Key-words:
Ancient schwannoma - Antoni A and Antoni B areas - intraosseous schwannoma - S-100
protein - Verocay bodies
Introduction
Schwannoma or neurilemmoma is a slow-growing, encapsulated benign tumor of neuroectodermal
origin arising from the perineural Schwann cells.[[1]],[[2]] It was first named neurinoma by Verocay in 1910. Term schwannoma was introduced
by Masson (1932). Later, the term neurilemmoma was coined by Stout (1935).[[3]] Approximately 25%–45% of cases are seen in the head and neck region, of which 0.5%–1%
are found within the oral cavity. Intraorally, tongue is the most common site involved.
It usually presents as an asymptomatic, solitary submucosal mass in the oral cavity.[[1]],[[4]] Intraosseous presentation is rare (less than 1%), with mandible being more commonly
involved.[[5]]
Histologically, schwannoma is usually an encapsulated tumor, consisting of mixture
of two cellular patterns: Antoni type A and Antoni type B.[[6]] Antoni type A tissue is characterized by streaming fascicles of palisaded spindle-shaped
Schwann cells with twisted nuclei around central acellular eosinophilic areas known
as Verocay bodies. Antoni type B areas are characterized by less cellular and less
organized spindle or oval cells within a loose myxoid hypocellular matrix.[[3]],[[6]] Conventional schwannomas that exhibit hyperchromatic, atypical, and pleomorphic
nuclei along with areas of hemorrhage and hemosiderin are diagnosed as ancient schwannoma.
Ancient schwannoma is an unusual variant of schwannoma.[[7]]
Material and methods
Archival data of seven cases of schwannoma were retrieved from the files of the Department
of Oral Pathology, Maulana Azad Institute of Dental Sciences, New Delhi. 3-μm thick
sections of paraffin-embedded tissues were stained with hematoxylin and eosin (H and
E) and reviewed.
Immunohistochemistry (S-100, vimentin, desmin, smooth muscle actin [SMA], and cytokeratin)
was performed by conventional standard technique with streptavidin–biotin immunoperoxidase
method to confirm diagnosis in instances where H and E stain was insufficient for
a confirmatory diagnosis.
In the present case series of seven cases, all lesions were soft in consistency, present
in different regions of head and neck including tongue, pterygomandibular raphe, mandibular
anterior tooth region, palate, preauricular region, and maxillary vestibule with one
intraosseous lesion involving anterior maxilla [[Figure 1]]. All the lesions were slow growing (range of 3 months to 9 years) with a mean size
of 2.5 cm × 2.2 cm. 71.4% of te patients were male with age ranging from 13 to 45
years.
Figure 1: Intraosseous schwannoma. (a) Solitary swelling of approximately 4 cm ͯ 3 cm was observed
in the anterior maxilla in relation to #11, 12, and 21 teeth. On palpation, it was
nontender and bony hard in consistency with no fluctuation or softening in any part
of the swelling. (b) Periapical radiograph mimicking a presentation of periapical
pathology. (c) Orthopantomograph revealed ill-defined radiolucent lesion with interrupted
corticated border in relation to #11, 12, and 21 teeth (Case 3)
Macroscopically, the cut sections showed homogenous gray-white areas [[Figure 2]]. The characteristic microscopic features of schwannoma were seen in all the cases.
71.4% of cases were encapsulated. The mixture of Antoni A and Antoni B areas was found
in all the cases. 85.7% of cases showed foci of myxoid changes and 57.1% of the cases
showed microcyst formation. Mild-to-moderate chronic inflammatory cell infiltrate
was found in all the cases [[Figure 3]]a and [[Figure 3]]b. One case (Case 1) showed degenerative features including nuclear atypia, pleomorphism,
and hyalinization and was thus diagnosed as ancient schwannoma [[Figure 3]]c and [[Figure 3]]d. To confirm the diagnosis, immunohistochemistry was done by using S-100, vimentin,
cytokeratin, desmin, and SMA [[Figure 3]]e and [[Figure 3]]f. The clinical findings are summarized in [[Table 1]] and histopathological findings are summarized in [[Table 2]].
Figure 2: (a and b) Macroscopically tissue was globoid shaped and tan in color. It consisted
of a capsule and homogeneously firm in texture (Case 3 and Case 4)
Figure 3: (a) Photomicrograph showing Antoni type A areas composed of spindle-shaped tumor
cells with palisaded nuclei surrounding the amorphous eosinophilic central Verocay
bodies. (b) Antoni type B areas composed of less cellular and less organized loosely
fibrous background. (c) Ancient schwannoma consisting of degenerative features ‒ microcyst
formation and (d) cellular pleomorphism and nuclear atypia. (e) S-100‒positive expression.
(f) Positive expression of vimentin
Table 1: Summary of clinical features of schwannomas
Table 2: Summary of histopathological features of schwannomas
Discussion
Schwannomas are solitary tumors and usually originate from the proliferation of Schwann
cells in the perineurium of the peripheral, cranial, or autonomic nerves, which usually
result in displacement and compression of the adjacent nerves.[[1]] The pathogenetic mechanism responsible for tumor is loss of function of merlin,
the protein encoded by the neurofibromatosis type 2 (NF2) gene. Loss of function of
merlin, either by direct genetic change involving the NF2 gene on chromosome 22 or
secondarily to merlin inactivation, results in downstreaming of its signaling pathways
which lead to formation of tumor.[[2]]
Schwannomas occur most frequently in middle-aged individuals (25–55 years).[[3]],[[4]] In our case series, the age ranged from 13 to 45 years with M: F ratio of 5:2 showing
male predominance. Similar studies were reported by Williams et al., where males were
affected more frequently.[[8]] Study by Lucas found a female predominance, while other studies found no gender
predilection.[[4]],[[9]] Based on their location, two types of schwannomas are described: peripheral (extraosseous)
and central (intraosseous).[[10]],[[11]] Many authors found that tongue is the most frequent site of occurrence.[[4]],[[12]],[[13]] In the present case series, all the lesions were present on different regions of
head and neck.
Clinically, these soft tissue tumors may be mistaken for other benign lesions such
as peripheral ossifying fibroma, traumatic fibroma, and pleomorphic adenoma. In the
current case series, the tumor was present as soft tissue submucosal nodule on different
sites in the head and neck region, with one case of central schwannoma. Less than
1% of lesions are intraosseous and predominantly involve the mandible. It is considered
that there are three mechanisms by which schwannomas may involve bone: (a) a tumor
may arise centrally within bone, (b) a tumor may arise within the nutrient canal and
cause canal enlargement, or (c) a soft tissue or periosteal tumor may cause secondary
erosion and penetration into bone.[[14]],[[15]] Radiographically, intraosseous schwannomas are difficult to differentiate from
other bone lesions such as fibrous dysplasia, neurofibroma, central giant cell lesion,
or periapical lesion.[[14]] Maxillary schwannomas are extremely rare. To the best of our knowledge, only 13
cases of maxillary schwannomas have been reported till date. The central schwannoma
in our case series involved the anterior maxilla (Case 3).
Histopathologically, schwannomas are unilocular encapsulated masses. In the current
case series, all the cases showed features of conventional schwannoma consisting of
two cellular patterns: Antoni type A and Antoni type B.[[2]],[[16]],[[17]]
Ancient schwannoma is an unusual variant of schwannoma. It was first described in
the thorax by Ackerman and Taylor in 1951.[[7]] It is a rare benign encapsulated long-standing tumor. Histologically, it consists
of degenerative changes along with presence of Antoni A and Antoni B cellular areas.
Degenerative features are represented by areas of hemorrhage, hemosiderin deposits,
inflammation, fibrosis, hyalinization, and nuclear atypia.[[2]],[[7]],[[16]],[[17]] It is believed that long history of the lesion could be the cause of the transformation
to an “ancient” variant.[[18]] Out of current case series, one case (Case 1) showed features of ancient schwannoma.
The duration of the lesion was 3 years. Histopathologically, it showed cellular atypia,
nuclear hyperchromasia, and pleomorphism with some areas showing interstitial hyalinization.
Immunohistochemically, the cases showing S-100 (highly reactive) and vimentin (weakly
reactive) positivity were diagnosis as schwannoma. S-100 is an acidic protein which
usually stains the neural crest derivatives. It is consistently expressed in schwannomas
as majority of the cells of schwannomas have Schwann cell antigenic phenotype.[[8]]
In conclusion, we recommend that intraosseous schwannoma should be considered in the
differential diagnosis of intraosseous lesions of the head and neck region. As ancient
schwannoma shows degenerative features, it is important to recognize the histopathological
findings to reach the correct diagnosis. The treatment option for schwannoma is surgical
removal, and recurrence after local excision is rare.
