Acanthomatous ameloblastoma is one of the rarest histological variants of ameloblastoma, a locally aggressive jaw tumor that arises from cells involved in tooth development. What sets the acanthomatous type apart under the microscope is its tendency to undergo squamous metaplasia, meaning the tumor’s central cells transform to resemble mature squamous epithelium and can produce keratin. That particular tissue pattern creates real diagnostic challenges, because it can look disturbingly similar to squamous cell carcinoma on a biopsy slide. Understanding the pathology and the tools available to tell these tumors apart has become more nuanced in recent years, especially as molecular findings open the door to targeted treatments.
What Makes This Variant Distinct
Ameloblastoma as a category is the most common tumor that originates from odontogenic (tooth-forming) tissue in the jawbone. It is classified as benign but behaves aggressively locally, infiltrating surrounding bone and soft tissue with a notable tendency to recur after surgery. Several histological subtypes exist, including follicular, plexiform, granular cell, desmoplastic, and acanthomatous. The acanthomatous variant stands out because the epithelial cells that make up the tumor undergo squamous differentiation in the central portions of the tumor islands, producing prominent intercellular bridges and sometimes frank keratin formation.
In practical terms, a pathologist looking at an acanthomatous ameloblastoma sees nests or islands of tumor cells where the peripheral layer still has the classic ameloblastoma appearance, with tall columnar cells arranged in a palisade pattern and nuclei pushed away from the basement membrane. But in the center of those islands, instead of the loose, stellate reticulum-like tissue seen in conventional follicular ameloblastoma, there are sheets of flattened squamous cells that may contain keratin pearls.
This squamous transformation is formally described as squamous epithelial metaplastic transformation of odontogenic tissue.1PubMed Central. Acanthomatous Ameloblastoma of Mandible in a Paediatric Patient Some researchers have speculated that chronic irritation from factors like calculus buildup or oral sepsis may promote this squamous metaplasia, though that explanation remains debated.2Journal of Research in Medical and Dental Science. Rare variant of Ameloblastoma – A Case report of Acanthomatous Ameloblastoma
Where and in Whom It Typically Appears
Like most ameloblastomas, the acanthomatous variant strongly favors the mandible over the maxilla. Reports put the mandibular occurrence at roughly four out of five cases, with the maxilla accounting for the remainder.2Journal of Research in Medical and Dental Science. Rare variant of Ameloblastoma – A Case report of Acanthomatous Ameloblastoma The posterior mandible is the classic location for ameloblastomas in general, though case reports document the acanthomatous type arising in less typical sites, including the anterior mandible crossing the midline.3PubMed Central. Acanthomatous ameloblastoma in anterior mandibular region of a young patient: A rare case report
The acanthomatous variant tends to be reported more often in older individuals, in contrast to ameloblastoma overall, which has a broad age range peaking in the third to fifth decades of life. That said, well-documented cases in young patients exist, and the age predilection is more of a general trend than a reliable rule.3PubMed Central. Acanthomatous ameloblastoma in anterior mandibular region of a young patient: A rare case report When maxillary tumors do occur, they tend to cluster in the molar region or extend into the maxillary antrum and nasal floor, which complicates both surgery and reconstruction.
Clinically, these tumors grow slowly for the most part. A patient may notice a painless swelling of the jaw that has been present for months or years before seeking care. By the time the tumor is diagnosed, it may have reached considerable size, with displacement of teeth, loosening of teeth, or visible expansion of the cortical bone.
How Ameloblastoma Destroys Bone
One of the defining behaviors of all ameloblastomas, including the acanthomatous variant, is their capacity to infiltrate and resorb bone. This is not a passive process. Ameloblastoma cells actively recruit and stimulate the formation of osteoclasts, the specialized cells responsible for breaking down bone tissue. Research has confirmed that ameloblastoma cells express and secrete RANKL, a signaling molecule that drives osteoclast formation, as well as TNF-alpha, another molecule that promotes bone resorption.4PubMed. Ameloblastoma induces osteoclastogenesis: a possible role of ameloblastoma in expanding in the bone
Laboratory experiments have shown that ameloblastoma cells can induce bone marrow cells to differentiate into functional osteoclasts capable of resorbing bone, and that RANKL plays a central role in this process. MMP-9, an enzyme that degrades the structural matrix of bone, is also expressed by these tumors.5PubMed. The role of RANKL and MMP-9 in the bone resorption caused by ameloblastoma This active bone-destroying mechanism explains why ameloblastomas can expand through cancellous bone, perforate cortical plates, and invade surrounding soft tissue, behaviors that make them far more clinically significant than their benign classification might suggest.
The Diagnostic Challenge of Squamous Differentiation
The central squamous metaplasia and keratin formation that define the acanthomatous variant are exactly what make diagnosis tricky. On a small biopsy specimen, the squamous elements can look convincingly like squamous cell carcinoma, a true malignancy with a fundamentally different prognosis and treatment plan. A pathologist who sees keratin pearls and sheets of squamous cells in a jaw lesion has to consider both possibilities carefully. Misdiagnosing an ameloblastoma as carcinoma can lead to unnecessarily aggressive treatment, while mistaking a carcinoma for an ameloblastoma can result in undertreatment.
The presence of the characteristic peripheral palisading, with tall columnar cells whose nuclei are polarized away from the basement membrane, is a critical clue that points toward ameloblastoma rather than carcinoma. But when biopsy material is limited or the architecture is disrupted, additional tools become necessary.
Calretinin Staining
Calretinin, a calcium-binding protein, has emerged as a useful immunohistochemical marker for ameloblastoma. Studies have found that ameloblastoma epithelium consistently stains positive for calretinin, while keratocystic odontogenic tumors (a common lesion in the differential diagnosis) rarely do. In one study, all 16 ameloblastoma cases showed positive calretinin staining, while only one keratocystic odontogenic tumor case was positive, and that positivity was limited to a narrow zone.6PubMed Central. Calretinin as a Diagnostic Adjunct for Ameloblastoma
The calretinin story gets more interesting when distinguishing ameloblastoma from squamous cell carcinoma in veterinary cases, which parallels the human diagnostic dilemma. In canine oral tumors, calretinin staining intensity was significantly different between acanthomatous ameloblastoma and oral squamous cell carcinoma, with carcinoma showing much stronger staining. Only a small proportion of canine acanthomatous ameloblastoma cases showed calretinin staining, and it was weaker in intensity.7PubMed. The expression of calretinin and cytokeratins in canine acanthomatous ameloblastoma and oral squamous cell carcinoma The direction of calretinin expression differs between human and canine tumors, which is an important caveat for researchers working across species.
Other Markers and Differential Considerations
Calcifying epithelial odontogenic tumor, another odontogenic lesion that can occur in similar jaw locations, sometimes enters the differential diagnosis. One immunohistochemical study examined insulin-like growth factor 1 expression in both calcifying epithelial odontogenic tumor and ameloblastoma and found no meaningful differences between them in expression values, rates, or staining intensity.8PubMed Central. Immunohistochemical study of insulin-like growth factor 1 in calcifying epithelial odontogenic tumor and ameloblastoma In other words, IGF-1 does not help distinguish between the two. The practical takeaway is that no single marker solves the diagnostic puzzle; pathologists rely on a combination of architectural features, immunohistochemistry results, and clinical context.
Molecular Genetics and What They Reveal
Over the past decade, the molecular landscape of ameloblastoma has come into sharper focus. Two recurrent mutations dominate: BRAF V600E and SMO. These mutations show a striking geographic pattern within the jaws. BRAF mutations are found almost exclusively in mandibular tumors, while SMO mutations predominate in maxillary ameloblastomas.9PubMed Central. Identification of recurrent SMO and BRAF mutations in ameloblastomas This site-specific mutation pattern has been confirmed across multiple independent studies, with additional rare mutations occasionally identified in genes like EGFR, KRAS, and NRAS.10PubMed Central. The landscape of genetic alterations in ameloblastomas relates to clinical features
The BRAF V600E mutation is the same mutation found commonly in melanoma and certain thyroid and colorectal cancers, and its presence in ameloblastoma has opened a doorway to targeted drug therapy. In one study of 74 ameloblastoma cases, roughly two-thirds showed BRAF V600E positivity. There was a nonsignificant trend toward earlier recurrence in BRAF-mutated tumors, with a shorter median disease-free survival compared to tumors without the mutation, but the difference did not reach statistical significance.11PubMed. Analysis of BRAF V600E expression and disease-free survival in patients with ameloblastoma Whether BRAF status will eventually prove to be a reliable prognostic marker is still an open question that needs larger studies to settle.
Targeted Therapy for Difficult Cases
Because most mandibular ameloblastomas carry the BRAF V600E mutation, the drugs developed to target this mutation in other cancers have been tried in ameloblastoma patients, particularly those with recurrent or advanced disease where repeat surgery would be disfiguring or impractical. The combination of dabrafenib (a BRAF inhibitor) and trametinib (a MEK inhibitor) has shown genuinely encouraging results in case reports and small series.
In one well-documented case, an individual with a recurrent mandibular ameloblastoma carrying the BRAF V600E mutation received 18 months of neoadjuvant therapy with dabrafenib and trametinib. The tumor shrank from about 73 mm to 56 mm, and on histopathological examination after treatment, the tumor showed extensive degenerative changes with essentially no remaining proliferative activity.12PubMed Central. Efficiency of B-RAF-/MEK-inhibitors in B-RAF mutated Ameloblastoma: Case report and review of literature A systematic review of reported cases found treatment results ranging from tumor size reduction to complete resolution, across patients with initial diagnoses, recurrences, and even metastatic disease.13PubMed Central. BRAF inhibitors in BRAF V600E-mutated ameloblastoma: systematic review of rare cases in the literature A broader systematic review confirmed that BRAF/MEK inhibitor therapy led to significant tumor regression and improved quality of life, though manageable side effects were noted.14PubMed Central. Molecular-targeted therapy in ameloblastoma: a systematic review
These drugs are not yet standard first-line treatment. Surgery remains the primary approach for most ameloblastomas. But for recurrent tumors, tumors in locations where surgery would cause severe functional or cosmetic deficits, or cases where a patient is not a surgical candidate, targeted therapy represents a real and growing option.
Surgical Management and Recurrence
The debate over how aggressively to treat ameloblastoma surgically has persisted for decades. Conservative approaches, such as enucleation (shelling out the tumor) with curettage, preserve more bone and cause less deformity, but they come with substantially higher recurrence rates. In one series, six of ten patients treated with enucleation and curettage experienced recurrence, compared to one of ten in the surgical resection group.15PubMed Central. Surgical management of ameloblastoma: Conservative or radical approach A larger institutional review found a recurrence rate of roughly 6% after radical resection versus about half of all cases treated with limited surgery.16PubMed Central. Ameloblastoma: 25 Year Experience at a Single Institution
A long-term follow-up study of 305 ameloblastoma cases found that disease-free survival was significantly better with resection including a bone margin compared to conservative treatment. There was no meaningful difference between resection with bone margin and full segmental resection, suggesting that an adequate safety margin matters more than how radical the procedure is. The same study noted that conservative approaches may still be reasonable for patients in their first decade of life, or for unicystic and plexiform variants that tend to be less infiltrative.17PubMed. Long-term follow up on recurrence of 305 ameloblastoma cases
For the acanthomatous variant specifically, the same general principles apply, though the rarity of the subtype means there are no large comparative surgical trials exclusive to it. The presence of squamous differentiation does not in itself change the surgical approach; the tumor’s location, size, and relationship to surrounding structures are what drive the decision.
Reconstruction After Jaw Resection
Resecting a portion of the mandible or maxilla to achieve adequate margins creates an obvious reconstructive challenge. Patients need a functional jaw for chewing, speaking, and maintaining facial contour. Several reconstructive options exist, including reconstruction plates, free bone grafts, and vascularized free tissue flaps, with the fibula free flap being the workhorse for mandibular reconstruction.
Comparative studies have found that free tissue flaps offer the most reliable long-term outcomes. In one review, complications like screw loosening occurred in cases reconstructed with plates, and graft exposure was seen with free bone grafts, while free flaps had a lower complication rate overall and produced better facial contour and speech outcomes.18PubMed Central. Management of Ameloblastoma with Free Tissue Flap in Comparison with Other Reconstructive Options Available Folded fibula flaps can restore the natural curvature and height of the mandible, and patients have been successfully rehabilitated with dental implants placed into the grafted bone.19Scientific Reports. The role of folded fibular flap in patients’ reconstruction of mandibular defects: a retrospective clinical study
Quality-of-life data after mandibular reconstruction following ameloblastoma resection show that most patients achieve good physical and social function scores. However, appearance and chewing ability remain the domains where patients report the most concern, particularly when defects involve the central segment of the mandible.20PubMed. Reconstruction of mandibular defects involving the central segment with fibula osteoseptocutaneous free flap following ameloblastoma resection These patient-reported outcomes are important context for surgical planning, because the decision between conservative and radical treatment is not just about recurrence rates; it is also about what the patient will live with afterward.
Pediatric Considerations
Managing ameloblastoma in children and adolescents introduces a unique tension. The jawbone is still growing, and aggressive surgical resection can disrupt facial development in ways that compound over years. At the same time, undertreating the tumor invites recurrence, which means additional surgeries down the road. Children also have higher cancellous bone content and more active periosteal growth, which some authors argue favors a more conservative initial approach that preserves bone architecture and function while minimizing growth disruption.21Bharati Vidyapeeth Journal of Dentistry and Allied Sciences. Global Insights into Pediatric and Adolescent Ameloblastoma: A Comprehensive Systematic Review
The histological subtype matters here. Unicystic ameloblastomas, which are more common in younger patients than the acanthomatous variant, are often adequately treated with conservative surgery. Multilocular or mural unicystic variants may still require more radical resection. When an acanthomatous pattern is identified in a pediatric patient, it is a genuinely rare finding, and treatment decisions have to weigh the tumor’s locally aggressive behavior against the long-term consequences of bone loss in a developing jaw.
Rare Malignant Transformation
Ameloblastoma is classified as benign, and the vast majority of cases, including the acanthomatous variant, behave accordingly: they infiltrate locally but do not spread to distant sites. Malignant transformation, where the tumor acquires features of true malignancy and metastasizes, is extraordinarily rare but documented. One well-studied case described a mandibular ameloblastoma that changed in appearance over multiple recurrences, progressing from a follicular pattern without atypia to undifferentiated and squamoid features. Eventually, the tumor metastasized to the orbit, and both the architectural and immunohistochemical profile of the metastatic deposit matched the most advanced recurrent tumor rather than the original primary.22SpringerLink / Virchows Archiv. Ameloblastoma of the mandible metastasizing to the orbit with malignant transformation
This sort of case is the exception, not the rule, but it underscores why long-term follow-up after ameloblastoma treatment is standard practice. Recurrences can appear years or even decades after initial surgery, and each recurrence carries at least a theoretical risk of progressive dedifferentiation. For the acanthomatous variant, the existing squamous differentiation might raise concern about confusion with malignant transformation, but the presence of keratin and squamous features alone does not indicate malignancy. Cellular atypia, high mitotic rates, and loss of the characteristic peripheral palisading pattern are what push a pathologist toward a malignant diagnosis.
Acanthomatous Ameloblastoma in Dogs
One of the more interesting wrinkles in the acanthomatous ameloblastoma story is that this variant is far more common in dogs than in humans. Canine acanthomatous ameloblastoma is one of the most frequently diagnosed oral tumors in dogs, occurring predominantly along the gingiva of the rostral mandible or maxilla. In veterinary literature, this tumor’s local aggressiveness is well recognized: it causes alveolar bone lysis and frequently extends beyond alveolar bone margins.23PubMed. Cytologic features of an acanthomatous ameloblastoma in a dog
The biological behavior of canine acanthomatous ameloblastoma shows substantial variability. A study of 45 cases using computed tomography and histopathology found that some tumors grew slowly with minimal bone loss, while others caused severe destruction of both cancellous and cortical bone. Tumors that grew predominantly within bone rather than extending outward were associated with more invasive behavior.24PubMed. Biological Behavior of Canine Acanthomatous Ameloblastoma Assessed With Computed Tomography and Histopathology: A Comparative Study The canine model has been useful for understanding the biology of this tumor type more broadly, and the comparative pathology has informed diagnostic approaches in both species, even as immunohistochemical markers like calretinin behave differently between human and canine tumors.