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Part III • White Cell Disorders

Figure 11.9 Acute myelomonocytic leukemia. (A) AMML with prominent monoblasts, promonocytes, and spectrum of myeloid/monocytic cells. (B) AMML with promonoblast, promonocytes, and eosinophil on edge of frame at arrow (AMMLe)

is associated with favorable prognosis, whereas survival rates for the other AMMLs vary.

Acute Monoblastic and Acute Monocytic Leukemia

Acute monoblastic leukemia accounts for 5% to 8% of AMLs, whereas acute monocytic leukemia comprises 3% to 6% of cases.27 The bone marrow in both of these leukemias shows greater than 20% blasts, with greater than 80% of the cells having monocytic origin, including monoblasts, promonocytes, and monocytes. The distinction between monoblastic and monocytic leukemia subtypes depends on the proportions of monoblasts and promonocytes present in the bone marrow. Acute monoblastic leukemia has a predominance of monoblasts, which are large cells with moderate to intensely basophilic, abundant cytoplasm, and prominent round nuclei with fine chromatin. A spectrum of monocytic cells is seen in acute monocytic leukemia, with the majority of cells being promonocytes. The nuclear chromatin of promonocytes is more condensed and they often have a convoluted or cerebriform configuration. The cytoplasm of promonocytes contain azurophilic granules and may be vacuolated. Less than 20% of the cells are of granulocytic origin. Auer rods are usually absent in acute monoblastic leukemia but are frequently seen in the promonocytes of acute mono-cytic leukemia (Fig. 11.10). In most cases, monoblasts

Figure 11.10 (A) Acute monoblastic leukemia with Auer rods. (B) Acute monocytic leukemia, one monoblast, three promonocytes. (C) Acute monocytic leukemia, monoblast, several promonocytes, and monocytes are present.

and promonocytes will stain intensely positive with NSE. Monoblasts are typically MPO or SBB negative; promonocytes may be very weakly positive with these staining reactions. The characteristic immunoreactivity of the monocytic leukemic cells for lysozyme is also a common finding.

Acute monoblastic leukemia may occur at any age, but the majority of patients tend to be younger, have increased blast percentages in the peripheral blood, and have a poor prognosis.32 Acute monocytic leukemia is more common in adults, with the median age being 49 years. A hallmark clinical feature of the monocytic leukemias is extramedullary disease, with the most predominant finding being the cutaneous and gum infiltration that results in gingival hypertrophy. Other clinical features include bleeding disorders due to DIC, as well as a high incidence of CNS or meningeal disease either at the time of diagnosis or as a manifestation of relapse during remission.33 A high WBC count is another common finding reported in 10% to 30% of patients.

Characteristic immunophenotypic markers for cells of monocytic differentiation include CD14, CD4,

CD11b, CD11c, CD 36, CD64, and CD68. The strong association between the acute monoblastic leukemia and deletions/translocations involving chromosome 11q23 have been previously described under AML with recurrent genetic abnormalities. In general, both acute monoblastic and acute monocytic leukemias have an unfavorable prognosis due to shorter duration of treatment response and poor prognostic factors.

Acute Erythroid Leukemia

Acute erythroid leukemias are predominantly characterized by abnormal proliferation of erythroid precursors. The additional presence or absence of a myeloid element defines the two subtypes, erythroleukemia and pure erythroid leukemia. More than 50% of the bone marrow cells are erythroid precursors and at least 30% are myeloblasts in erythroleukemia (erythroid/myeloid) (Fig. 11.11). Pure erythroid leukemia is defined by the majority of marrow cells (>80%) being comprised of erythroid precursors, without a myeloid proliferation.19 Erythroleukemia is usually found in patients 50 years of age or older and accounts for approximately 5%

Figure 11.11 (A) and (B) Acute erythroid leukemia. (C) Acute erythroid leukemia, note Auer rod in myeloblast. (D) Acute erythroid leukemia, left frame shows binucleated pronormoblasts and dysplastic features, right frame shows PAS block positiv-ity in a ring around the nucleus.

Figure 11.11 (A) and (B) Acute erythroid leukemia. (C) Acute erythroid leukemia, note Auer rod in myeloblast. (D) Acute erythroid leukemia, left frame shows binucleated pronormoblasts and dysplastic features, right frame shows PAS block positiv-ity in a ring around the nucleus.

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