Morphologic Classification

The FAB group in 197614 proposed a uniform classification system for ALL. Three subtypes of ALL are distinguished on the basis of cell size, nuclear shape, number and prominence of nucleoli, and the relative amount of cytoplasm. L1 defines a uniform population of small blast cells with high nuclear/cyto-plasmic (N/C) ratio, a small inconspicuous nucleolus, and a regular nucleus (Plate 14-1), whereas L2 is designated as moderate to large cells with irregular nuclear membrane, one or

Table 14-3. Proposed WHO Classification of Acute Myeloid Leukemias (AML)

Acute myeloid leukemias with recurrent cytogenetic translocations AML with t(8;21)(q22;q22), AML1/ETO Acute promyelocytic leukemia [AML with t(15;17)(q22;q11 — 12) and variants], PML/RARa AML with abnormal marrow eosinophils [inv(16)(p13q22) or t(16;16)(p13;q11)], CBFb/MYHl 1 AML with 11q23 (MLL) abnormalities Acute myeloid leukemia with multilineage dysplasia With prior myelodysplastic syndrome Without prior myelodysplastic syndrome Therapy-related acute myeloid leukemia Alkylating-agent-related

Epipodophyllotoxin related (some may be lymphoid) Acute myeloid leukemia not otherwise categorized AML without maturation (FAB M0) AML with minimal maturation (FAB M1) AML with maturation (FAB M2) Acute myelomonocytic leukemia (FAB M4) Acute monocytic leukemia (FAB M5) Acute erythroid leukemia (FAB M6) Acute megakaryocytic leukemia (FAB M7) Acute basophilic leukemia (FAB M2Baso) Acute biphenotypic leukemia more nucleoli, more cytoplasm, and lower N/C ratio (Plate 14-2). A rare type seen in approximately equal percentages in children and adults (about 3 percent), referred to as L3, is morphologically identical to the cell characteristics of Burkitt's lymphoma. The cells are large and homogeneous with round to oval nuclei. The chromatin is finely dispersed with prominent nucleoli. The cytoplasm is intensely basophilic with or without vacuoles (Plate 14-3). Modifications for the assessment of lym-phoblasts were introduced to improve reproducibility and interobserver concordance.15 A simplified scoring system is given in Table 14-4. In this system, the sum may vary from +2 to -4. A total score of 0 to +2 defines L1, while a score of -1 to -4 establishes L2.

The relative frequency of the ALL morphologic subtypes varies with age. In children, approximately 80 percent have L1 morphology. In contrast more than 60 percent of adults are classified as L2. In earlier studies it was shown that L1 morphology was associated with a better event-free survival and induction rate than L2.16 However, classification of ALL using

Table 14-2. Proposed WHO Classification of Acute Lymphoblastic Leukemias

Precursor B-cell acute lymphoblastic leukemia (cytogenetic subgroups) t(9;22)(a34;q11); BCR/ABL t(v;11q23);MLL rearranged t(1;19)(q23;p13) PBX/E2A t(12;21)(p12;q22) ETV/CBFa Precursor T-cell acute lymphoblastic leukemia Burkitt cell leukemia

Table 14-4. Simplified Scoring System for FAB L1 and L2 Criteria Score

High N/C ratio > 75 percent of cells +

Low N/C ratio > 25 percent of cells -

Irregular nuclear membrane > 25 percent of cells -

Large cells > 50 percent of cells -

Abbreviations: N/C, nuclear/cytoplasmic (ratio).

FAB criteria into L1 and L2 types is becoming less relevant because of a poor correlation with immunological and genetic findings and the lack of consistent prognostic significance.17'18 Nonetheless, hematopathologists and hematologists should be aware of morphological variability in ALL blasts.19'20 The recent WHO classification11 has proposed to combine L1 and L2 subgroups into one group. FAB-L3, however, remains a clinically important ALL type, which is associated with MYC gene disregulation and mature B-cell phenotype.21,22

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