Ameloblastoma, conventional
Odontogenic
epithelial neoplasm characterized by slow, expansile growth of ameloblast-like
cells and stellate reticulum. Is caused by dysregulated MAPK and Hedgehog
signaling pathways, BRAF p.V600E mutation is the most common activating
mutation in the MAPK pathway, followed by mutually exclusive mutations in RAS
genes (KRAS, NRAS, HRAS) and FGFR2.
Hyperchromatic
palisaded tumor cells (blue arrows), stellate reticulum-like arrangement (black
arrows), expected location of subnuclear vacuoles (red arrows), 200x.
Classified as
a benign neoplasm; ameloblastoma behaves in a locally aggressive manner with a
tendency to recur, most commonly
occurs in mandible, there are multiple microscopic variants:
·
Follicular
·
Plexiform
·
Acanthomatous
·
Granular cell
·
Basal cell
·
Desmoplastic
Localization
Up to 87% of
AM arise in the mandible, predominantly in the posterior molar area, ~20% cases
occur in maxilla, especially Desmoplastic ameloblastoma in anterior jaws,
especially in anterior maxilla.
Clinical
Features
Painless,
slow-growing mass that, if untreated, reaches a large size, displaces and
loosens teeth, expands and perforates the cortices, may cause paresthesia and
ultimately causes disfigurement and risks adjacent vital structures.
Bone
formation in desmoplastic AMs may producing a fine honeycomb mixed radiolucent
appearance resembling a fibro-osseous
lesion.
Large
ameloblastoma of the mandible causing facial deformity.
Panoramic
radiograph of an ameloblastoma forming a typical radiolucent, multilocular,
soap-bubble pattern lesion with cortical expansion, tooth displacement and root
resorption.
3D
CT reconstruction highlights expansion and multilocularity of the tumor.
Macroscopic
Features
Solid and
yellowish-white to multicystic tumors with little intervening solid tissue.
Solid
and cystic (star annotation) tumor appearance. Location of inferior alveolar nerve
(blue arrow).
Microscopic
Features
Follicular
subtype, there are islands of epithelium resembling the epithelial component of
the enamel organ in a fibrous stroma.
Peripheral cells are columnar-to-cuboidal (ameloblast-like), with
hyperchromatic nuclei arranged in a palisading pattern with reverse polarity,
and often subnuclear vacuolation. The central epithelium is reminiscent of
stellate reticulum, with loosely arranged angular cells and often undergoes
cystic change.
Mandibular
ameloblastoma with follicular growth (arrow outline), cystic change (star
annotation) and plexiform pattern growth (solid black arrow), 40x.
Plexiform,
composed of anastomosing strands of ameloblastomatous epithelium with an
inconspicuous stellate reticulum, less prominent ameloblast-like cells and
cyst-like degeneration in the stroma rather than the epithelium. Mitoses are
usually scattered but posterior maxillary ameloblastoma may have high
cellularity and frequent mitoses.
Higher
magnification ameloblastoma with plexiform growth, 200x.
Acanthomatous
subtype has squamous differentiation centrally in islands but maintains the
reverse polarization of the nuclei in peripheral columnar cells. In the
granular cell subtype the central epithelium develops abundant eosinophilic
granular cytoplasm.
Basal cell
subtype comprises islands and strands of basaloid cells with scanty cytoplasm
and peripheral palisading, reminiscent of basal cell carcinoma.
The
desmoplastic subtype has more widely dispersed islands with a spiky outline,
cuboidal to flat peripheral cells and central spindle shaped cells in a densely
collagenous stroma, sometimes with osteoplasia.
Prognosis and
prediction
AM recurs if
inadequately removed. The standard of
care is complete excision with negative margins at least 10mm
beyond the radiographic margin to ensure removal of AM permeating medullary
bone, usually a segmental resection, mandibulectomy or maxillectomy, depending
on size
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