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Ameloblastoma

(from the early English word
amel, meaning enamel + the Greek word
blastos, meaning germ
) is a rare, benign tumor of odontogenic epithelium (ameloblasts, or outside portion, of the teeth during development) much more commonly appearing in the lower jaw than the upper jaw.
It was recognized in 1827 by Cusack. This type of odontogenic neoplasm was designated as an
adamantinoma in 1885 by the French physician Louis-Charles Malassez. It was finally renamed to the modern name
ameloblastoma in 1930 by Ivey and Churchill.
While these tumors are rarely malignant or metastatic (that is, they rarely spread to other parts of the body), and progress slowly, the resulting lesions can cause severe abnormalities of the face and jaw.
Additionally, because abnormal cell growth easily infiltrates and
destroys surrounding bony tissues, wide surgical excision is required to
treat this disorder.
Subtypes
There are three main clinical subtypes of ameloblastoma: unicystic, multicystic, peripheral
. The peripheral subtype composes 2% of all ameloblastomas. Of all ameloblastomas in younger patients, unicystic ameloblastomas represent 6% of the cases.
A fourth subtype, malignant, has been considered by some oncologic
specialists, however, this form of the tumor is rare and may be simply a
manifestation of one of the three main subtypes. Ameloblastoma also
occurs in long bones, and another variant is Craniopharyngioma (Rathke's pouch tumour, Pituitary Ameloblastoma.)
Clinical features
Ameloblastomas are often associated with the presence of unerupted teeth.
Symptoms include painless swelling, facial deformity if severe enough,
pain if the swelling impinges on other structures, loose teeth, ulcers,
and periodontal (gum) disease. Lesions will occur in the mandible and
maxilla,although 75% occur in the ascending ramus area and will result
in extensive and grotesque deformitites of the mandible and maxilla. In
the maxilla it can extend into the maxillary sinus and floor of the
nose. The lesion has a tendency to expand the bony cortices because slow
growth rate of the lesion allows time for periosteum to develop thin
shell of bone ahead of the expanding lesion. This shell of bone cracks
when palpated and this phenomenon is referred to as "Egg Shell Cracking"
or crepitus, an important diagnostic feature. Ameloblastoma is tentatively diagnosed through radiographic examination and must be confirmed by histological examination (e.g., biopsy).
Radiographically, it appears as a lucency in the bone of varying size
and features—sometimes it is a single, well-demarcated lesion whereas it
often demonstrates as a multiloculated "soap bubble" appearance.
Resorption of roots of involved teeth can be seen in some cases, but is
not unique to ameloblastoma. The disease is most often found in the
posterior body and angle of the mandible, but can occur anywhere in
either the maxilla or mandible.
Ameloblastoma is often associated with bony-impacted wisdom teeth—one
of the many reasons some dentists recommend having them extracted.
Histopathology
Histopathology will show cells that have the tendency to move the nucleus away from the basement membrane.
This process is referred to as "Reverse Polarization". The follicular
type will have outer arrangement of columnar or palisaded ameloblast
like cells and inner zone of triangular shaped cells resembling stellate reticulum in bell stage.
The central cells sometimes degenerate to form central microcysts. The
plexiform type has epithelium that proliferates in a "Fish Net Pattern".
The plexiform ameloblastoma shows epithelium proliferating in a 'cord
like fashion', hence the name 'plexiform'. There are layers of cells in
between the proliferating epithelium with a well-formed desmosomal
junctions, simulating spindle cell layers.
Variants
The six different histopathological variants of ameloblastoma are
desmoplastic, granular cell, basal cell, plexiform, follicular, and
acanthomatous
.
The acanthomatous variant is extremely rare.
One-third of ameloblastomas are plexiform, one-third are follicular.
Other variants such as acanthomatous occur in older patients.
In one center, desmoplastic ameloblastomas represented about 9% of all ameloblastomas encountered.
Treatment
While chemotherapy, radiation therapy,
curettage and liquid nitrogen have been effective in some cases of
ameloblastoma, surgical resection or enucleation remains the most
definitive treatment for this condition. In a detailed study of 345
patients, chemotherapy and radiation therapy seemed to be contraindicated for the treatment of ameloblastomas.
Thus, surgery
is the most common treatment of this tumor. Because of the invasive
nature of the growth, excision of normal tissue near the tumor margin is
often required. Some have likened the disease to basal cell carcinoma
(a skin cancer) in its tendency to spread to adjacent bony and sometimes
soft tissues without metastasizing. While not a cancer that actually
invades adjacent tissues, ameloblastoma is suspected to spread to
adjacent areas of the jaw bone via marrow space. Thus, wide surgical
margins that are clear of disease are required for a good prognosis.
This is very much like surgical treatment of cancer. Often, treatment
requires excision of entire portions of the jaw.
Radiation is ineffective
in many cases of ameloblastoma. There have also been reports of sarcoma being induced as the result of using radiation to treat ameloblastoma.Chemotherapy is also often ineffective. However, there is some controversy regarding this and some indication that some ameloblastomas might be more responsive to radiation that previously thought.
While the Mayo Clinic
recommends surgery for almost all ameloblastomas, there are situations
in which a Mayo Clinic physician might recommend radiation therapy.
These include malignancy, inability to completely remove the
ameloblastoma, recurrence, unacceptable loss of function, and
unacceptable cosmetic damageIn the case of radiotherapy, oncologists at the Mayo Clinic would use intensity-modulated radiotherapy.
Molecular biology
There is evidence that suppression of matrix metalloproteinase-2 may inhibit the local invasiveness of ameloblastoma, however, this was only demonstrated
in vitro. There is also some research suggesting that α
5β
1 integrin may participate in the local invasiveness of ameloblastomas.
Recurrence
Recurrence is common, although the recurrence rates for block resection followed by bone graft are lower than those of enucleation and curettage. Follicular variants appear to recur more than plexiform variants.Unicystic tumors recur less frequently than "non-unicystic" tumors. Persistent follow-up examination is essential for managing ameloblastoma. Follow up should occur at regular intervals for at least 10 years.Follow up is important, because 50% of all recurrences occur within 5 years postoperatively.Recurrence within a bone graft (following resection of the original tumor) does occur, but is less common. Seeding to the bone graft is suspected as a cause of recurrence.The recurrences in these cases seem to stem from the soft tissues, especially the adjacent periosteum. Recurrence has been reported to occur as many as 36 years after treatment.
To reduce the likelihood of recurrence within grafted bone, meticulous surgery with attention to the adjacent soft tissues is required.
Epidemiology
The annual incidence rates per million for ameloblastomas are 1.96,
1.20, 0.18 and 0.44 for black males, black females, white males and
white females respectively. Ameloblastomas account for about one percent of all oral tumors
and about 18% of odontogenic tumors.
Men and women tend to be equally affected, although women tend to be 4
years younger than men when tumors first occur and tumors appear to be
larger in females.
The resected left half of a mandible containing an ameloblastoma, initiated at the third
A CT scan of a patient suffering from an ameloblastoma
Ameloblastoma |
Classification and external resources |

Micrograph of an ameloblastoma showing the characteristic nuclear palisading and stellate reticulum. H&E stain.
|
ICD-10 |
D16.5 |
ICD-9 |
213.1 |
ICD-O: |
9310/0 |
DiseasesDB |
31676 |
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