WHO Classification of Haematolymphoid Tumours, 5th Edition (Khoury et al., Leukemia 2022) & ICC 2022 (Arber et al., Blood 2022) · Source-verified
Source: Khoury JD, Solary E, Hochhaus A et al. WHO 5th Edition. Leukemia 2022;36:1703–1719 · Tables 1–15. Full classification: tumourclassification.iarc.who.int/chapters/63 · ⚠ APL = medical emergency — start ATRA empirically while FISH pending
Source: Arber DA, Orazi A, Hasserjian RP et al. ICC 2022. Blood 2022;140:1200–1228 · Tables 1–26. Key: ICC-only entities = CMUS/CCMUS, MDS/AML category, MDS with mutated TP53, AML with mutated TP53, JMML-like, RCC retained name, aCML retained name, MDS/MPN with i(17q), SM-AMN terminology
| Entity | Lineage | Flow Cytometry Key Markers | IHC Key Markers | Critical Molecular / Genetics |
|---|---|---|---|---|
| Myeloproliferative Neoplasms (MPN) | ||||
| CML · BCR::ABL1+ | Pluripotent HSC; granulocytic | CD13+CD33+CD34+ (blasts)MPO+ (granulocytes)TdT− (CP)
⚑ BC transformation: lymphoid BC = CD19+/TdT+; myeloid BC = MPO+. WHO 5 eliminates AP; ICC retains AP (10–19% blasts + major route ACA) |
BCR::ABL1 FISH/RT-PCR (IS) — diagnosticMPO+CD34+ increases in AP (ICC) or BC
★ RT-PCR BCR::ABL1 International Scale (IS): aim for MMR (MR3.0); deep MR4/MR4.5 for TFR attempt. T315I = ponatinib only; Y253H/E255K = resist 1st/2nd gen TKI |
t(9;22)(q34;q11) · p210 BCR::ABL1 (CML) · ASXL1 co-mut adverse in CP · Sokal/ELTS score · WHO 5: CP + BP only. ICC: CP + AP + BP. ABL1 kinase domain mutations guide TKI selection |
| Polycythaemia vera | Multipotent HSC; erythroid | CD71+CD235a (GPA)+CD36+
★ Flow limited role in PV. Gold standard: JAK2 mutation + low serum EPO + BM biopsy (panmyelosis). JAK2 exon 12: may have isolated erythrocytosis without panmyelosis |
JAK2 V617F or exon 12 mutationCD71++ (erythroid hyperplasia)CD34 not increased
★ BM: panmyelosis · near-100% cellularity · fat reduced · pleomorphic megakaryocytes ("staghorn" nuclear lobation) · loose scattered clusters · BM biopsy may not be required if Hb >18.5g/dL male or >16.5g/dL female + JAK2 mutation |
JAK2 V617F (~98%) · JAK2 exon 12 (~2%) · TET2 · DNMT3A · ASXL1 · LNK(SH2B3) · Post-PV MF: IDH1/2 · TP53 clonal evolution · Phlebotomy + aspirin ± HU · Ruxolitinib high-risk/refractory |
| ET vs pre-PMF — THE critical MPN distinction | Multipotent HSC; megakaryocytic | CD41+ (GPIb)CD61+ (GPIIIa)Circulating CD34+ progenitors: <10/μL = ET · >10/μL = PMF
★ Circulating CD34+ progenitor count on PB flow = SINGLE MOST DISCRIMINATING flow finding between ET and pre-PMF when BM morphology is equivocal |
Driver mutation: JAK2 V617F / CALR type 1 or 2 / MPL W515MF-0 on reticulin (ET)
★ ET megakaryocytes: large mature hyperlobated ≥5 lobes ("staghorn"), loosely scattered/small clusters · pre-PMF: TIGHT dense clusters (>6 cells), bulbous hyperchromatic nuclei, "naked nuclei" (extruded segments). CALR type MUST be reported (type 1=best OS, lower thrombosis; type 2=intermediate) |
JAK2 V617F (~55%) · CALR type 1 del52bp (best OS) · CALR type 2 ins5bp · MPL W515L/K (~4%) · Triple-negative (worst outcome) · HMR co-muts: ASXL1/SRSF2/EZH2/IDH1-2/U2AF1-Q157 · ET pre-PMF distinction drives different risk/treatment approach |
| Primary myelofibrosis (overt) | Multipotent HSC; megakaryocytic + stromal | CD34+ circulating progenitors >10/μL in PBCD41+/CD61+
⚑ Teardrop cells + leukoerythroblastosis in PB = myelofibrosis until proven otherwise. Dry tap = fibrosis |
Reticulin stain MF grade (0–3) mandatory in ALL MPN trephine reportsCD61+ (atypical mega clusters)
★ Overt PMF: MF-2/3 · osteosclerosis · sinusoidal hematopoiesis · naked megakaryocyte nuclei |
JAK2 V617F (~55%) · CALR type 1 (best OS) · CALR type 2 · MPL (~5%) · Triple-negative (worst) · Ruxolitinib (JAK1/2) · Pacritinib (plt <50) · Momelotinib (anemia) · AlloSCT curative intent |
| Systemic mastocytosis (all SM subtypes) | Mast cell progenitor → CD34+ BM progenitor | CD117++ (c-KIT, very bright)CD25+ (ABERRANT — absent on normal mast cells)CD33+CD45+CD2+ (subset)CD30+ (new minor criterion WHO 5 + ICC)
★ CD25+/CD117++ = SM until proven otherwise. CD25 absent on reactive MCs. CD30 added as minor criterion in BOTH WHO 5 and ICC |
Tryptase IHC — highlights MC aggregatesCD117++ (KIT IHC)CD25+CD30+
★ WHO SM criteria (both WHO 5 + ICC agree): Major = multifocal dense MC aggregates ≥15 MCs/cluster. 4 minor criteria (1 major + 1 minor OR 3 minor for diagnosis). WHO 5 adds BMM as new SM subtype. WDSM: round MCs, D816V absent, CD25−, CD30+ — fails standard minor criteria |
KIT D816V (>95% SM) · Avapritinib (BLU-285) FDA 2021 for advanced SM/D816V · Midostaurin (broader KIT) · KIT D816V VAF ≥10% = B-finding (new WHO 5) · TET2/SRSF2/ASXL1/RUNX1 (SMAR mutations) → SM-AHN risk |
| MLN-TK (all subtypes) | Multipotent HSC; eosinophilic predominance | CD34+CD33+CD25+ (PDGFRA — MC component)
⚑ FIP1L1::PDGFRA is CRYPTIC on karyotype — FISH with PDGFRA break-apart probe or RT-PCR mandatory. NEVER diagnose CEL without excluding all MLN-TK fusions |
FISH/RT-PCR: PDGFRA/B/FGFR1/JAK2/FLT3/ABL1 break-apart probes — mandatory before TKI
★ MLN-TK supersedes all other myeloid/lymphoid diagnoses (both WHO 5 + ICC). PCM1::JAK2: BM fibrosis prominent. FGFR1: most aggressive, may present as T-LBL before myeloid phase |
PDGFRA: imatinib dramatic · T674I resistance (avapritinib) · PDGFRB: imatinib · FGFR1: pemigatinib/futibatinib · JAK2 (PCM1::JAK2): ruxolitinib · FLT3: midostaurin/quizartinib · ETV6::ABL1: dasatinib |
| Myelodysplastic Neoplasms (MDS) | ||||
| MDS-5q / MDS-del(5q) | Multipotent HSC; megakaryocytic + erythroid | CD41+CD61+CD71+ (dysplastic erythropoiesis)
★ FISH confirmation of del(5q) required before lenalidomide. Isolated = del(5q) ± 1 additional cytogenetic abnormality (NOT −7/del7q) |
Monolobate megakaryocytes on CD61 IHC — pathognomonic hallmarkCD34 not increased
★ Count nuclear lobes on CD61 IHC: del(5q) = predominantly 1 lobe (monolobate). Normal = ≥3 lobes. FISH + TP53 sequencing mandatory before lenalidomide (TP53 mutation = resistance mechanism) |
del(5q) isolated or +1 (not −7/del7q) · RPS14 haploinsufficiency → macrocytic anemia · miR-145/146a loss → thrombocytosis · Lenalidomide targets CRBN · TP53 acquisition after lenalidomide = transformation risk |
| MDS-SF3B1 (ring sideroblast MDS) | Multipotent HSC; erythroid predominance | CD71+CD36+CD235a+Aberrant CD71/CD36 asynchronous maturationCD34 not increased | Ring sideroblasts on Perl's Prussian blue stain (≥15% or ≥5% if SF3B1 mut)CD71++ (erythroid hyperplasia)
★ Ring sideroblast: ≥5 iron granules encircling ≥1/3 of nucleus on Perl's stain. Perl's stain MANDATORY in all MDS workups — never report without it. SF3B1 mutation causes aberrant 3' splice site → mitochondrial iron accumulation |
SF3B1 splice factor mutation · Luspatercept (TGF-β trap, activin receptor ligand trap) FDA-approved for SF3B1-mutated MDS-RS · Co-mut JAK2/CALR + thrombocytosis → reclassify as MDS/MPN-SF3B1-T |
| MDS-biTP53 (WHO 5) / MDS with mutated TP53 (ICC) | Multipotent HSC; multi-lineage dysplasia | Variable blast % and myeloid phenotypeCD34+
⚑ WHO 5: biallelic TP53 required for MDS entity (2 muts OR 1 mut + del17p/cnLOH). ICC: multi-hit required for MDS; any TP53 VAF>10% sufficient for MDS/AML and AML. VAF >50% = presumptive biallelic on NGS |
p53 IHC strong >10% cells = missense gain-of-function surrogatep53 IHC null = nonsense/frameshift loss-of-function
★ p53 IHC NULL does NOT exclude biallelic TP53 — molecular testing always required. Complex karyotype virtually universal. Eprenetapopt/APR-246 + aza · Magrolimab (anti-CD47) |
TP53 biallelic (2 muts OR 1 mut + del17p/cnLOH) · Complex karyotype (−5, −7, −17p, +8) · Very poor OS (<12 months) · Venetoclax+aza · APR-246/eprenetapopt |
| MDS-LB / MDS-IB1 / MDS-IB2 / MDS-F / MDS-h | Multipotent HSC; multi-lineage dysplasia | CD34+: MDS-LB <5% · IB1 5–9% · IB2 10–19%CD117+CD33+Aberrant CD5/CD7/CD56 on blasts
★ Ogata/Wells-Ogata FC score: validated quantitative MDS scoring. Score ≥2 = high sensitivity/specificity for MDS. MDS-h: BM ≤25% cellularity; T-cell immune attack; overlaps AA/PNH; IST-responsive subset |
CD34 IHC — quantify blast %p53 IHC (if TP53 suspected)Reticulin stain (mandatory · MF grade 0–3)
★ Auer rod in ANY blast = MDS-IB2 regardless of blast % (confirm with MPO stain). ALIP = clusters of ≥3–5 CD34+/MPO+ blasts in central intertrabecular space = adverse marker. Reticulin grading mandatory in all MDS trephine reports |
TET2 · DNMT3A · ASXL1 · SRSF2 · U2AF1 · ZRSR2 · RUNX1 · EZH2 · STAG2 · IPSS-R and IPSS-M risk stratification · HMA (azacitidine/decitabine) · Luspatercept (SF3B1-mutated) · Imetelstat (LR-MDS) · AlloSCT high/very-high risk |
| MDS/MPN Overlap | ||||
| CMML (Types 1 & 2) · MD-CMML vs MP-CMML | Monocyte-committed progenitor / HSC | Classical monocytes (CD14+CD16−) ≥94% of monocyte gate — most specific flow finding for CMMLCD14+CD64+CD163+CD56− (aberrant CD56 = worse prognosis)
★ WHO 5 lowers monocytosis threshold to ≥0.5×10⁹/L. ICC: same threshold + requires clonality (≥1 myeloid mut VAF ≥10%). Both WHO 5 and ICC eliminate CMML-0. MP-CMML (WBC ≥13) = RAS pathway mutations + worse prognosis |
CD14+CD163+Lysozyme+CD34+ (increases with CMML type)
★ CMML-1: <5% PB / <10% BM blasts · CMML-2: 5–19% PB / 10–19% BM or Auer rods · Promonocytes = blast equivalents · monoblasts + promonocytes ≥20% = AML transformation |
TET2 (~60%) · SRSF2 (~50%) · ASXL1 (~40%) · RAS pathway (NRAS/KRAS/CBL) · TET2+SRSF2 co-mutation highly specific for CMML · RUNX1 (adverse) · Ruxolitinib (WBC >13) · Azacitidine · AlloSCT high-risk |
| Acute Myeloid Leukaemia | ||||
| APL with PML::RARA ⚠ EMERGENCY | Promyelocyte-stage progenitor | CD33+++ (very bright)CD13+CD34− (hypergranular)HLA-DR− (KEY absence)CD11b−CD2+ (microgranular M3v)
⚑ HLA-DR−/CD34− + coagulopathy = APL → START ATRA EMPIRICALLY. M3v mimics monocytic AML — CD34−/HLA-DR− pattern is the clue. Both WHO 5 and ICC: ≥10% blasts required |
PML::RARA FISH+ (t(15;17))PML IHC: cytoplasmic diffuse pattern (normal = nuclear dots)MPO++ (very strong)
★ ATRA + ATO = chemo-free standard for standard-risk APL. High-risk (WBC >10): add GO. MRD by PML::RARA RT-PCR (IS) mandatory. ZBTB16::RARA (PLZF) = ATRA-resistant — always identify fusion partner |
t(15;17)(q24.1;q21.2) · ATRA+ATO (standard-risk) · ATRA releases HDAC repression → differentiation · ATO degrades PML::RARA · MRD by RT-PCR IS at each treatment milestone |
| AML with RUNX1::RUNX1T1 · t(8;21) | GMP / myeloid progenitor | CD34+CD117+CD33+CD19+ (aberrant B-marker on myeloid blasts)CD56+ (subset — adverse)MPO+
★ CD19+ on myeloid blasts = classic t(8;21). Any CD19+ myeloid blast → RUNX1::RUNX1T1 FISH/RT-PCR urgently. WHO 5: any blast %; ICC: ≥10% |
MPO+ (strong)CD34+CD19+
★ Salmon-pink granules + long single Auer rods + CD19+ aberrant = classic triad. RUNX1::RUNX1T1 can be cryptic on ~30% of standard karyotypes → FISH mandatory. MRD by RT-PCR IS |
RUNX1::RUNX1T1 · KIT D816V/Y (~25%) adverse · NRAS/KRAS · HiDAC ×4 cycles consolidation · GO benefit in CBF-AML |
| AML with CBFB::MYH11 · inv(16) | GMP / myeloid + monocytic + eosinophilic | CD34+CD117+CD33+CD2+ (aberrant T-marker on myeloid blasts)MPO+CD64+
★ CD2+ on myeloid blasts = classic inv(16). CBFB FISH break-apart probe required — inv(16) frequently missed on G-banding |
MPO+Abnormal eosinophils in BM: enlarged purple/basophilic granules in eosinophilic cells — essentially pathognomonic for inv(16)
★ Abnormal mixed-granule eosinophils (purple+eosinophilic granules in same cell) on BM aspirate = pathognomonic for inv(16) |
CBFB::MYH11 · KIT exon 8 (~40%) · NRAS · HiDAC consolidation · GO benefit · MRD by RT-PCR IS |
| AML with NUP98 rearrangement — NEW in WHO 5 · NOT in ICC | HSC / myeloid progenitor; variable differentiation | CD34+CD117+Variable differentiation · NUP98::KDM5A = megakaryoblastic (CD41/CD61+)
⚑ NUP98 rearrangements CRYPTIC on standard karyotype — RNA-seq or NUP98 break-apart FISH required. WHO 5-specific entity: NOT in ICC 2022. Any pediatric/young adult AML with poor prognosis and no identified genetic driver should have NUP98 testing |
MPO+ (most)CD34 variable
★ NUP98 has >30 fusion partners. Common: NUP98::NSD1 (pediatric/young adult poor prognosis), NUP98::HOXA9, NUP98::KDM5A (pediatric AMKL). All deregulate HOX gene expression. WHO 5: diagnosed at any blast %. This is a WHO 5-exclusive entity |
NUP98 11p15.4 rearrangement · >30 partners · NUP98::NSD1 (poor prognosis, young adults) · NUP98::KDM5A (pediatric AMKL, CBFA2T3::GLIS2 spectrum) · NUP98::HOXA9 · HOX deregulation common mechanism · NOT in ICC 2022 |
| AML with NPM1 mutation · ~30% of AML | GMP / mixed differentiation | CD33+CD13+HLA-DR+CD34− (paradox — most NPM1-mutated AML is CD34 negative)CD14+/CD64+ if monocytic
★ CD34-negative AML with normal karyotype = NPM1 mutation until proven otherwise. CD34-negativity occurs because mutant NPM1c exported to cytoplasm. CD34-negative AML must automatically trigger NPM1 testing |
NPM1 IHC: cytoplasmic staining (NPM1c+) — reliable surrogate on trephineCD34− (most)Cup-like nuclear invaginations on Romanowsky stain
★ Normal NPM1 = nuclear dots; mutant NPM1c = cytoplasmic diffuse. NPM1 IHC can be performed on decalcified trephine sections. Revumenib (menin inhibitor) Phase III AUGMENT-101 for NPM1-mutated AML |
NPM1 exon 12 insertion (type A ~80%) · FLT3-ITD allelic ratio (AR): >0.5 = high AR = adverse · DNMT3A · IDH1/2 · TET2 · Midostaurin + 7+3 if FLT3+ · NPM1 ddPCR/RQ-PCR for MRD = gold standard |
| AML with CEBPA mutation (in-frame bZIP) | GMP / granulocytic progenitor | CD34+CD117+CD13+HLA-DR+CD7+ (aberrant T-marker)CD14−
⚑ WHO 5 requires ≥20% blasts · ICC requires ≥10% blasts for this entity — a key classification-dependent blast threshold difference. Both require in-frame bZIP (C-terminal) mutation |
MPO+CD34+CD7+
★ WHO 5 updated: monoallelic bZIP also qualifies (favorable). ICC: bZIP mutations included (≥10% blasts). Both: in-frame bZIP domain required. Germline CEBPA: autosomal dominant AML predisposition — test all family members |
CEBPA in-frame bZIP (C-terminal) · Mono- or bi-allelic bZIP both favorable (WHO 5) · Germline CEBPA risk · GATA2 co-mut · TET2/WT1 · Excellent prognosis (~60% 5-yr OS) · AlloSCT CR1 often deferred |
| AML with KMT2A rearrangement · 11q23.3 | HSC / myeloid-monocytic progenitor | CD33+++ (characteristically very bright)CD64+CD14+ (monocytic)NG2 (clone 7.1)+ — highly specific for KMT2A-rearranged AML and ALLCD34±
★ CD33 characteristically very bright in KMT2A-r AML. NG2 (clone 7.1) = highly specific for KMT2A rearrangement in AML AND ALL contexts. Any blast % in both WHO 5 and ICC |
NG2 (clone 7.1)+CD33+Lysozyme+
★ t(9;11) MLLT3::KMT2A = intermediate · t(6;11) AFDN::KMT2A = poor · t(10;11) = variable. Revumenib (menin inhibitor) FDA-approved Oct 2024 for r/r KMT2A-rearranged AML/ALL. Mixed-lineage surface markers (CD19, CD2) can occur — check MPAL criteria before reclassifying |
KMT2A rearrangement 11q23.3 · >80 partners · Revumenib FDA 2024 · Ziftomenib (menin) Phase III · FLT3 co-mut frequent · AlloSCT CR1 recommended for most KMT2A-r AML |
| AML-MR (WHO 5 8 genes) · AML with MR gene mutations (ICC 9 genes) | HSC; myelodysplastic evolution | CD34+CD117+CD33+/CD13+Aberrant CD5/CD7/CD56 on blasts
⚑ WHO 5 AML-MR = 8-gene list (NO RUNX1): ASXL1, BCOR, EZH2, SF3B1, SRSF2, STAG2, U2AF1, ZRSR2. ICC = 9-gene list (adds RUNX1). This is a clinically important difference — affects CPX-351 eligibility. Both require ≥20% blasts for AML designation |
CD34+p53 IHC (if TP53 co-mutation)
★ CPX-351 (liposomal daunorubicin+cytarabine) = standard induction for AML-MR (FDA-approved, superior to 7+3). WHO 5: ≥20% blasts required. ICC: 10–19% blasts = MDS/AML with MR gene mutations (different treatment implications) |
WHO 5 AML-MR 8 genes: ASXL1, BCOR, EZH2, SF3B1, SRSF2, STAG2, U2AF1, ZRSR2 (NO RUNX1) · ICC adds RUNX1 = 9 genes · CPX-351 standard · Venetoclax+HMA unfit patients · AlloSCT eligible |
| BPDCN | Plasmacytoid dendritic cell precursor | CD123++ (exceptionally bright)CD4+CD56+HLA-DR+CD3−CD19−MPO−CD14−
★ CD123++ CD4+ CD56+ without other lineage markers = BPDCN. WHO 5 Table 15 adds CD303 (BDCA-2) and CD304 (BDCA-4) as expected positive pDC markers in updated criteria |
TCF4 (E2-2) IHC+ — most specific BPDCN markerSPIB IHC+ — highly sensitiveCD123++CD4+
★ WHO 5 BPDCN diagnostic criteria (Table 15): CD123 + 1 other pDC marker (TCF4/TCL1/CD303/CD304) + CD4 and/or CD56 OR any 3 pDC markers + all negatives absent. Tagraxofusp-erzs (CD123 × diphtheria toxin) FDA 2018 |
TET2 (~50%) · NRAS/KRAS · ASXL1 · ZEB2 · MYC amplification · CDKN2A/B del · Tagraxofusp-erzs FDA 2018 · AlloSCT consolidation in CR1 · CAR-T (CD123) in trials |
Purple border = hallmark/discriminating · Green = positive · Red-orange = negative · Amber = variable
| Entity | BM Biopsy Architecture | Aspirate / PB Cytology | Key Diagnostic Features | Pitfalls / Must Not Miss |
|---|---|---|---|---|
| MPN — Megakaryocyte Morphology (the Critical MPN Trephine Challenge) | ||||
| CML | Near 100% cellularity · Granulocytic hyperplasia overwhelming fat · Pseudo-Gaucher cells (lipid-laden macrophages) · Sea-blue histiocytes · No significant fibrosis in CP · WHO 5: only CP and BC. ICC: AP at 10–19% blasts | Left-shifted granulocytosis · "Myelocyte bulge": myelocytes > metamyelocytes (inverse of reactive) · Basophilia (hallmark — virtually no other condition causes this degree) · Eosinophilia · NAP score very LOW (contrast reactive = HIGH) |
|
⚑ Leukemoid reaction vs CML: reactive lacks basophilia, has high NAP, BCR::ABL1 negative. Always test BCR::ABL1 for any unexplained WBC >25×10⁹/L |
| ET vs pre-PMF — Critical MPN Morphological Distinction | ET: normocellular or mildly hypercellular · Megakaryocytic hyperplasia ONLY · Fat preserved · MF-0 pre-PMF: Hypercellular · Granulocytic + megakaryocytic hyperplasia · TIGHT dense mega clusters · MF-0/1 at most Overt PMF: MF-2/3 · osteosclerosis · sinusoidal hematopoiesis · dry tap |
ET: Thrombocytosis >450 · large/giant platelets · No left shift pre-PMF: May look like ET early · Circulating CD34+ >10/μL (discriminating by flow) Overt PMF: Leukoerythroblastic PB · teardrop cells · circulating megakaryocyte nuclear fragments |
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⚑ pre-PMF vs ET = SINGLE most critical MPN morphological distinction. pre-PMF has higher MF transformation risk, different prognosis and therapy implications. Many pre-PMF cases are incorrectly called ET — requires expert trephine evaluation |
| Systemic mastocytosis | Multifocal dense MC aggregates ≥15 MCs/cluster = major criterion · Paratrabecular or perivascular clustering · Fibrosis around aggregates · MCL: diffuse MC infiltrate ≥20% of nucleated BM cells · SM-AHN/SMN: concurrent AML/MDS/MPN morphology | Spindle-shaped (fusiform) mast cells on BM aspirate (≥25% = minor criterion) · Normal MCs = round/oval; neoplastic = spindle · Metachromatic granules (Giemsa/toluidine blue positive) · Eosinophilia in PB (PDGFRA-associated) |
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⚑ MC aggregates mimicked by fibroblasts or plasma cell clusters on H&E — tryptase IHC + CD25 IHC mandatory for confirmation before diagnosing SM |
| MDS-5q | Variable cellularity · Megakaryocytic hyperplasia · Small MONOLOBATE megakaryocytes (pathognomonic) · Erythroid hypoplasia + dysplasia · Granulopoiesis preserved | Macrocytic anemia · Thrombocytosis or normal platelet count · Normal or mildly elevated WBC · Rare circulating blasts |
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⚑ Monolobate megakaryocytes + macrocytic anemia in elderly woman = del(5q) until proven otherwise. Additional del(7q)/−7 changes entity — review full karyotype before lenalidomide |
| MDS-LB / MDS-IB1 / MDS-IB2 / MDS-F / MDS-h | Variable cellularity · Dysplasia ≥10% in ≥1 lineage · ALIP (central intertrabecular CD34+/MPO+ clusters) · MDS-F: MF-2/3 · MDS-h: ≤25% cellularity + clonal dysplasia | Cytopenias · Pseudo-Pelger-Huët cells (bilobed, hypogranular) · Hypogranular neutrophils · MDS-IB: circulating blasts · Auer rods (→ always MDS-IB2 regardless of blast %) · Dimorphic RBCs |
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⚑ Dysplasia on aspirate can be artifactual (poor smear quality, prolonged EDTA) — always assess smear quality before counting. Acquired pseudo-Pelger vs inherited Pelger-Huët anomaly: inherited = bilobate but normally granulated + no other cytopenias |
| CMML | Variable cellularity · Monocytic expansion (interstitial CD68/CD163+) · Granulocytic dysplasia · CD34+ blasts increase with type · Fibrosis in subset | Monocytosis ≥0.5×10⁹/L AND ≥10% WBC · Promonocytes (blast equivalents) = large cells with irregular nuclear folds, fine chromatin, pale gray cytoplasm · Dysplastic neutrophils · Proliferative CMML: WBC ≥13 |
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⚑ Promonocytes vs monocytes: promonocytes have irregular nuclear folds, fine chromatin, ground-glass cytoplasm — distinct from mature kidney-shaped monocyte nuclei. Under-counting promonocytes underestimates blast equivalents |
| APL with PML::RARA ⚠ MEDICAL EMERGENCY | Hypercellular BM · Sheets of abnormal promyelocytes · Hypergranular: coarse granules obscuring nuclear outline · M3v: bilobed nuclei + fine dusty granules | Hypergranular promyelocytes (coarse azurophilic granules, bilobed "butterfly" nuclei) · FAGGOT CELLS: bundles of multiple Auer rods per cell = PATHOGNOMONIC · DIC evidence: schistocytes + thrombocytopenia · M3v: bilobed cells with fine granules — mimics monocytic AML |
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⚑ MOST DANGEROUS morphological pitfall in hematopathology: missing APL = fatal DIC. ANY highly granular myeloid blast + coagulopathy → START ATRA IMMEDIATELY ⚑ M3v mimics monocytic AML — CD34−/HLA-DR− on flow + coagulopathy = APL regardless of granule appearance |
| AML with RUNX1::RUNX1T1 · t(8;21) | BM replaced by blasts · Large blasts with abundant cytoplasm · Salmon-pink primary granules · Long single Auer rods prominent · Residual dysplastic granulocytes | Salmon-pink cytoplasm + long single Auer rods (solitary per cell) + CD19+ aberrant on myeloid blasts = CLASSIC TRIAD |
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⚑ Do NOT mistake long single Auer rods for Faggot cells (APL). Single rod per cell = t(8;21); bundled multiple rods per cell = APL. If any doubt, rule out APL first |
| AML with CBFB::MYH11 · inv(16) | Hypercellular BM · Myeloid + monocytic + eosinophilic components | Abnormal eosinophils = PATHOGNOMONIC: immature cells containing BOTH eosinophilic AND enlarged purple/basophilic granules in same cell |
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⚑ inv(16) is the cytogenetic finding most commonly missed on standard G-banding. FISH with CBFB break-apart probe is mandatory in ALL myelomonocytic AML with eosinophilia regardless of karyotype result |
| AML with NUP98 rearrangement (WHO 5 ONLY entity) | Variable cellularity · Blast infiltration · No specific BM architectural pattern | Variable cytology by fusion partner · NUP98::KDM5A = megakaryoblastic (CD41/CD61+) · NUP98::HOXA9 = poorly differentiated · No pathognomonic cytomorphological feature |
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⚑ NUP98 rearrangements will be missed without RNA sequencing or targeted FISH. Any pediatric/young adult AML with normal karyotype + poor response + no other identified genetic driver should have NUP98 testing |
| AML with NPM1 mutation | Hypercellular BM · Usually normal karyotype · Often monocytic/myelomonocytic differentiation · CD34-negative blast clusters on IHC (unusual) | Cup-like nuclear invaginations (punched-out nuclear indentation) in blasts = characteristic NPM1 AML feature on Romanowsky stain · Monocytic differentiation common · Occasional Auer rods |
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⚑ CD34-negative AML should AUTOMATICALLY trigger NPM1 mutation testing — this is standard of care ⚑ Always test FLT3 allelic ratio concurrently with NPM1 — allelic ratio (>0.5 high; <0.5 low) determines prognosis and guides alloSCT decision in CR1 |
⚑ = diagnostic pitfall · ★ = high-yield discriminating feature · ⚠ APL = emergency — ATRA empirically while FISH pending
| Topic | WHO 5 (Khoury et al., Leukemia 2022) | ICC 2022 (Arber et al., Blood 2022) | Divergence Level | Clinical Impact / Pitfall |
|---|---|---|---|---|
| CML — Phase Classification | ||||
| CML phases | WHO 5: Eliminates accelerated phase (AP). Only chronic phase (CP) and blast phase (BP). Emphasis on risk features in CP. BP: ≥20% blasts OR extramedullary blasts OR ↑ lymphoblasts in PB/BM | ICC: Retains accelerated phase (AP). AP = blasts 10–19% OR PB basophils ≥20% OR major route ACA in Ph+ cells (second Ph, +8, i17q, +19, complex, abn 3q26.2) | MAJOR | ⚑ WHO 5: A patient with 15% blasts in CML = still "chronic phase with high-risk features" + TKI optimization. ICC: same patient = accelerated phase, may qualify for different trial eligibility or alloSCT consideration. Report both classifications in pathology report when divergence matters clinically |
| JMML Categorization | ||||
| JMML classification placement | WHO 5: JMML is placed under MPN. Reflects RAS pathway molecular pathogenesis and absence of bona fide MDS stigmata. KMT2A rearrangements excluded from JMML diagnosis in WHO 5 | ICC: JMML placed under Pediatric and/or germline mutation-associated disorders. ICC also defines JMML-like neoplasms (no RAS mutation) and Noonan syndrome-associated MPD as formal entities | Minor | ⚑ Placement differs between WHO 5 (MPN) and ICC (Pediatric). Clinical criteria are essentially the same but ICC provides additional guidance on JMML-like neoplasms and Noonan-MPD which WHO 5 doesn't formalize as separate entities. ICC Table 22 requires monocytes ≥1×10⁹/L (not ≥0.5 like CMML); ~7% JMML cases may not meet this criterion |
| AML — NUP98 Rearrangement | ||||
| AML with NUP98 rearrangement | WHO 5 Table 7: NEW standalone AML entity. Any blast % (KMT2A/MECOM/NUP98 are the three rearrangement-based entities that allow any blast %). WHO 5 explicitly states NUP98 rearrangements may be cryptic on conventional karyotyping | ICC 2022: NUP98 rearrangement is NOT a separate AML entity. Cases with NUP98 fusions would fall into AML with other MDS-related cytogenetic abnormalities, AML-NOS, or possibly MDS/AML depending on blast count | MAJOR | ⚑ Critical practical pitfall: A pediatric patient with 12% blasts and NUP98::NSD1 fusion = "AML with NUP98 rearrangement" (WHO 5, any blast %) vs "MDS/AML with MR cytogenetics or MDS-IB2" (ICC). Treatment intensity differs dramatically. Institutions using ICC must have an institutional protocol to test for and flag NUP98 rearrangements given their clinical importance |
| AML — TP53 Mutated AML | ||||
| AML with mutated TP53 | WHO 5: AML with mutated TP53 is NOT a standalone separate AML entity. Biallelic TP53 inactivation is recognized as MDS-biTP53 (in MDS) and within AML-MR (cytogenetic criteria). WHO 5 does not create a standalone "AML with mutated TP53" category | ICC 2022 (Table 21): Creates a standalone TP53-mutated spectrum category. MDS with mutated TP53 (multi-hit required) · MDS/AML with mutated TP53 (10–19% blasts, any TP53 VAF >10%) · AML with mutated TP53 (≥20% blasts, any TP53 VAF >10%). Groups these together due to similar aggressive biology regardless of blast % | MAJOR | ⚑ ICC AML with mutated TP53 = any pathogenic TP53 mutation VAF >10% (monoallelic allowed for AML). WHO 5 handles TP53-mutated AML within AML-MR cytogenetic category. This ICC-specific framing groups MDS, MDS/AML, and AML with TP53 together for unified treatment strategy. Pure erythroid leukemia in ICC is classified within AML with mutated TP53 (TP53 mutations very common in AEL) |
| AML — CEBPA Blast Threshold | ||||
| AML with CEBPA mutation — blast threshold | WHO 5: Requires ≥20% blasts (exception along with BCR::ABL1). "There is insufficient data to support any change in the blast cutoff criterion for AML with CEBPA mutation" (Khoury et al., 2022). In-frame bZIP; monoallelic bZIP now qualifies | ICC 2022 (Table 25): Requires ≥10% blasts (same threshold as most other genetic AML entities). ICC also requires in-frame bZIP mutations. Unlike WHO 5, ICC lowers the blast threshold for CEBPA | MAJOR | ⚑ A patient with 15% blasts and monoallelic bZIP CEBPA mutation: WHO 5 = NOT AML with CEBPA (only MDS-IB2 since <20%). ICC = AML with in-frame bZIP CEBPA mutations (favorable prognosis entity). This directly affects whether a patient receives HMA (MDS) vs intensive induction chemotherapy. Additionally, WHO 5 updated to include monoallelic bZIP as qualifying (equivalent prognosis to biallelic bZIP-containing cases). Both WHO 5 and ICC now agree that monoallelic in-frame bZIP mutations qualify for the favorable entity |
| AML-MR / AML-MRC — Gene List | ||||
| Myelodysplasia-related gene mutation list | WHO 5 AML-MR (Table 8): 8 somatic genes — ASXL1, BCOR, EZH2, SF3B1, SRSF2, STAG2, U2AF1, ZRSR2. RUNX1 is NOT on the WHO 5 AML-MR list (WHO 5 explicitly states: "RUNX1 mutations in AML overlap with such a broad range of defining molecular features that it was determined to lack enough specificity to define a standalone AML type") | ICC 2022 (Table 25): 9 somatic genes — ASXL1, BCOR, EZH2, RUNX1, SF3B1, SRSF2, STAG2, U2AF1, ZRSR2. RUNX1 IS on the ICC list (now encompasses the prior provisional entity of "AML with mutated RUNX1"). Blast threshold: ≥20% for AML; 10–19% = MDS/AML | MAJOR | ⚑ A patient with ≥20% blasts and a RUNX1 mutation only (no other MR feature): WHO 5 = AML-NOS (not AML-MR since RUNX1 not on WHO 5 list). ICC = AML with myelodysplasia-related gene mutations (eligible for CPX-351). A patient with 15% blasts and SRSF2 mutation: WHO 5 = MDS-IB2 (HMA standard). ICC = MDS/AML with MR gene mutations (potentially CPX-351 eligible). These are clinically high-stakes differences for treatment decisions |
| MDS — MDS/AML Bridging Category and Blast Thresholds | ||||
| MDS-IB2 vs MDS/AML (10–19% blasts) | WHO 5: 10–19% BM blasts without defining genetic lesion = MDS-IB2 (not AML). WHO 5 retains ≥20% blast threshold for MDS → AML transition. "Broad agreement that MDS-IB2 may be regarded as AML-equivalent for therapeutic considerations" (Khoury et al., 2022) | ICC 2022: Introduces MDS/AML as a new category for 10–19% blasts (adults; pediatric cases retain MDS-EB with 10–19% blasts). Eligible for both MDS and AML clinical trials. Sub-classified as MDS/AML-TP53, MDS/AML-MR gene muts, MDS/AML-MR cytogenetics, MDS/AML-NOS | MAJOR | ⚑ Most clinically important classification divergence for treatment decisions. At 15% blasts with SRSF2 mutation: WHO 5 = MDS-IB2 (HMA standard, may consider CPX-351 off-label). ICC = MDS/AML with MR gene mutations (CPX-351 appropriate, eligible for AML trials). Approved drug labeling (CPX-351, venetoclax) uses ≥20% blasts — the ICC MDS/AML framing currently exceeds regulatory approvals, creating a treatment gap |
| MDS — SLD vs MLD Distinction | ||||
| Single vs multilineage dysplasia | WHO 5: MDS-LB merges single lineage dysplasia (SLD) and multilineage dysplasia (MLD) into one category. "The distinction between single lineage and multilineage dysplasia is now considered optional" (Khoury et al., 2022) | ICC 2022: Retains SLD and MLD as separate subtypes within MDS-NOS. "Although there is poor reproducibility in distinguishing single lineage vs multilineage dysplasia in MDS, this distinction has been retained" (Arber et al., 2022) | Minor | ⚑ WHO 5 MDS-LB does not distinguish SLD from MLD — loses prognostic granularity used in clinical trials and some registry databases. Institutions using WHO 5 that report only "MDS-LB" cannot satisfy trial eligibility criteria that specify SLD vs MLD. Consider documenting number of dysplastic lineages even when using WHO 5 MDS-LB designation |
| MDS/MPN — aCML Naming | ||||
| aCML / MDS/MPN with neutrophilia | WHO 5: Renames atypical CML to "MDS/MPN with neutrophilia" (MDS/MPN-N). The name "atypical CML" is formally retired to avoid confusion with CML | ICC 2022: Retains "atypical CML, BCR::ABL1-negative" as the preferred name. Drops the "BCR::ABL1-negative" qualifier but keeps "aCML". Same diagnostic criteria | Minor — naming only | ⚑ Same entity, same criteria, different names. Pathology reports using "aCML" (ICC) vs "MDS/MPN-N" (WHO 5) for the same case may appear as different diagnoses in registries. Reports should ideally cross-reference both terms when the two classifications assign different names |
| MDS/MPN — ICC-only entities | ||||
| CMUS / CCMUS · MDS/MPN with i(17q) | WHO 5: CCUS is defined (CHIP + unexplained cytopenia) but CMUS (with monocytosis) is not a distinct WHO 5 entity. WHO 5 acknowledges "additional studies needed to determine optimal approach to classifying individuals with unexplained clonal monocytosis who do not fit the new CMML diagnostic criteria." MDS/MPN with i(17q) not listed as provisional entity in WHO 5 | ICC 2022: CMUS (clonal monocytosis of undetermined significance) = formal ICC entity for cases with monocytosis ≥0.5×10⁹/L + clonality but NO BM findings of CMML. CCMUS if cytopenia also present. MDS/MPN with i(17q) = new provisional subentity under MDS/MPN-NOS | Minor — ICC provides more operational guidance | ⚑ ICC CMUS/CCMUS provides explicit guidance for a common diagnostic grey zone (monocytosis + clonal mutations + no CMML morphology). WHO 5 acknowledges the grey zone but doesn't formalize a diagnostic category. In practice, ICC's CMUS concept is clinically useful and many centers apply it even when using WHO 5 terminology |
| SM — AHN vs AMN Terminology | ||||
| SM-AHN (WHO 5) vs SM-AMN (ICC) | WHO 5: Retains SM-AHN (SM with associated haematological neoplasm) — "haematological" allows for both myeloid AND lymphoid associated neoplasms. SM can have associated lymphoid neoplasm in WHO 5 | ICC 2022: Renames to SM-AMN (SM with associated myeloid neoplasm). ICC restricts the associated neoplasm to myeloid context only, based on evidence that shared KIT mutations are present with myeloid but not lymphoid neoplasms | Minor | ⚑ Rare but important: SM + concurrent lymphoma (e.g., CLL/SLL) = "SM-AHN" in WHO 5 (formally recognized as an SM subtype) but would NOT be SM-AMN in ICC (which restricts to myeloid neoplasms). Clinically uncommon but the name matters for registry coding |
| Histiocytic/DC Neoplasms — FDCS Placement | ||||
| Follicular dendritic cell sarcoma (FDCS) placement | WHO 5: FDCS and fibroblastic reticular cell tumour are moved OUT of histiocytic/DC neoplasms to a new chapter: "stroma-derived neoplasms of lymphoid tissues" | ICC 2022: Changes to acute leukemias of ambiguous lineage not addressed in the CAC meeting. ICC does not specifically address FDCS placement (lymphoid neoplasms handled separately) | Minor — WHO 5 reorganization | ⚑ Practical impact: FDCS should no longer be coded/filed under histiocytic neoplasms in WHO 5-based laboratory information systems. Reclassification from "histiocytic/DC" to "stroma-derived" has registry and coding implications |
| Summary: Where Each Classification Misses the Mark | ||||
| WHO 5 — areas of concern |
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| ICC 2022 — areas of concern |
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WHO 5th Ed = Khoury JD, Solary E, Hochhaus A et al. Leukemia 2022;36:1703–1719 · ICC 2022 = Arber DA, Orazi A, Hasserjian RP et al. Blood 2022;140:1200–1228. Both effective simultaneously. Pathology reports should document which classification was applied. Clinical trials specify which classification was used for eligibility determination.