Ameloblastic Fibroma: Clinical features, Radiology and Histopathology

Ameloblastic fibroma (AF) is a rare odontogenic tumor characterized by simultaneous neoplastic proliferation of epithelial and ecto-mesenchymal tissues without dental hard tissue formation. This tumor is hence categorized as a mixed odontogenic tumor. Okay…so what does this mean? Well, odontogenic tumors usually have either a proliferative epithelial component like in an ameloblastoma or a proliferative mesenchymal component like in an odontogenic myxoma. Ameloblastic fibroma however, essentially has a proliferative epithelial and a mesenchymal component. The mesenchyme or the connective tissue resembles the ecto-mesenchymal dental papilla of a tooth germ. Strands or small nests of epithelia proliferating in the mesenchymal tissue resemble the dental lamina present in a tooth germ. But in spite of their striking resemblance
to components in the tooth germ, there is no formation of hard tissue in this tumor. In the past, investigators suggested AFs to be hamartomas and were considered to be lesions in the immature end of the spectrum of complex odontomas. What this meant was, an AF would differentiate to become an ameloblastic fibro-odontoma (AFO) which would later mature to become a complex odontoma (CO). However, this theory was disputed on the basis of demographic evidence. If the theory was true, it would mean that patients with AF should be younger than patients with AFO and CO. On the contrary, evidence states that on an average AFs occurs at an older age than AFOs. AFs are also known to occur beyond the age of completion of odontogenesis (>20 years). Furthermore, AFs are known to recur more than the other two lesions and can also grow to large sizes. Hence AF is considered to be a true neoplastic odontogenic tumor whereas CO is considered to be a hamartoma. The World Health Organization (WHO) in their recent classification of odontogenic tumors in 2017 have removed the entities ameloblastic fibro-dentinoma and AFO as these lesions are now considered to represent part of the spectrum of changes seen in developing COs. AFs are rare tumors representing around 2% of all odontogenic tumors. They are predominantly lesions of childhood and adolescence with 77% of these lesions occurring before the age of 20 with a mean age of 14.6 years. They have a slight male predilection and are usually asymptomatic. They may however grow to large sizes and cause considerable swelling. These lesions most commonly (75%) occur in the posterior mandible followed by the posterior maxilla and are 75% of the times associated with an impacted tooth. Radiographically AF manifests as an unilocular lesion but could also be multilocular when large in size. It is a well circumscribed lesion with corticated borders. Most AFs are seen to be associated with an impacted tooth making it difficult to differentiate them from dentigerous cysts. Under the microscope, AFs show cords, strands or small nests and islands of cuboidal odontogenic epithelium resembling the dental lamina. These strands or cords are composed usually of two layers of epithelium. The nests or islands resemble follicular islands seen in ameloblastoma, with a peripheral layer of cuboidal or columnar cells surrounding
a central mass of cells resembling the stellate reticulum. The connective tissue is loose and very primitive (immature) with delicate collagen fibrils and angular or spindle shaped mesenchymal cells. The connective tissue very starkly resembles the dental papilla in a developing tooth germ. Sometimes juxta-epithelial hyalinization surrounding the epithelial strands or islands is seen in the connective tissue. This may be because of an inductive effect of the epithelium on the mesenchymal tissue, much like the effect the inner enamel epithelium has over the dental papilla. As far as the treatment and prognosis is concerned, AF is a lesion with good prognosis and conservative treatment is usually sufficient. Surgical excision with thorough curettage
is usually recommended. Though the recurrence rate for the lesion
is low, there are studies reporting recurrence rates of 16-18 % for these lesions. There are also reports of recurrent AFs transforming to become ameloblastic fibro-sarcomas. Hence a complete excision with regular clinical and radiographic follow up is recommended for treating AF.

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