Histological subtype

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Ancient schwannoma

Nuclear pleomorphism, including bizarre forms with cytoplasmic-nuclear inclusions, and the occasional mitotic figure may be seen, but should not be misinterpreted as indicating malignancy

Low Ki-67 proliferation index in the enlarged atypical cells

Ancient schwannoma. A Marked degenerative atypia may induce concem expression. C The markedly atypical tumour cells are negative for Ki-67, providing further support that they are degenerative in nature. Fig. 9.06 for malignant transformation, but other features of malignancy are lacking. B Diffuse SIOO — Ancient schwannoma. A Marked degenerative atypia may induce concern for malignant transformation, but other features of malignancy are lacking. B Diffuse S100 expression. C The markedly atypical tumour cells are negative for Ki-67, providing further support that they are degenerative in nature.

Cellular schwannoma

Definition

Hypercellular schwannoma composed exclusively or predominantly of Antoni A tissue

Devoid of well-formed Verocay bodies

Location

Paravertebral sites in the

Pelvis
Retroperitoneum
Mediastinum

CN 5 & 8

Clinical presentation: similar to that of conventional schwannoma

Histopathology

Hypercellularity

Fascicular cell growth

Occasional nuclear hyperchromasia and atypia

Fig. 9.07 Cellular schwannoma composed entirely of compact Antoni A tissue, but with other common features of conventional schwannoma, such as hyalinized blood vessels. Fig. 9.08 Cellular schwannoma. A An increased mitotic index is often seen in cellular schwannoma and should not be taken as evidence of malignant peripheral nerve sheath tumour (MPNST) when other classic features of schwannoma are also found. B Diffuse expression of SIOO protein helps to distinguish cellular schwannoma from MPNST. C Extensive nuclear positivity for SOXIO helps distinguish this cellular schwannoma from MPNST. D A moderate Ki-67 proliferation index is not uncommon in cellular schwannoma, but the proliferation index is usually still lower than that encountered in most MPNSTs. — Cellular schwannoma composed entirely of compact Antoni A tissue, but with other common features of conventional schwannoma, such as hyalinized blood vessels.

Fig. 9.07 Cellular schwannoma composed entirely of compact Antoni A tissue, but with other common features of conventional schwannoma, such as hyalinized blood vessels. Fig. 9.08 Cellular schwannoma. A An increased mitotic index is often seen in cellular schwannoma and should not be taken as evidence of malignant peripheral nerve sheath tumour (MPNST) when other classic features of schwannoma are also found. B Diffuse expression of SIOO protein helps to distinguish cellular schwannoma from MPNST. C Extensive nuclear positivity for SOXIO helps distinguish this cellular schwannoma from MPNST. D A moderate Ki-67 proliferation index is not uncommon in cellular schwannoma, but the proliferation index is usually still lower than that encountered in most MPNSTs. — Cellular schwannoma. A An increased mitotic index is often seen in cellular schwannoma and should not be taken as evidence of malignant peripheral nerve sheath tumour (MPNST) when other classic features of schwannoma are also found. B Diffuse expression of S100 protein helps to distinguish cellular schwannoma from MPNST. C Extensive nuclear positivity for SOX10 helps distinguish this cellular schwannoma from MPNST. D A moderate Ki-67 proliferation index is not uncommon in cellular schwannoma, but the proliferation index is usually still lower than that encountered in most MPNSTs.

Low to medium mitotic activity

Usually < 4 mitoses per 10 high-power fields, but occasionally as many as > 10 mitoses per 10 high power fields
Some may have high mitotic rate that are misdiagnosed as malignancy (as malignant peripheral nerve sheath tumour)

Prognosis

Benign

Never metastasize

High local recurrence (5-40%)

Notably in intracranial, spinal, and sacral

DDx vs malignant peripheral nerve sheath tumours

Schwannomas has

Schwannian whorls
A peritumoural capsule
Subcapsular lymphocytes
Macrophage-rich infiltrates
Absence of fascicles
Strong, widespread expression of S100, SOX10, neurofibromin, and CDKN2A (p16)

Diffuse strong S100 protein expression

Diffuse S100 protein expression is exceedingly uncommon in spindled MPNST

Ki-67 proliferation index < 20%

Plexiform schwannoma

Definition

A schwannoma growing in a plexiform or multinodular manner and can be of either conventional or cellular type

Involving multiple nerve fascicles or a nerve plexus, the vast majority arise in skin or subcutaneous tissue of an extremity, the head and neck, or the trunk

Both in childhood and at birth

Despite an often rapid growth, hypercellularity, and increased mitotic activity, the behaviour is that of a benign tumour, prone to local recurrence but with no metastatic potential

Rare association with NF2 (but not with NF1) and has also been noted to occur in patients with schwannomatosis

9.09 Plexiform schwannoma.A Multi le I • ' — Multiple fascicle involvement (plexiform pattern) in a schwannoma.

9.09 Plexiform schwannoma.A Multi le I • ' — At higher magnification, Verocay bodies can be seen.

Cranial and spinal nerves are usually spared.

C, Plexiform neurofibroma debulking.

The plexiform neurofibroma has caused fusiform dilatation of the affected nerve.
Intraneurally, the tumor has enveloped many fascicles.
The tumor is debulked, but it cannot be fully resected.

Schwannoma Neurofibroma Plexiform neurofibroma c FIGURE 203-5 Schematic illustration of surgery for benign peripheral nerve sheath tumors. A, Schwannoma resection. The tumor causes well- circumscribed dilatation of the nerve. Internal neurolysis shows that the tumor has thinned and displaced the fascicles. A single small fascicle may be enveloped by tumor. The fascicles surrounding the schwannoma are dissected free, the tumor capsule is opened, and the schwannoma, and any enveloped fascicles, are resected en bloc. B, Neurofibroma resection. Neurofibromas cause well-circumscribed nerve dilatation (similar to schwanno- mas). However, unlike schwannomas, neurofibromas envelop, rather than displace, the majority of fascicles. After internal neurolysis, the tumor is seen enveloping multiple nerve fascicles. The neurofibroma and enveloped fascicles are resected en bloc. Nerve grafts may be required to repair functional fascicles that were resected. C, Plexiform neurofibroma debulking. The plexiform neurofibroma has caused fusiform dilatation of the affected nerve. Intraneurally, the tumor has enveloped many fascicles. The tumor is debulked, but it cannot be fully resected. — Schematic illustration of surgery for benign peripheral nerve sheath tumors.

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