Oral and Maxillofacial Tumors, Cysts, and Tumor‐Like Lesions

Oral and Maxillofacial Tumors, Cysts, and Tumor‐Like Lesions

Jason Soukup1 and John Lewis2

1 School of Veterinary Medicine, University of Wisconsin‐Madison, Madison, WI, USA

2 Veterinary Dentistry Specialists, Chadds Ford, PA

Knowledge of clinical appearance and biological behavior of maxillofacial tumors is an important component of everyday veterinary practice. This chapter reviews the important clinical information of maxillofacial tumors, cysts, and tumor‐like lesions of dogs and cats, including odontogenic tumors, non‐odontogenic tumors, odontogenic cysts, bone‐associated lesions, and tumor‐like lesions of the gingiva and oral mucosa. Staging and incisional biopsy techniques will also be discussed.

7.1 Introduction: Terminology

The term “tumor” is derived from the Latin tumere, “to swell.” The term has evolved to describe benign or malignant neoplastic disease. Tumors are generally considered to be either benign or malignant. Benign tumors may be locally invasive but do not metastasize to distant sites. Malignant tumors are not only locally invasive but also have the potential to metastasize to other sites in the body. In this chapter we will broadly define tumors and similar lesions of the maxillofacial region as non‐odontogenic or odontogenic and then describe their biological nature as either benign or malignant. Odontogenic tumors are defined by the World Health Organization (WHO) as “lesions derived from epithelial, ectomesenchymal, and/or mesenchymal elements that still are, or have been, part of the tooth‐forming apparatus.” Odontogenic tumors comprise a large portion of oral tumors seen in dogs and are less common in cats than non‐odontogenic tumors. Some odontogenic tumors, such as odontomas, are not true tumors. They are hamartomas, which are disorganized accumulations of histologically normal tissues. Any tumor that does not arise from odontogenic tissues is considered to be non‐odontogenic. Cysts, on the other hand, are not tumors but rather epithelium lined potential spaces, which, in the maxillofacial region, are generally of odontogenic origin.

“Epulis” is a non‐specific, clinical term used to describe tumors and tumor‐like masses of the gingiva. The term is derived from the Greek epi‐oulon meaning “on the gum.” In the human literature, the term is used to describe any focal growth of the gingiva. Within the veterinary literature, this term has been used indiscriminately in various contexts and it is the authors’ opinion that the term “epulis” should be avoided in lieu of more specific descriptions.

7.2 Maxillofacial Oncological Principles

7.2.1 Clinical Presentation

Oral tumors account for approximately 5% of all canine tumors and approximately 6–10% of feline tumors [1, 2]. In dogs, the oral cavity is the fourth most common site of neoplasia. There is not a definitive gender or breed predilection, but many purebred dogs have been reported to have a higher risk of developing oral tumors, including Cocker Spaniels, Chow Chows, Retrievers, and Poodles.

Dogs and cats are presented once an oral mass is noticed by the pet owner or by its veterinarian during routine physical examination. Some patients have a history of mobile teeth in the area of the tumor, and the cause of tooth mobility may have originally been presumed to be due to periodontal disease. Animals having neoplasia of the caudal oral cavity or tonsils are usually diagnosed later in the course of the disease. Clinical signs of oral tumors include halitosis, oral hemorrhage (spontaneous or after eating, drinking, chewing, or playing with objects), drooling, maxillofacial deformity, difficulty or pain upon opening the mouth, weight loss, or exophthalmos. With advanced maxillary tumors, there is invasion of the nasal cavity with subsequent sneezing, nasal discharge/hemorrhage. Some patients are diagnosed during a professional dental cleaning procedure or are presented for an enlarged mandibular lymph node.

7.2.2 Principles of Tumor Staging

The TNM (Tumor‐Node‐Metastasis) System devised by the WHO is the standard system for most tumors in veterinary medicine. Currently the TNM is developed and maintained by the Union for International Cancer Control (UICC) to achieve consensus on one globally recognized standard for classifying the extent of spread of cancer. The TNM classification is also used by the American Joint Committee on Cancer (AJCC) and the International Federation of Gynecology and Obstetrics (FIGO). In 1987, the UICC and AJCC staging systems were unified into a single staging system (www.uicc.org/resources/tnm). The system is used to assess the extent of tumor involvement based upon primary tumor (T), regional nodes (N), and metastasis (M). Information about the primary tumor, regional lymph nodes, and distant metastasis are used to provide a stage of I, II, III, or IV (Table 7.1).

Table 7.1 Clinical staging (TNM) of oral tumors in dogs and cats. TX or NX can be used initially if the tumor or node cannot be assessed at the time of diagnosis. T0 or N0 can be used if there is no sign of tumor (T0) or if tumor cells are absent from lymph nodes (N0).



Tumor in situ (pre‐invasive carcinoma)

Tumor 2 cm or less at maximum diameter

T1a Without bone involvement

T1b With bone involvement

Tumor 2–4 cm maximum diameter

T2a Without bone involvement

T2b With bone involvement

Tumor more than 4 cm maximum diameter

T3a Without bone involvement

T3b With bone involvement

Regional lymph nodes (mandibular, retropharyngeal, and parotid)

Regional lymph nodes not palpable

Movable ipsilateral lymph nodes

N1a No evidence of lymph node metastasis

N1b Evidence of lymph node metastasis

Movable contralateral lymph nodes

N2a No evidence of lymph node metastasis

N2b Evidence of lymph node metastasis

Fixed lymph nodes

Distant metastasis

M0 No evidence of distant metastases

M1 Distant metastases present
Stage grouping
Stage grouping T N M
I T1 N0, N1a or N2a M0
II T2 N0, N1a or N2a M0
III T3 N0, N1a or N2a

Any T N1b M0
IV Any T N2b or N3 M0

Any T Any N M1

TX or NX can be used initially if the tumor or node cannot be assessed at the time of diagnosis.

T0 or N0 can be used if there is no sign of tumor (T0) or if tumor cells are absent from lymph nodes (N0).

Dogs and cats with an oral or maxillofacial tumor should be thoroughly evaluated. A thorough examination and staging can be done under general anesthesia at the time of incisional biopsy. The tumor is palpated and measurements are taken in three dimensions. The tumor’s attachment to adjacent tissues can be assessed and categorized as pedunculated or broad‐based. The teeth are assessed for mobility and the bone is assessed radiographically. Regional lymph nodes are palpated to assess size and mobility. Mandibular lymph node enlargement may be associated with an oral mass, especially in cases of tonsillar squamous cell carcinoma (SCC) and melanoma. Medial retropharyngeal lymph node enlargement can be difficult to discern because this lymph node is deep in the neck. The parotid lymph node is located at the rostral aspect of the parotid salivary gland and is typically not palpable. Assessment of the retropharyngeal lymph nodes is important, since both the parotid and mandibular nodes drain to the medial retropharyngeal lymph node and anastomotic connections exist from the medial retropharyngeal lymph node to the contralateral medial retropharyngeal node in the dog [3].

Depending on tumor location, the full extent of tumors may be difficult to assess with dental radiography. Computed tomography (CT) is particularly helpful for maxillary, caudal mandibular, and pharyngeal masses. Head and neck CT may diagnose enlargement of the medial retropharyngeal lymph nodes, which may not be evident on clinical examination. CT provides a more sensitive evaluation than three‐view thoracic radiographs for detection of small nodule pulmonary metastasis [4]. Three‐dimensional CT reconstructions of the primary tumor can be very helpful for surgical planning. Magnetic resonance imaging (MRI) provides assessment of various soft tissues, but is limited in its usefulness in evaluating bone. Since evaluation of bony structures is often important with oral and maxillofacial tumors, CT is often the test of choice for advanced imaging of the mouth, face, and head.

7.2.3 Principles of Diagnostic Imaging

Intraoral radiography is essential in evaluating oral tumors. Malignant tumors often show a combination of bone lysis and periosteal proliferation. Teeth in the area of a malignant tumor often show evidence of root resorption by the encroaching tumor. In contrast, benign tumors may show no lysis or central mineralization (e.g., peripheral odontogenic fibromas (POFs)), or focal lysis (e.g., acanthomatous ameloblastoma), little or no periosteal proliferation, and displacement of teeth rather than root resorption. Radiographic evidence of demineralization may not be evident until greater than 40% of bone becomes demineralized [5]. Since obtaining surgical margins around a tumor may require removal of radiographically normal bone, ostectomy may still be necessary in the absence of significant bony changes, if tumor type dictates aggressive treatment. Intraoral dental radiographs are most helpful when performed with a size 4 or larger phosphor plate or conventional size 4 dental radiographic films.

7.2.4 Principles of Biopsy Acquisition

Incisional biopsy is an important first step to discern the biological behavior of an oral tumor and treatment options. Even when excisional surgery is expected to be the best option, an initial incisional biopsy will help to determine appropriate surgical margins. The goal of incisional biopsy is to obtain a representative sample without increasing morbidity and without making definitive treatment of the tumor more difficult. For most oral tumors, an incisional biopsy is best taken through the oral mucosa rather than through the skin. A variety of instruments may be used, including a number 15 blade, a biopsy punch, a Tru‐cut™ biopsy instrument, and in some cases rongeurs. The authors prefer use of a number 15 blade to obtain one or more deep but narrow wedges that may be easily sutured with 4‐0 or 5‐0 absorbable monofilament on a tapered needle. If the tumor is too friable to hold suture, a variety of hemostatic agents may be used at the biopsy site in lieu of suturing the site. The pet owner should be warned of potential for post‐operative bleeding. Some oral masses may exhibit necrotic or severely ulcerated areas: these sites should be avoided in favor of sites without necrosis or infection. If the mass is small and pedunculated, such as in cases of suspected POFs, removal at the level of the normal gingiva is appropriate with careful documentation of where the mass arose from on the dental record to ensure no confusion of the original site when/if further surgery is necessary. When performing regional nerve blocks prior to biopsy, it is important to avoid seeding of the tumor into the deeper tissues via travel of the needle. For example, if a suspected melanoma is centered over the infraorbital foramen, then a maxillary nerve block may be more appropriate than a deep infraorbital block.

The anesthetic event for the incisional biopsy is an opportunity to perform a thorough examination of the patient. Regional lymph node aspirates and photos of the primary tumor site can be obtained for planning of future curative‐intent surgery. Debulking of aggressive oral masses is generally unrewarding. In fact, in some cases, debulking may actually cause more morbidity than doing nothing, due to lack of normal healing of tumor tissue and increased bleeding from the debulked site. One exception where debulking may be helpful is with pedunculated tumors: this may provide the opportunity to exchange a large area of tumor ulceration for a smaller area of ulceration at the pedunculated incision site.

7.3 Non‐odontogenic Tumors

7.3.1 Malignant Oral and Maxillofacial Tumors

The most common malignant oral tumor in the dog is malignant melanoma (MM), followed by SCC and fibrosarcoma (FSA). In the cat, SCC is the most common oral tumor, accounting for 60–70% of cases, followed by FSA. Malignant Melanoma

MM is the most common oral and maxillofacial tumor in dogs, but oral MM is rare in cats. Breeds with pigmented oral mucosa appear to be predisposed to MM. Melanoma is usually ulcerated and friable, readily bleeding and often pigmented. However, it is not uncommon for MM to be amelanotic (Figure 7.1). Necrosis is common in larger tumors. Melanoma is usually located on the tongue, gingiva, and mucosa (alveolar, labial, or buccal mucosa). Large pigmented tumors are likely to be MM (Figure 7.2). Histologically, MM may show a lack of differentiation and be diagnosed as undifferentiated malignant tumor or anaplastic sarcoma. A benign variant of oral melanoma, often referred to as oral melanocytoma, has been described. Melanocytomas tend to be very well circumscribed and less than 1 cm in diameter at the time of diagnosis (Figure 7.3). They respond well to marginal or wide excision rather than radical mandibulectomy or maxillectomy [6]. Oral MM has a variable metastatic rate, with metastasis to the nodes and lungs being common with progressed tumors. There seems to be a great degree of variation in the survival time in dogs treated for MM.

Image described by caption.

Figure 7.1 Amelanotic melanoma of the left mandibular labial mucosa of a 12‐year‐old Coonhound. The amelanotic melanoma is surrounded by smaller satellites of melanotic and amelanotic melanoma.

Image described by caption.

Figure 7.2 Large malignant melanoma of the tongue in a large mixed breed dog.

Image described by caption.

Figure 7.3 Melanocytoma (benign melanocytic tumor) upper lip margin in a dog. Squamous Cell Carcinoma

SCC is the most common oral tumor in cats and the second most common malignant oral tumor in dogs. It usually has the appearance of a pink or red, verrucous, ulcerated mass that may bleed very easily. SCC may be evident as ulceration of the soft tissues of the upper or lower jaw, with no evidence of a mass. Occasionally, cats or dogs with SCC will present with the clinical finding of a loose tooth or teeth, which may be mistaken for periodontal disease. Risk factors in cats may include use of flea collars, eating canned tuna, and canned cat food. Exposure to environmental tobacco smoke increased risk of SCC by two times but was not found to be statistically significant [7]. Tumor biopsy samples from cats exposed to environmental tobacco smoke were 4.5 times more likely to overexpress p53 than were tumors from unexposed cats [8]. SCC rapidly infiltrates bone, especially in the cat. In dogs, the metastatic rate is considered to be low for rostral tumors and high for lingual and tonsillar tumors. Lingual SCC, if small at the time of diagnosis, may be cured by radical glossectomy, but micrometastasis to regional lymph nodes may become apparent after surgery. Though glossectomy may cause significant functional issues in both canine and feline patients, dogs appear to adapt better than cats to radical glossectomy. A multicenter study of 44 dogs with tonsillar SCC showed a median survival time of 179 days (Figure 7.4). Clinical signs of anorexia and lethargy were significantly associated with a poor prognosis [9]. Metastasis rates in feline SCC has historically been considered to be low, but more recent studies suggest a metastatic rate of 31% to the mandibular lymph nodes [10, 11].

Image described by caption.

Figure 7.4 Tonsillar squamous cell carcinoma of the left tonsil in a dog. Fibrosarcoma

FSA is the second most common malignant oral and maxillofacial tumor in cats and the third most common in dogs. It occurs more commonly in large breed dogs than in small breed dogs. FSA usually presents as a broad based mass causing a diffuse firm swelling. In some cases, FSA can appear benign histologically and may be misdiagnosed as a fibroma or a low‐grade FSA, even with large incisional biopsies, since it may not show typical histopathological criteria of malignancy [12]. Pathologists with an interest in the manifestations of oral neoplasia will often provide the best chance for a correct diagnosis. Treatment of histologically low‐grade, biologically high‐grade FSA (also called “high‐low FSA”) must be very aggressive. FSA is infiltrative, and even with advanced diagnostics, it is challenging to discern the full extent of the tumor in dogs and cats. Radical surgery, sometimes in conjunction with post‐surgical radiation therapy, provides the best chance for a cure. In one study, prevalence of pulmonary metastasis in dogs with high–low FSA was 12% (3/25 dogs) and regional lymph node metastasis was 20% [12]. Malignant Peripheral Nerve Sheath Tumor

Malignant peripheral nerve sheath tumor (MPNST) is an uncommon tumor of the oral cavity, but when it occurs it manifests as a diffuse, poorly delineated mass involving a large portion of the mandible or maxilla. The authors have seen MPNST arising in the area rostral to the infraorbital foramen and as a fleshy mass arising from the mandibular neurovascular bundle, as seen on dental radiographs as bone loss in the area of the mandibular canal. MPNST may be the most challenging oral tumor to prevent local recurrence due to tracking of the tumor along nerve sheaths. Therefore, radical surgery is necessary (2 cm or greater margins) for a possible cure. Prevalence of regional and distant metastasis for MPNST has not been determined. Osteosarcoma

Osteosarcoma (OSA) of the oral cavity often presents as a soft tissue mass, red, and friable, with surrounding bone lysis rather than hard tissue proliferation. Mandibular OSA appears to have a better prognosis than appendicular OSA [13]. In a recent study of canine mandibular OSA, median survival time was 525 days and 58% of dogs developed metastatic disease [14]. Hemangiosarcoma

Hemangiosarcoma (HSA) often presents similarly to OSA: a rapidly growing, red, vascular mass with or without bone lysis seen on dental radiographs. In the authors’ experience, maxillary and mandibular HSA has been locally invasive with the potential for regional and distant metastasis. However, a study of 20 cases of lingual HSA showed a median overall survival time of 553 days. This suggests lingual HSA may be less aggressive that those arising in other areas of the oral cavity [15]. The authors have rarely seen a case where an HSA in the mouth appeared to be a metastatic lesion from a primary HSA elsewhere in the body. Therefore, staging of oral HSA patients is important prior to performing oral surgery. Multilobular Tumor of Bone

Osteochondrosarcoma (multilobular tumor of bone, chondroma rodens) is a hard, nonulcerated mass, occurring preferentially on the caudal mandible, hard palate, zygomatic arch, base of the skull, and calvarium. The radiographic appearance is a characteristic combination of well‐circumscribed mineralized and non‐mineralized tissue, which has been referred to as a “popcorn ball” appearance. One study of 39 dogs showed a metastatic rate of 56% and 47% of treated dogs had local recurrence. However, median survival times were good at 797 days [16]. Mast Cell Tumor

A mast cell tumor (MCT) occurs rarely in the oral cavity, though the authors have seen MCT arising from the gingiva, the skin lateral to the nasal planum, lips, and tongue. There appears to be a predilection for older male cats to develop cutaneous MCTs, especially in the head and neck region [17]. Cutaneous MCT that occur in the head and neck region of cats tend not to metastasize or show local recurrence after excision. However, aggressive cutaneous MCTs in cats are possible. MCT should be considered as a differential with any waxing and waning oral/perioral mass. A retrospective study of 44 cases of canine oral and perioral MCTs showed 59% of dogs had metastasis at the time of presentation. Overall median survival time was 52 months, which dropped to 14 months for dogs with the presence of lymph node metastasis. Median survival time in dogs without lymph node metastasis was not achieved during the study period [18]. Lymphosarcoma

When lymphosarcoma occurs in the oral cavity, it often manifests as a diffuse process referred to as epitheliotropic lymphoma, also called cutaneous T‐cell lymphoma. In earlier literature, it was referred to as mycosis fungoides. Clinically, epitheliotropic lymphoma shows diffuse depigmentary changes of the mucosa, adjacent skin, and nasal planum. Gingiva may also be affected and may be diffusely red with punctate areas of increased redness/petechiation [19] (Figure 7.5). Survival times, even with chemotherapy, rarely exceed six months with epitheliotropic lymphoma. Low‐grade (indolent) T‐cell lymphoma (T‐zone lymphoma) may also occur in the oral cavity, which has a much better prognosis, with an overall mean survival time of 4.4 years in one study [20] (Figure 7.6).

Image described by caption.

Figure 7.5 Cutaneous T‐cell lymphoma in a 10‐year‐old Golden Retriever. Note the depigmentation of pigmented mucosa and severe hyperemia of the full height of the gingiva.

Image described by caption.

Figure 7.6 T‐zone lymphoma of the tongue. In contrast to T‐cell lymphoma, T‐zone lymphoma is an indolent lymphoma with a good long‐term prognosis.

7.3.2 Benign Non‐odontogenic Oral Tumors Osteoma

Osteomas may occur in dogs or cats. They have been reported on the hard palate, zygomatic arch, or caudal mandible. Osteomas tend to be very well‐circumscribed. Biopsy results may not show obvious characteristics of neoplasia and may show a histopathological diagnosis of normal woven bone. Of four cats treated in one study, surgical treatment of mandibulectomy, maxillectomy, or debulking resulted in greater than one‐year survival in all cats [21]. Plasmacytoma

Plasmacytomas may arise at various locations, but common locations include the dorsal surface of the tongue and the mucosa of the lips, which manifests as a well‐circumscribed, red mass, often less than 1 cm in diameter (Figure 7.7). Surgical margins of 1 cm or slightly less have been curative in cases treated by the authors.

Image described by caption.

Figure 7.7 Plasmacytoma of the gingiva lateral to the left mandibular canine tooth in a Parson Russell Terrier. Unilateral rostral mandibulectomy was curative in this case. Papilloma

Papillomas are often seen in young dogs as multiple verrucous lesions on the tongue, lips, and cheeks. The untraumatized lesions have a characteristic “sea anemone” appearance. These lesions often regress on their own once young dogs’ immune response allows for clearance. Severe refractory cases of oral papillomas may be seen in elderly dogs with concurrent illnesses such as lymphoma or other immunocompromising disease. A recombinant canine oral papillomavirus vaccine has been described for use in refractory cases [22].

7.4 Odontogenic Tumors

7.4.1 Pathogenesis

Possession of a clear understanding of the embryological development of odontogenic structures aids in the understanding of odontogenic tumors in dogs and cats. This process is described in detail in Chapter 1 – Oral Anatomy and Physiology. However, for the purposes of this chapter, there are some important concepts in understanding the development of odontogenic tumors worth discussing here.

The first concept relates to the continued presence of epithelial rests from the degenerating dental lamina and epithelial root sheath upon completion of odontogenesis and tooth eruption. These epithelial rests normally remain quiescent. However, if some carcinogenic or inflammatory mechanism stimulates their proliferation they can develop into an odontogenic tumor or cyst.

The second concept relates to the reciprocal inductive interaction between neural crest‐derived ectomesenchymal cells and epithelium that lines the oral cavity. Odontogenic potential depends upon the continued interaction between differentiating odontogenic epithelium and differentiating ectomesenchyme. The degree of interaction, and thus differentiation, of these cell lines to a large degree determines the type of odontogenic tumor that may develop. This process is particularly underscored when considering the histogenesis of ameloblastic fibromas described later in this chapter.

7.4.2 Classification of Odontogenic Tumors

Advances in our understanding of animal odontogenic tumors have transpired since the most recent veterinary classification system was created more than a decade ago [23]. Veterinary pathologists and clinicians have successfully implemented the more inclusive WHO classification system, utilized to classify human odontogenic tumors, to classify most odontogenic tumors in dogs and cats [2426]. However, the current human classification system does not accommodate a few odontogenic tumors that are described as unique to animals. For the purposes of this chapter, a modification of the 2005 WHO classification system is utilized [27] (Table 7.2).

Table 7.2 Comparison between the WHO 2005 classification of odontogenic tumors in man and the author’s modified classification of odontogenic tumors in animals.

2005 WHO classification – Human Author’s modified classification – Animals
Malignant Tumors Malignant Tumors
Odontogenic carcinomas Odontogenic carcinomas
 Metastasizing (malignant) ameloblastoma
 Ameloblastic carcinoma – primary type  Ameloblastic carcinomaa
 Ameloblastic carcinoma – secondary type (dedifferentiated), intraosseous
 Ameloblastic carcinoma – secondary type (dedifferentiated), peripheral
 Primary intraosseous squamous cell carcinoma – solid type  Primary intraosseous squamous cell carcinomaa
 Primary intraosseous squamous cell carcinoma derived from keratocystic odontogenic tumor
 Clear cell odontogenic carcinoma
 Ghost cell odontogenic carcinoma
Odontogenic Sarcomas Odontogenic Sarcomas
 Ameloblastic fibrosarcoma  Ameloblastic fibrosarcoma
 Ameloblastic fibrodentino‐ and fibro‐odontosarcoma  Ameloblastic fibrodentino‐ and fibro‐odontosarcoma
Benign Tumors Benign Tumors
Odontogenic epithelium with mature, fibrous stroma without odontongenic ectomesenchyme Odontogenic epithelium with mature, fibrous stroma without odontongenic ectomesenchyme
 Ameloblastoma, solid/multicystic type  Ameloblastoma, solid/multicystic type
 Ameloblastoma, extraosseous/peripheral type
 Ameloblastoma, desmoplastic type  Ameloblastoma, desmoplastic type
 Ameloblastoma, unicystic type
 Squamous odontogenic tumor
 Calcifying epithelial odontogenic tumor  Calcifying epithelial odontogenic tumor
 Adenomatoid odontogenic tumor
 Keratocystic odontogenic tumor

 Amyloid producing odontogenic tumor
Odontogenic epithelium with odontogenic ectomesenchyme, with or without hard tissue formation Odontogenic epithelium with odontogenic ectomesenchyme, with or without hard tissue formation
 Ameloblastic fibroma  Ameloblastic fibroma
 Ameloblastic fibrodentinoma  Ameloblastic fibrodentinoma
 Ameloblastic fibro‐odontoma  Ameloblastic fibro‐odontoma
 Odontoma  Odontoma
  Odontoma, complex type   Odontoma, complex type
  Odontoma, compound type   Odontoma, compound type
 Calcifying cystic odontogenic tumor  Calcifying cystic odontogenic tumor
 Dentinogenic ghost cell tumor

 Feline inductive odontogenic tumor
Mesenchyme and/or odontogenic ectomesenchyme with or without odontogenic epithelium Mesenchyme and/or odontogenic ectomesenchyme with or without odontogenic epithelium
 Odontogenic fibroma  Peripheral odontogenic fibroma
 Odontogenic myxoma/myxofibroma  Odontogenic myxoma/myxofibroma
Bone‐related lesions Bone‐related lesions
 Ossifying fibroma  Ossifying fibroma
 Fibrous dysplasia  Fibrous dysplasia
 Osseous dysplasia
 Central giant cell lesion (granuloma)  Central giant cell lesion (granuloma)
 Aneurysmal bone cyst  Aneurysmal bone cyst
 Simple bone cyst

a Due to lack of appropriate documentation, no distinction between PIOSCC variants is made.

7.4.3 Odontogenic Tumors Tumors of Odontogenic Epithelium with Mature Fibrous Stroma, Without Odontogenic Ectomesenchyme Ameloblastomas

Ameloblastomas are a group of slowly growing, locally invasive tumors of odontogenic epithelial origin. The WHO currently recognizes four distinct biologic subtypes of ameloblastoma based on their clinical behavior in humans: solid/multicystic ameloblastoma, unicystic ameloblastoma, desmoplastic ameloblastoma, and peripheral ameloblastoma [27]

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