, and 36 were also missing.

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Figure 2: Orthopantomogram of jaws showing no such gross abnormality or severe bone loss in 34-38 region (red arrow).

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Histopathological findings

Histopathological section stained with an eosin revealed the presence of two bits of tissue showing a hyperplastic, nondysplastic, parakeratinized stratified squamous epithelium with elongated rete pegs [Figure 3]. The underlying connective tissue stroma had a “grenz zone” beneath which lay the lesion proper. One side of the lesion proper in the stroma had mixed hypo and hypercellular areas. The hypocellular areas showed thin spindle-shaped fibroblasts and collagen fibers, whereas the hypercellular areas contained numerous plump proliferating fibroblasts and collagen fiber bundles. Islands and cords of darkly stained odontogenic epithelial cells were interspersed within the stroma [Figure 4]. Blood vessels and chronic inflammatory cells were also noted. Focal areas of calcification were appreciated. The other side of the lesion proper in the stroma exhibited spindle-shaped fibroblasts arranged in a fascicular manner and storiform pattern [Figure 5]. Multiple polygonal and plump histiocytic cells were present between the fibroblasts [Figure 6]. Numerous blood vessels, extravasated RBCs, and chronic inflammatory cells were also seen.

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Figure 3: Hematoxylin and eosin-stained (10x view) section showing hyperplastic, non-dysplastic, parakeratinized stratified squamous epithelium with elongated rete pegs (marked with black arrow). The underlying connective tissue stroma had a “grenz zone.” (purple arrow).

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Figure 4: Hematoxylin & eosin stained section (40x view) Islands and cords of darkly stained odontogenic epithelial cells interspersed within the stroma (green arrow).

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Figure 5: Hematoxylin & eosin stained section (10x view) spindleshaped fibroblasts arranged in a fascicular manner and storiform pattern (black arrow).

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Figure 6: Hematoxylin & eosin stained section (40x) multiple polygonal and plump histiocytic cells (marked with blue arrow).

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Based on the above features, a histopathological diagnosis of POF along with benign fibrous histiocytoma (BFH) like areas was made [Table 1].

Table 1: Features and findings of the case.

Features	POF	BFH	Present case
Location	Gingiva	Buccal mucosa, tongue, and gingiva	Gingiva
Age	30–50 years	40–60 years	52 years
Clinical findings	Slow growing, firm, pedunculated or sessile, non-ulcerated, lobulated, non-tender growth	Slow-growing, sessile, non-tender, well-circumscribed growth with intact overlying mucosa	Slow growing, firm, pedunculated, non-ulcerated, lobulated, non-tender growth
Histopathology	Parakeratinized stratified squamous surface epithelium backed by connective tissue stroma Stroma exhibits spindle-shaped, plump, proliferating fibroblasts and collagen fiber bundles Islands and cords of darkly stained odontogenic epithelial cells. Blood vessels and chronic inflammatory cells. Focal areas of calcification were appreciated	Parakeratinized stratified squamous surface epithelium backed by connective tissue stroma. Stroma exhibits spindle-shaped fibroblasts and collagen fibers arranged in a fascicular manner and storiform pattern. Presence of multiple polygonal, plump histiocytic cells Numerous blood vessels and chronic inflammatory cells are also seen.	Both POF and BFH-like areas are present.

POF: Peripheral odontogenic fibroma, BFH: Benign fibrous histiocytoma

The post-operative phase revealed proper healing with no sign of recurrence.

DISCUSSION

POF is an odontogenic tumor and is of ectomesenchymal origin. The common and potential sources of POF are the rests of dental lamina.^[4] According to Farman, the epithelial ectomesenchymal interaction was the main cause for the secondary growth of the dental lamina rests.^[5] POF usually does not cause resorption of the underlying bone. Rarely, superficial erosion of the underlying bone is seen.^[6] Neither is there any tooth displacement.

The clinical differential diagnosis of POF is peripheral ossifying fibroma, pyogenic granuloma, focal gingival hyperplasia, peripheral giant cell granuloma (PGCG), peripheral ossifying fibroma, and peripheral ameloblastoma. Differentiation among these lesions was done during the histopathological study. Peripheral ossifying fibroma was excluded because it usually does not contain odontogenic epithelial rests. Pyogenic granuloma was also not considered because pyogenic granuloma exhibits vascular proliferation similar to granulation tissue. Focal gingival hyperplasia usually exhibits proliferation of the gingival epithelium along with proliferation of the connective tissue. But neither the odontogenic islands nor the storiform pattern is seen. Hence, focal gingival hyperplasia was not considered. PGCG was excluded because of the lack of numerous giant cells. Histopathological diagnosis of peripheral ameloblastoma was also not given because the odontogenic epithelial islands were inactive, small, and resting in nature. Stellate reticulum-like cells were not appreciable.

BFH itself is a benign mesenchymal tumor consisting of biphasic cell population, i.e., fibroblasts and histiocytes.^[7] Collagen fibers are arranged in a fascicular and storiform pattern. Prominent vascularity and inflammatory infiltrate are also seen.

The striking feature in this case report of POF is its concurrent presence of BFH-like areas. Such type of presentation in POF is a novel finding.

OF is more prone to mixed histopathological presentations. There are many reported cases of OF with concurrent features of giant cell granuloma,^[8] dentigerous cyst,^[9] and granular cell odontogenic tumor.^[10] In this case, we have got POF with features of BFH. This appearance may be due to some reactive response of the body toward developing POF. It may also be said that as both POF and BFH have the same source of mesenchymal component, POF may exhibit features of BFH. Or else, it may be a collision tumor which may develop due to synchronous occurrence of POF and BFH in the same area.

CONCLUSION

POF, being a rare odontogenic neoplasm, is usually less talked about and also not commonly included as a differential diagnosis of benign gingival growths. This case is a novel finding of POF with BFH-like areas, which the pathologist should always keep in mind.