Definition/General

Introduction:
-Colorectal schwannoma is a rare benign nerve sheath tumor arising from Schwann cells
-It represents less than 1% of all gastrointestinal mesenchymal tumors
-Most commonly occurs in the rectosigmoid region
-Typically presents as a submucosal or intramural mass.
Origin:
-Originates from Schwann cells of the peripheral nerve sheaths
-Arises from autonomic nerve plexuses in the colorectal wall
-Can develop from Auerbach plexus (myenteric) or Meissner plexus (submucosal)
-Shows encapsulated growth pattern without malignant potential.
Classification:
-Classified as conventional schwannoma (most common)
-Ancient schwannoma (degenerative changes)
-Cellular schwannoma (high cellularity)
-Melanotic schwannoma (rare variant)
-Plexiform schwannoma (multiple nodules).
Epidemiology:
-Peak incidence in 5th-6th decades
-Slight female predominance
-Most common location is rectum and sigmoid colon
-Account for 2-5% of all gastrointestinal schwannomas
-Indian population shows similar distribution as Western countries.

Clinical Features

Presentation:
-Asymptomatic in many cases (incidental finding)
-Abdominal pain (most common symptom)
-Change in bowel habits
-Rectal bleeding (occasional)
-Palpable mass (large tumors)
-Bowel obstruction (rare).
Symptoms:
-Lower abdominal discomfort (40-50% cases)
-Constipation or diarrhea
-Rectal bleeding (10-20% cases)
-Tenesmus
-Anal discomfort (rectal lesions)
-Rarely: bowel perforation.
Risk Factors:
-Neurofibromatosis type 1 (increased risk)
-Sporadic occurrence (most cases)
-Previous radiation exposure
-Genetic predisposition
-No known dietary factors.
Screening:
-Colonoscopy for detection
-CT colonography for evaluation
-Endoscopic ultrasound (EUS) for depth assessment
-MRI for large lesions.

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Gross Description

Appearance:
-Well-circumscribed, encapsulated mass
-Firm consistency
-Gray-white to yellow cut surface
-May show cystic degeneration in large tumors
-Whorled appearance on cut section.
Characteristics:
-Smooth external surface
-Fibrous capsule typically present
-Cut surface shows fascicular pattern
-Areas of hemorrhage and necrosis (ancient schwannomas)
-Myxoid areas may be present.
Size Location:
-Size ranges from 1-15 cm
-Rectum (40-50% cases)
-Sigmoid colon (30-40% cases)
-Right-sided colon (10-20% cases)
-Usually solitary lesions.
Multifocality:
-Usually solitary
-Multiple lesions in neurofibromatosis type 1
-Plexiform variant shows multiple nodules
-No metastatic potential.

Microscopic Description

Histological Features:
-Spindle cells arranged in fascicles
-Verocay bodies (pathognomonic feature)
-Alternating Antoni A and Antoni B areas
-Thick-walled vessels with hyalinization
-Lymphocytic infiltration may be present.
Cellular Characteristics:
-Spindle-shaped cells with elongated nuclei
-Wavy, serpentine nuclei
-Eosinophilic cytoplasm
-Nuclear palisading around acellular zones
-Low mitotic activity (<2 per 10 HPF).
Architectural Patterns:
-Antoni A areas: highly cellular with Verocay bodies
-Antoni B areas: loosely arranged cells in myxoid matrix
-Fascicular arrangement
-Encapsulation by fibrous tissue
-Degenerative changes in ancient schwannomas.
Grading Criteria:
-Benign lesions (no grading system)
-Mitotic count typically <2 per 10 HPF
-Absence of necrosis
-No cellular atypia
-Malignant transformation extremely rare (<1%).

Immunohistochemistry

Positive Markers:
-S-100 protein (100% positive, strong and diffuse)
-GFAP (glial fibrillary acidic protein)
-SOX-10 (nuclear staining)
-Calretinin (variable)
-p75 nerve growth factor receptor.
Negative Markers:
-CD117 (c-kit) (negative, helps exclude GIST)
-CD34 (negative)
-Smooth muscle actin (negative)
-Desmin (negative)
-Melanoma markers (negative).
Diagnostic Utility:
-S-100 positivity is essential for diagnosis
-CD117 negativity helps exclude GIST
-Smooth muscle marker negativity excludes leiomyoma
-SOX-10 positivity supports neural origin
-Combination pattern confirms schwannoma.
Molecular Subtypes:
-No molecular subtypes recognized
-NF2 gene mutations (some cases)
-SMARCB1 loss (rare cases)
-LZTR1 mutations (schwannomatosis-associated)
-Chromosomal alterations (22q loss).

Molecular/Genetic

Genetic Mutations:
-NF2 gene mutations (chromosome 22q12)
-SMARCB1 inactivation (rare)
-LZTR1 mutations (schwannomatosis)
-Sporadic mutations (most cases)
-Loss of heterozygosity at 22q.
Molecular Markers:
-S-100 protein expression (universal)
-GFAP expression (most cases)
-SOX-10 nuclear expression
-p75 NGFR expression
-Collagen IV (basement membrane).
Prognostic Significance:
-Excellent prognosis (benign tumor)
-Complete excision curative
-Local recurrence rare (<5%)
-Malignant transformation extremely rare
-NF1-associated cases may have multiple tumors.
Therapeutic Targets:
-Surgical excision (treatment of choice)
-Endoscopic resection (small lesions)
-No targeted therapy needed
-Observation for asymptomatic small lesions
-Regular surveillance in NF1 patients.

Differential Diagnosis

Similar Entities:
-Gastrointestinal stromal tumor (GIST) (most important differential)
-Leiomyoma (smooth muscle tumor)
-Neurofibroma (nerve sheath tumor)
-Inflammatory fibroid polyp
-Ganglioneuroma.
Distinguishing Features:
-Schwannoma: S-100 positive
-Schwannoma: CD117 negative
-Schwannoma: Encapsulated
-GIST: CD117 positive
-GIST: S-100 negative
-Leiomyoma: Smooth muscle actin positive
-Neurofibroma: CD34 positive.
Diagnostic Challenges:
-Distinguishing from GIST (immunohistochemistry crucial)
-Differentiating from leiomyoma (smooth muscle markers)
-Separating from neurofibroma (S-100 intensity, encapsulation)
-Ancient schwannoma with atypia vs malignancy.
Rare Variants:
-Ancient schwannoma (degenerative changes, atypia)
-Cellular schwannoma (hypercellular)
-Melanotic schwannoma (melanin pigment)
-Plexiform schwannoma (multiple nodules)
-Epithelioid schwannoma (rare).

Sample Pathology Report

Template Format

Sample Pathology Report

Complete Report: This is an example of how the final pathology report should be structured for this condition.

Specimen Information

[specimen type], measuring [size] cm in greatest dimension

Diagnosis

Schwannoma

Location

Location: [anatomical site in colorectum]

Histological Features

Shows spindle cells in fascicular arrangement with Antoni A and B areas. Verocay bodies present. Nuclear palisading noted.

Size and Extent

Size: [X] cm, well-circumscribed and encapsulated

Cellular Features

Spindle cells with wavy nuclei and eosinophilic cytoplasm. Mitotic activity: [X] per 10 HPF

Immunohistochemistry

S-100 protein: Positive (strong, diffuse)

CD117: Negative

Smooth muscle actin: Negative

SOX-10: Positive (nuclear)

Margins

Margins: [involved/uninvolved] (if resection specimen)

Final Diagnosis

Colorectal Schwannoma - Benign nerve sheath tumor