WHO Classification of Thymic Tumours and Current Concept in Morphology and Molecular Pathology

PART 1: WHO CLASSIFICATION OF THYMIC TUMOURS (5 marks)

Introduction

WHO Histologic Classification (2004/2015, updated 5th ed. 2022)

The A→AB→B1→B2→B3 sequence reflects an increasing ratio of epithelial cells to lymphocytes, with increasing malignant potential.

Macroscopic Features

• Lobulated, firm, gray-white masses up to 15-20 cm
• May show areas of cystic necrosis and calcification
• Most are encapsulated; 20-25% penetrate the capsule
• Arise in the anterior superior mediastinum (account for 20-30% of anterosuperior mediastinal tumours)

Microscopic Features (Key Points)

Simplified Clinical Classification (Robbins)

1. Cytologically benign and noninvasive
2. Cytologically benign but invasive/metastatic (5-year survival <50% with extensive invasion)
3. Cytologically malignant = thymic carcinoma

Masaoka-Koga Staging (used alongside WHO grading)

Associated Paraneoplastic Syndromes

• Myasthenia gravis (most common; ~30-45% of thymomas)
• Pure red cell aplasia
• Hypogammaglobulinemia
• SLE, Sjogren syndrome, polymyositis, rheumatoid arthritis
• Hashimoto thyroiditis, Addison disease

PART 2: CURRENT CONCEPT IN MORPHOLOGY AND MOLECULAR PATHOLOGY (5 marks)

The "Currency Concept" - Morphology as the Gold Standard

"Morphology remains the gold standard against which any claim based on new technology needs to be measured... a novel diagnostic technique ought to be suspect if it does violence to a universally agreed-upon diagnosis arrived at by more traditional means."

How Molecular Information Complements Morphology

1. Neoplasm Classification

• Many tumours have characteristic molecular alterations that are now definitional
• Example: Oligodendroglioma was previously defined by morphology alone; it is now further defined by IDH mutation + 1p/19q co-deletion (WHO CNS 5th ed.)
• Example: GIST - morphologically spindle cell tumour, but confirmed/classified by KIT/PDGFRA mutations
• Molecular findings now drive the WHO classification of haematolymphoid, CNS, and soft tissue tumours

2. Therapeutic Targeting (Precision Oncology)

• Identifying actionable mutations guides targeted therapy
• Example: EGFR mutations in lung adenocarcinoma → tyrosine kinase inhibitors
• Example: BRAF V600E in melanoma → vemurafenib

Genetics → Morphology vs. Morphology → Genetics

Molecular Landscape of Thymic Tumours (Application to Thymoma)

Key principle: Since most thymomas can be diagnosed on morphology alone, molecular analysis is reserved for small biopsies, ambiguous cases, or therapeutic decision-making - illustrating the "currency concept" in practice.

Rosai's Framework: Integration is the Future

"Future discoveries will not likely be made by morphologists ignorant of molecular biologic findings, or biologists scornful of morphologic data, but by those willing and capable of integrating them through a team approach." - Rosai and Ackerman's Surgical Pathology, 10th ed.

• H&E morphology remains the fastest, most cost-effective, and most information-rich first-line tool
• Immunohistochemistry (IHC) bridges morphology and molecular (protein expression)
• Molecular testing (PCR, FISH, NGS) provides classification, prognostication, and therapeutic guidance
• Together, they form a hierarchical, integrated diagnostic pyramid

Summary Table

• Robbins, Cotran & Kumar - Pathologic Basis of Disease (10th ed.), p. 592
• Fischer's Mastery of Surgery (8th ed.), Chapter 92
• Rosai J. "The Continuing Role of Morphology in the Molecular Age." Mod Pathol 2001;14:258-260
• Marx A et al. "Molecular pathology of thymomas." Virchows Arch 2021;478:101-110 [PMID: 33674910]
• Fishman's Pulmonary Diseases and Disorders (5th ed.), Chapter 82

Odontogenic Tumours: WHO Classification, History, Cytology, and Molecular Pathology

INTRODUCTION

PART 1: HISTORY OF WHO CLASSIFICATION

PART 2: WHO 2022 CLASSIFICATION OF ODONTOGENIC TUMOURS

A. MALIGNANT ODONTOGENIC TUMOURS

• Ameloblastic carcinoma
• Primary intraosseous carcinoma, NOS
• Sclerosing odontogenic carcinoma
• Clear cell odontogenic carcinoma (CCOC)
• Ghost cell odontogenic carcinoma

• Ameloblastic fibrosarcoma
• Ameloblastic fibro-dentinosarcoma / fibro-odontosarcoma

B. BENIGN ODONTOGENIC TUMOURS

I. Benign Epithelial Odontogenic Tumours

II. Benign Mixed Epithelial and Mesenchymal Odontogenic Tumours

• Ameloblastic fibroma
• Primordial odontogenic tumour
• Odontoma (compound and complex types)
• Dentinogenic ghost cell tumour

III. Benign Mesenchymal Odontogenic Tumours

• Odontogenic fibroma
• Odontogenic myxoma/myxofibroma
• Cementoblastoma
• Cemento-ossifying fibroma

PART 3: DETAILED CYTOLOGY AND HISTOLOGY OF KEY TUMOURS

1. AMELOBLASTOMA

• Peripheral palisading of columnar cells
• Reverse polarity of nuclei (Vickers-Gorlin sign)
• Subnuclear vacuolation resembling pre-ameloblasts
• Central loose stellate reticulum-like cells

• Type I (luminal): cyst lining only
• Type II (intraluminal): polypoid projections into lumen
• Type III (intramural): mural nodular growth - behaves like SMA, requires radical treatment

2. CALCIFYING EPITHELIAL ODONTOGENIC TUMOUR (CEOT / Pindborg Tumour)

• ~3% of odontogenic tumours; mandible:maxilla = 2:1; peak 40 years
• Histology: Sheets and islands of polyhedral cells with:
  • Marked nuclear pleomorphism including giant/monster nuclei - but no mitotic figures (key: pleomorphism without mitoses)
  • Eosinophilic hyaline/amyloid material within/around cells
  • Liesegang rings - concentric calcification
  • Congo red positive amyloid; shows apple-green birefringence under polarised light

3. ADENOMATOID ODONTOGENIC TUMOUR (AOT)

• Histology:
  • Well-encapsulated with thick fibrous capsule
  • Nodular, trabecular, cribriform patterns
  • Columnar/cuboidal cells form duct-like (glandular) structures - hence "adenomatoid"
  • Nuclei palisade away from lumen
  • Eosinophilic amorphous droplets; calcifications
• Treatment: Enucleation curative; virtually no recurrence

4. ODONTOGENIC KERATOCYST (OKC) / formerly KCOT

• Among the most common odontogenic cysts/tumours
• Peak: 2nd-3rd decade; M:F = 1.6:1; 75% mandible (molar-angle-ramus)
• Hallmark histology:
  • Thin uniform parakeratinized epithelium (6-8 cells thick)
  • Palisaded, hyperchromatic, polarised basal cells (tombstone appearance)
  • Flat epithelial-connective tissue interface (no rete ridges)
  • Satellite/daughter cysts in wall
  • Lumen contains keratin squames (creamy content)
• PTCH1 mutation (Gorlin-Goltz syndrome) - multiple OKC + basal cell carcinomas + skeletal anomalies
• High recurrence rate (~25-60%) due to:
  • Thin fragile wall prone to perforation
  • Budding of basal cells → daughter cysts
  • Satellite cysts in wall

5. ODONTOGENIC MYXOMA

• Mesenchymal tumour from dental papilla/follicle
• Locally invasive; mandible > maxilla
• Histology: Loosely arranged stellate/spindle cells in abundant myxoid/mucoid stroma; few collagen fibres
• Radiology: Multilocular "tennis racket" / "soap bubble" / "honeycomb" pattern with fine bony septa

6. CEMENTOBLASTOMA

• Benign tumour of functional cementoblasts fused to root of a vital tooth (usually lower first molar)
• Histology: Cementum-like mineralised tissue in sheets/trabeculae; reversal lines; peripheral fibrovascular zone with active cementoblasts; no encapsulation
• Complete excision with involved tooth required to prevent recurrence

PART 4: MOLECULAR PATHOLOGY OF ODONTOGENIC TUMOURS

Ameloblastoma - MAPK Pathway Activation (~80%)

Clinical implication: BRAF V600E-mutant ameloblastomas respond to vemurafenib (BRAF inhibitor) - demonstrated in Stage IV metastatic/unresectable cases. This is the first targeted therapy for odontogenic tumours.

Odontogenic Keratocyst / OKC

• PTCH1 inactivation (chromosome 9q22) - tumour suppressor; component of Hedgehog signalling pathway
• Sporadic OKC: somatic PTCH1 mutations
• Gorlin-Goltz syndrome: germline PTCH1 mutation

CEOT (Pindborg Tumour)

• Hyaline material is odontogenic amyloid (derived from amelogenin)
• Congo red positive; apple-green birefringence under polarised light

Odontogenic Myxoma

• USP6::CDH11 fusion in ~70% of cases (similar to aneurysmal bone cyst)

Adenoid Ameloblastoma (new 2022 entity)

• Characterised by CTNNB1 (beta-catenin) mutations - activating Wnt signalling

Odontomas

• Represent hamartomas rather than true neoplasms
• Associated with aberrant activation of Wnt/beta-catenin signalling and decreased Sema3A expression

PART 5: IMMUNOHISTOCHEMISTRY IN DIAGNOSIS

SUMMARY TABLE

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, 8th ed., Chapter 25
• Robbins, Cotran & Kumar - Pathologic Basis of Disease, 10th ed.
• WHO Classification of Head and Neck Tumours, 5th ed. (2022)
• Speight PM & Takata T. Virchows Arch 2018;472:331-339
• Kurppa KJ et al. High frequency of BRAF V600E mutations in ameloblastoma. J Pathol 2014;232:492
• Khoshkam V et al. WHO Classification of Odontogenic Lesions 2022. PMC9999699

ODONTOGENIC TUMOURS — Rapid Revision Sheet (MD Path, 10 marks)

1. WHO CLASSIFICATION — HISTORY (5 editions)

2. WHO 2022 CLASSIFICATION SKELETON

MALIGNANT
├── Odontogenic carcinomas: Ameloblastic Ca, Primary intraosseous Ca NOS,
│   Sclerosing OC, Clear cell OC, Ghost cell OC
├── Odontogenic carcinosarcoma
└── Odontogenic sarcomas: Ameloblastic fibrosarcoma ± dentinoid/enameloid

BENIGN EPITHELIAL
├── Ameloblastoma (conventional, unicystic, peripheral, metastasizing)
├── Adenoid ameloblastoma (NEW 2022)
├── Squamous odontogenic tumour (SOT)
├── CEOT (Pindborg tumour)
└── AOT

BENIGN MIXED (epithelial + mesenchyme)
├── Ameloblastic fibroma
├── Primordial odontogenic tumour
├── Odontoma (compound / complex)
└── Dentinogenic ghost cell tumour

BENIGN MESENCHYMAL
├── Odontogenic fibroma
├── Odontogenic myxoma
├── Cementoblastoma
└── Cemento-ossifying fibroma

3. KEY TUMOURS — COMPRESSED DETAILS

AMELOBLASTOMA

CEOT (Pindborg Tumour)

• 3% of OGT; mandible:maxilla = 2:1; peak 40 yr; mixed radiolucent-opaque ± unerupted tooth
• Histology: Polyhedral cells + marked nuclear pleomorphism + NO mitoses + eosinophilic amyloid globules (Congo red +ve, apple-green birefringence) + Liesegang ring calcifications
• Recurrence ~15%; higher after enucleation

AOT (Adenomatoid Odontogenic Tumour)

• Dentigerous radiolucency + discrete radiopaque foci
• Histology: Encapsulated; nodular/trabecular/cribriform; duct-like (glandular) structures; columnar cells with palisaded nuclei away from lumen; eosinophilic droplets
• Enucleation curative; virtually no recurrence

ODONTOGENIC KERATOCYST (OKC) / formerly KCOT

• 2nd-3rd decade; M:F 1.6:1; 75% mandible (molar-angle-ramus)
• Unilocular or scalloped multilocular radiolucency; creamy keratin in lumen
• Histology:
  • Thin uniform parakeratinized epithelium (6-8 cells)
  • Palisaded hyperchromatic "tombstone" basal cells
  • Flat epithelium-connective tissue interface (no rete ridges)
  • Satellite/daughter cysts in wall
• PTCH1 mutation → Gorlin-Goltz syndrome (multiple OKC + BCC + skeletal anomalies)
• High recurrence (25-60%): thin wall fragments; daughter cysts; basal cell budding

ODONTOGENIC MYXOMA

• Mesenchymal; locally invasive; mandible > maxilla
• X-ray: "Tennis racket" / "soap bubble" fine bony septa (multilocular)
• Histology: Stellate/spindle cells in abundant myxoid stroma; sparse collagen
• Molecular: USP6::CDH11 fusion (~70%)

CEMENTOBLASTOMA

• Only odontogenic tumour fused to root of vital tooth (lower 1st molar)
• X-ray: radiopaque mass fused to root with radiolucent halo
• Histology: Cementum-like trabeculae + reversal lines + active cementoblasts at periphery
• Treatment: Excision with involved tooth (recurs otherwise)

SQUAMOUS ODONTOGENIC TUMOUR (SOT)

• From rests of Malassez in periodontal ligament
• Triangular radiolucency between roots
• Histology: Rounded islands of well-differentiated squamoid cells in fibrous stroma; no atypia

4. MOLECULAR PATHOLOGY — MASTER TABLE

Key principle: BRAF V600E → confirmed 2014 by 3 independent groups; ~94% of mandibular unicystic ameloblastomas carry it. First odontogenic tumour with proven targeted therapy response.

5. IHC QUICK REFERENCE

6. ONE-LINE DIFFERENTIATORS (Memory Anchors)

1. Give WHO classification skeleton (malignant → benign epithelial → mixed → mesenchymal)
2. History: 1971 → 1992 → 2005 (KCOT added) → 2017 (KCOT removed back) → 2022 (adenoid ameloblastoma)
3. Elaborate ameloblastoma (follicular vs plexiform; 3 cytological hallmarks; unicystic subtypes; BRAF)
4. CEOT: pleomorphism without mitoses + amyloid
5. AOT: two-thirds rule + glandular structures
6. OKC: tombstone basal cells + PTCH1 + Gorlin
7. Molecular table: BRAF/MAPK, PTCH1, USP6::CDH11, CTNNB1

ODONTOGENIC TUMOURS — Rapid Revision Sheet (MD Path, 10 marks)

1. WHO 2022 CLASSIFICATION SKELETON

MALIGNANT
├── Odontogenic carcinomas: Ameloblastic Ca, Primary intraosseous Ca NOS,
│   Sclerosing OC, Clear cell OC, Ghost cell OC
├── Odontogenic carcinosarcoma
└── Odontogenic sarcomas: Ameloblastic fibrosarcoma ± dentinoid/enameloid

BENIGN EPITHELIAL
├── Ameloblastoma (conventional, unicystic, peripheral, metastasizing)
├── Adenoid ameloblastoma (NEW 2022)
├── Squamous odontogenic tumour (SOT)
├── CEOT (Pindborg tumour)
└── AOT

BENIGN MIXED (epithelial + mesenchyme)
├── Ameloblastic fibroma
├── Primordial odontogenic tumour
├── Odontoma (compound / complex)
└── Dentinogenic ghost cell tumour

BENIGN MESENCHYMAL
├── Odontogenic fibroma
├── Odontogenic myxoma
├── Cementoblastoma
└── Cemento-ossifying fibroma

2. KEY TUMOURS — COMPRESSED DETAILS

AMELOBLASTOMA

CEOT (Pindborg Tumour)

• 3% of OGT; mandible:maxilla = 2:1; peak 40 yr; mixed radiolucent-opaque ± unerupted tooth
• Histology: Polyhedral cells + marked nuclear pleomorphism + NO mitoses + eosinophilic amyloid globules (Congo red +ve, apple-green birefringence under polarised light) + Liesegang ring calcifications
• Recurrence ~15%; higher after enucleation

AOT (Adenomatoid Odontogenic Tumour)

• Dentigerous radiolucency + discrete radiopaque foci
• Histology: Thick fibrous capsule; nodular/trabecular/cribriform; duct-like (glandular) structures; columnar cells with palisaded nuclei away from lumen; eosinophilic amorphous droplets
• Enucleation curative; virtually no recurrence

ODONTOGENIC KERATOCYST (OKC)

• 2nd-3rd decade; M:F 1.6:1; 75% mandible (molar-angle-ramus)
• Unilocular or scalloped radiolucency; creamy keratin in lumen
• Histology:
  • Thin uniform parakeratinized epithelium (6-8 cells thick)
  • Palisaded hyperchromatic "tombstone" basal cells
  • Flat epithelium-CT interface (no rete ridges)
  • Satellite/daughter cysts in wall
• High recurrence (25-60%): thin wall fragments + daughter cysts + basal cell budding
• PTCH1 mutation → Gorlin-Goltz syndrome (multiple OKC + basal cell carcinomas + skeletal anomalies)

ODONTOGENIC MYXOMA

• Mesenchymal; locally invasive; mandible > maxilla
• X-ray: "Tennis racket" fine bony septa (multilocular)
• Histology: Stellate/spindle cells in abundant myxoid stroma; sparse collagen
• Molecular: USP6::CDH11 fusion (~70%)

CEMENTOBLASTOMA

• Only odontogenic tumour fused to root of vital tooth (lower 1st molar)
• X-ray: radiopaque mass fused to root with radiolucent halo
• Histology: Cementum-like trabeculae + reversal lines + active cementoblasts at periphery
• Treatment: Excision with involved tooth (recurs otherwise)

SQUAMOUS ODONTOGENIC TUMOUR (SOT)

• From rests of Malassez in periodontal ligament; mandible > maxilla
• Triangular radiolucency between roots of vital erupted teeth
• Histology: Rounded islands of well-differentiated squamoid cells in fibrous stroma; no atypia; no palisading

3. MOLECULAR PATHOLOGY — MASTER TABLE

BRAF V600E confirmed in 2014 by 3 independent groups simultaneously. ~94% of mandibular unicystic ameloblastomas carry it. First odontogenic tumour with proven targeted therapy.

4. IHC QUICK REFERENCE

5. ONE-LINE DIFFERENTIATORS (Memory Anchors)