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WHO Recent Advances and Updates in Tumor Classification

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1. CNS Tumor Classification (WHO CNS5, 2021) with Molecular Biology of Gliomas

Key Paradigm Shift

Adult-Type Diffuse Gliomas (3 types only)

• The term "GBM, IDH-mutant" is abolished - these are now called Astrocytoma, IDH-mutant, grade 4
• IDH-wildtype astrocytomas with TERT promoter mutation / EGFR amplification / +7/-10 chromosomal changes are now classified as GBM even without histologic grade 4 features
• Biallelic CDKN2A/B deletion in IDH-mutant astrocytoma = automatic grade 4 designation

Pediatric-Type Diffuse Gliomas (new category)

Circumscribed Astrocytic Gliomas

• Pilocytic astrocytoma: BRAF-KIAA1549 fusion (most common), NF1
• High-grade astrocytoma with piloid features: New entity - BRAF/NF1/ATRX, poor prognosis despite "piloid" morphology
• Subependymal giant-cell astrocytoma: TSC1/TSC2

Ependymal Tumors - Location + Molecular Subtype

Key New Entities in CNS5

• Intracranial mesenchymal tumor, FET-CREB fusion-positive (provisional)
• CIC-rearranged sarcoma (CNS)
• Primary intracranial sarcoma, DICER1-mutant
• Astroblastoma, MN1-altered (molecular modifier added)
• "Hemangiopericytoma" retired - now Solitary fibrous tumor only

Grading Changes

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2. Molecular Classification of Endometrial Cancer + PTEN

The TCGA Classification (now integrated into WHO 2020)

Clinical Surrogate Testing (ProMisE Algorithm)

PTEN in Endometrial Cancer

• PTEN (Phosphatase and Tensin Homolog) on chromosome 10q23.3 is the most commonly mutated gene in endometrial cancer (~75-85% of low-grade endometrioid carcinomas)
• PTEN is a tumor suppressor phosphatase that degrades PIP3, thereby suppressing the PI3K/AKT/mTOR pathway
• Loss of PTEN leads to constitutive AKT activation, promoting cell survival and proliferation
• PTEN loss occurs early in endometrial carcinogenesis - found in 20% of endometrial hyperplasia (the precursor), rising to 85% in frank carcinomas
• PTEN inactivation occurs by: somatic mutation > promoter methylation > LOH > deletion
• Found in all 4 TCGA subtypes but most prevalent in POLE-mutant (94%) and dMMR (88%) groups
• PTEN-null IHC is a useful surrogate marker - when combined with ARID1A, PIK3CA, beta-catenin aberrations, it constitutes the "endometrioid" molecular landscape

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3. Updates on Lung Tumors (WHO 2021, 5th Edition)

Major Structural Changes

• Adenocarcinoma in situ (AIS): Pure lepidic, no invasion, ≤3 cm, 100% cure rate - now retained and better defined
• Minimally invasive adenocarcinoma (MIA): ≤3 cm, ≤5 mm invasion component, near 100% cure
• Invasive adenocarcinoma subtypes: Lepidic, acinar, papillary, micropapillary, solid, invasive mucinous, colloid, fetal, enteric
• Micropapillary and solid patterns retain prognostic significance (high-grade patterns)

• LCC is now a diagnosis of exclusion requiring resection specimen
• Tumors formerly classified as LCC are now assigned to adenocarcinoma, SCC, or neuroendocrine categories after IHC
• Lymphoepithelioma-like carcinoma renamed lymphoepithelial carcinoma and classified separately
• Large cell neuroendocrine carcinoma (LCNEC) moved to neuroendocrine tumor family

• Unified group: typical carcinoid, atypical carcinoid, SCLC, LCNEC
• Typical carcinoid = well-differentiated NET grade 1
• Atypical carcinoid = well-differentiated NET grade 2
• SCLC/LCNEC = poorly differentiated NEC grade 3

• Classified as: keratinizing, non-keratinizing, basaloid (subtype)
• In situ carcinoma = squamous cell carcinoma in situ (SCIS)

• EGFR (exon 19 del, L858R, T790M, exon 20 insertions)
• ALK rearrangement
• ROS1 rearrangement
• BRAF V600E
• MET exon 14 skipping and amplification
• KRAS G12C (actionable target)
• RET rearrangement
• NTRK fusions (pan-cancer)
• PD-L1 expression (predictive for immunotherapy)

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4. New Renal Tumor Entities (WHO 2022, 5th Edition)

Key Structural Innovation

New/Renamed Established Entities

New Molecularly Defined Entities (7 distinct entities)

1. TFE3-rearranged RCC - various fusion partners (ASPSCR1, PRCC, SFPQ, NONO)
2. TFEB-altered RCC - TFEB fusion or amplification, aggressive behavior
3. ELOC (TCEB1)-mutated RCC - indolent, small clear cell tumors
4. SDH-deficient RCC - SDHB/C/D mutations, mitochondrial dysfunction
5. FH-deficient RCC - fumarate hydratase loss, part of HLRCC syndrome
6. SMARCB1-deficient renal medullary carcinoma - sickle cell trait association, SMARCB1/INI1 loss
7. ALK-rearranged RCC - novel entity, seen in children/young adults with sickle cell trait

New/Emerging Entities (Other renal tumors category)

• Biphasic hyalinizing psammomatous RCC (BHP-RCC): NF2 mutations, papillary with psammoma bodies
• Papillary renal neoplasm with reverse polarity (PRNRP): KRAS mutations, very indolent
• Eosinophilic solid and cystic RCC (ESC-RCC): TSC mutations, predominant in women
• Eosinophilic vacuolated tumor (EVT): mTOR/TSC pathway, eosinophilic cells with vacuoles

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5. New Soft Tissue Tumor Entities (WHO 2020, 5th Edition)

Newly Recognized Entities by Category

• EWSR1-SMAD3-positive fibroblastic tumor (provisional): acral location, young women, EWSR1::SMAD3 fusion
• CIC-rearranged sarcoma: aggressive round cell sarcoma, CIC::DUX4 fusion, distinct from Ewing sarcoma
• BCOR-rearranged sarcomas: BCOR::CCNB3 (X-linked), pediatric bone/soft tissue
• Inflammatory myofibroblastic tumor (IMT): ALK-rearranged (50%), ROS1/RET/NTRK fusions in others - targetable with crizotinib

• Pseudomyogenic hemangioendothelioma: FOSB rearrangement, WWTR1::FOSB; multifocal, young men
• Epithelioid hemangioendothelioma: now formally classified with WWTR1::CAMTA1 or YAP1::TFE3 fusions distinguishing two subtypes

• Hybrid nerve sheath tumors: combinations of schwannoma + neurofibroma + perineurioma
• Melanotic schwannoma: PRKAR1A mutations, Carney complex association
• Malignant melanotic nerve sheath tumor: renamed from "melanotic schwannoma, malignant"

• Ewing sarcoma: EWSR1/FUS::ETS fusions (EWSR1-FLI1 most common, ~85%)
• CIC-rearranged sarcoma: more aggressive than Ewing, CIC::DUX4
• BCOR-rearranged sarcomas: BCOR::CCNB3, BCOR internal tandem duplication
• EWSR1-non-ETS fused sarcomas: EWSR1::NFATc2, etc.

• "Hemangiopericytoma" - fully retired
• "Solitary fibrous tumor" - graded on a 3-tier risk stratification model
• DFSP with fibrosarcomatous transformation = distinct entity with CDKN2A deletion

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6. Serrated Lesions of Colon and Rectum (WHO 2019/2022)

WHO Classification of Serrated Lesions

Key WHO 2022 Updates

• Terminology change: "Sessile serrated adenoma/polyp (SSA/P)" is now officially "Sessile Serrated Lesion (SSL)" - removing "adenoma" because dysplasia is not required for the diagnosis
• SSL without dysplasia is diagnosed by architectural criteria alone: distorted/dilated crypt bases, L-shaped or boot-shaped crypts, horizontal spread at muscularis mucosa
• SSL with dysplasia is the immediate precursor to CRC and should be reported separately
• Serrated polyposis syndrome (WHO criteria): ≥5 serrated polyps proximal to sigmoid, with ≥2 >10 mm; OR any serrated polyp proximal to sigmoid in 1st-degree relative with serrated polyposis; OR >20 serrated polyps of any size distributed throughout colon
• The "serrated pathway" accounts for ~15-30% of colorectal cancers (MSI-H, BRAF-mutant CRCs)

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7. Intraductal Carcinoma of the Prostate (WHO 2022)

Definition

Diagnostic Criteria (WHO 2022)

• Dense cribriform glands (most common)
• Loose cribriform with comedonecrosis
• Solid nests
• Micropapillary

• Marked nuclear enlargement (≥6x normal), prominent nucleoli
• Non-luminal mitoses
• Comedonecrosis

Clinical Significance - Critical Update

• IDC-P in prostate biopsies is strongly associated with high-grade, advanced-stage prostate cancer (Gleason 8-10)
• When found in isolation on biopsy (without adjacent invasive carcinoma), it should be reported and treated as a surrogate for high-grade disease - radical treatment may be indicated
• A small subset (~5-10%) represents a true precursor lesion (de novo IDC-P) rather than retrograde spread
• IDC-P should be excluded from Gleason grading but its presence should be explicitly noted as it worsens prognosis independently
• WHO 2022 now requires IDC-P to be reported separately from Gleason score
• Associated with BRCA2 mutations and homologous recombination repair defects - clinical significance for PARP inhibitor eligibility

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8. Tumors Arising from Vestigial Remnants

Craniopharyngioma

• Arises from Rathke's pouch remnants (Rathke cleft)
• Adamantinomatous type: CTNNB1 (beta-catenin) mutations; children; "machine oil" cholesterol fluid; calcification; wet keratin
• Papillary type: BRAF V600E mutation; adults; solid, non-calcified; responds to BRAF inhibitors
• WHO CNS5 now uses molecular markers to classify these as distinct entities

Chordoma

• Arises from notochordal remnants along the axial skeleton
• Locations: sacrococcygeal (50%) > skull base/clivus (35%) > vertebral column (15%)
• Subtypes: Conventional chordoma, chondroid chordoma (better prognosis), dedifferentiated chordoma (worst)
• IHC: S100+, EMA+, brachyury (T-box gene T) - pathognomonic nuclear marker
• WHO 2020 (Soft Tissue): Poorly differentiated chordoma - new entity with SMARCB1 (INI1) loss, pediatric patients, aggressive

Branchial Cleft Cysts / Branchiomal

• Arise from branchial cleft remnants
• Branchial cleft carcinoma - now recognized that many represent metastatic HPV+ oropharyngeal carcinoma in cystic nodes; true primary branchial cleft carcinoma is rare and diagnosis of exclusion

Thyroglossal Duct Cyst / Carcinoma

• Arise from thyroglossal duct remnants
• Thyroglossal duct cyst carcinoma (almost exclusively papillary thyroid carcinoma) - ~1% of thyroglossal duct cysts
• BRAF V600E in ~50%

Urachal Carcinoma (Vestigial Remnant - Bladder)

• Arises from urachal remnant at bladder dome
• Adenocarcinoma (most common), enteric type
• WHO 2022: Separate entity from bladder adenocarcinoma; must occur at dome/anterior wall, in/beyond bladder wall, no widespread CIS

Rathke Cleft Cyst

• Benign; arises from remnants of Rathke's pouch
• Must be distinguished from craniopharyngioma: no CTNNB1 or BRAF mutations, simple cyst epithelium

Pilonidal Sinus / Sacrococcygeal Teratoma

• Sacrococcygeal teratoma (SCT): most common neonatal tumor; isochromosome 12p (mature) or i(12p) in malignant GCT; OCT3/4, SALL4, AFP

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9. CNS Lesions in HIV and AIDS

Spectrum of CNS Lesions in HIV/AIDS

• Most common CNS tumor in AIDS (CD4 <50 cells/µL)
• Almost universally EBV-driven diffuse large B-cell lymphoma (DLBCL)
• Imaging: ring-enhancing, periventricular, crossing corpus callosum - difficult to distinguish from toxoplasmosis
• EBV DNA in CSF by PCR is diagnostic
• Thallium-201 SPECT: positive in PCNSL (negative in toxoplasmosis)

• Most common CNS opportunistic infection in AIDS
• Multiple ring-enhancing lesions, basal ganglia, gray-white junction
• Toxoplasma gondii reactivation; CD4 <100

• JC virus (polyomavirus) reactivation in oligodendrocytes
• Non-enhancing white matter lesions, no mass effect
• CD4 <200; poor prognosis (pre-HAART)
• IRIS (immune reconstitution inflammatory syndrome) after HAART initiation can worsen PML initially

• Direct HIV invasion of microglia/macrophages
• Pathology: microglial nodules, multinucleated giant cells, myelin pallor, perivascular inflammation
• WHO grading: asymptomatic neurocognitive impairment → mild neurocognitive disorder → HIV-associated dementia

• Cryptococcus neoformans; CD4 <100
• "Soap bubble" appearance (pseudocysts) in basal ganglia on MRI
• India ink stain of CSF; cryptococcal antigen

• CD4 <50; periventricular enhancement
• CMV inclusions ("owl-eye" cells)

• Basilar meningitis, tuberculomas
• Ring-enhancing lesions (similar to toxoplasmosis)

• Rare; HHV-8 driven; spindle cells with slit-like vascular spaces

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10. Pediatric Soft Tissue Sarcomas

WHO 2020 Classification - Pediatric Entities

• Embryonal RMS (ERMS): Most common (60%); loss of heterozygosity at 11p15 (IGF2 imprinting); KRAS, NRAS, HRAS, FGFR4 mutations; intermediate prognosis
• Alveolar RMS (ARMS): PAX3::FOXO1 (most common, ~55%) or PAX7::FOXO1 fusions; worst prognosis; small cells with alveolar pattern; now WHO 2020 recognizes fusion-positive ARMS and fusion-negative ARMS (latter behaves more like ERMS)
• Pleomorphic RMS: Adults; no specific fusion; worst adult STS
• Spindle cell/sclerosing RMS: MYOD1 L122R mutation (aggressive, head-neck); VGLL2/NCOA2 fusions (infantile, favorable)
• Intraosseous RMS: New entity

• NEW entity in WHO 2020: infantile fibrosarcoma spectrum
• ETV6::NTRK3 fusion (most common) - previously called congenital fibrosarcoma
• Responds dramatically to TRK inhibitors (larotrectinib, entrectinib)
• Pan-cancer NTRK testing now recommended

• Replaces "Ewing-like sarcoma CIC-DUX4"
• More aggressive than Ewing sarcoma; older children/young adults
• CD99 negative (unlike Ewing); WT1 positive

• BCOR::CCNB3 (X-linked; boys)
• BCOR internal tandem duplication (ITD)
• Overlaps with clear cell sarcoma of kidney

• Age <2 years; ETV6::NTRK3; now part of NTRK-rearranged spindle cell neoplasm spectrum
• Excellent prognosis; responds to TRK inhibitors

• EWSR1::WT1 fusion; peritoneal/abdominal; adolescent males
• Multi-lineage differentiation (desmin+, keratin+, WT1+)

• SS18::SSX1/SSX2; biphasic or monophasic; younger patients better prognosis

• Undifferentiated pleomorphic sarcoma (UPS) in pediatrics is rare; complex karyotype
• Alveolar soft part sarcoma (ASPS): ASPSCR1::TFE3 fusion
• Clear cell sarcoma of soft tissue: EWSR1::ATF1/CREB1; "melanoma of soft parts"; S100+

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11. Classification of Salivary Gland Tumors with Recent WHO 5th Edition Advances

Overall Classification Framework (WHO 5th Edition, Head & Neck Tumors 2022)

Benign Salivary Gland Tumors

NEW Benign Entities (WHO 5th Edition):

• Sclerosing polycystic adenoma: Previously considered non-neoplastic; now recognized as true benign neoplasm; PTEN/PIK3CA/AKT pathway alterations; can show dysplasia
• Keratocystoma: Rare; cystic tumor with keratin-filled cysts; primarily parotid; BRAF or PIK3CA mutations
• Intercalated duct hyperplasia and adenoma: Spectrum entity; precursor to some low-grade carcinomas
• Striated duct adenoma: New entity; ducts with striated/granular cytoplasm

Malignant Salivary Gland Tumors

NEW Malignant Entities (WHO 5th Edition):

• Microsecretory adenocarcinoma: Very recently described; intercalated duct origin; MEF2C::SS18 fusion; low-grade; microcystic pattern with secretory material
• Sclerosing microcystic adenocarcinoma: Very rare low-grade; infiltrative microcystic pattern; no specific fusion yet
• Mucinous adenocarcinoma: Primary salivary mucinous carcinoma without features of other types; negative MAML2 (unlike colloid MEC)

Reorganized/Reclassified Entities:

• Intraductal carcinoma: Previously known as "low-grade cribriform cystadenocarcinoma" and "low-grade salivary duct carcinoma" - now unified as intraductal carcinoma based on specific tumor fusions (RET, NCOA4::RET, TRIM27::RET); two types: intercalated duct type and apocrine type
• Warthin-like and oncocytic mucoepidermoid carcinoma: New subtypes of MEC recognized by molecular sequencing

Key Fusion Gene Summary for Salivary Gland Carcinomas:

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Summary: Overarching Themes Across WHO 5th Edition Updates

1. Molecular markers define entities, not just histology - integrated diagnoses are mandatory
2. Fusion gene identification by RNA sequencing is now central to soft tissue, salivary, and lung tumor diagnosis
3. Pediatric vs. adult tumors are biologically distinct (clearest in CNS and soft tissue)
4. New "molecularly defined" categories created in renal, CNS, and soft tissue tumors
5. Terminology harmonization across all Blue Books (e.g., hemangiopericytoma retired, variants → subtypes)
6. Prognostic molecular subgroups (POLE, dMMR, p53abn in endometrial; IDH status in gliomas) now drive treatment decisions
7. Targetable alterations (NTRK, ALK, BRAF, HER2, RET, MET, ROS1) are explicitly noted across all classification systems