What is the major criterion for the diagnosis of acute leukemia?

Acute Myelomonocytic Leukemia

In acute myelomonocytic leukemia [AMML], the sum of myeloblasts, monoblasts, and promonocytes is 20% or more. Twenty percent to 79% of the bone marrow cells are of monocyte lineage, often demonstrated by reactivity with the non-specific esterase stain; however, cytochemical studies are not necessary for diagnosis when the morphologic identity of the monocyte lineage is obvious. Numerous monocytes may be present in the peripheral blood and may mimic MDS/MPN, especially chronic myelomonocytic leukemia. Both granulocytic and monocytic differentiation are observed in varying proportions in the bone marrow. The major criterion distinguishing AMML from AML with maturation is the proportion of neoplastic cells with monocytic features, which collectively must equal 20% or more. The immunophenotype of AMML generally reflects the dual differentiation pattern of the leukemic cells, with some populations expressing fairly typical myeloid antigens and others expressing more monocytic antigens, including CD14 and CD64.

Careful distinction of promonocytes from abnormal monocytes in the bone marrow is essential to separate AMML from chronic myelomonocytic leukemia.18 Promonocytes retain fine chromatin, indistinct nucleoli, and delicate nuclear folds, reflecting their immaturity. In contrast, abnormal immature-appearing monocytes of chronic myelomonocytic leukemia have more condensed chromatin and generally more folded or convoluted nuclear contours. In a new diagnosis, the distinction between chronic myelomonocytic leukemia and AMML may not be possible with a peripheral blood smear. Correlation with bone marrow findings is essential to resolve the diagnosis because the immature populations of AMML are more readily identified in marrow. A reliable discriminating immunophenotype is not available because promonocytes typically lack CD34.

DIFFERENTIAL DIAGNOSIS

Acute Myelomonocytic and Acute Monocytic Leukemia

Acute leukemia must always be considered in the differential diagnosis of CMML, particularly CMML-2. A bone marrow aspirate and biopsy are crucial in distinguishing between these entities because blasts and promonocytes are usually more prominent in the bone marrow than in the blood. Even in the marrow specimens, the blasts may be irregularly distributed, and inspection of the biopsy and aspirate together yields the most useful information. Moreover, the distinction between monocytes, abnormal [immature] monocytes, promonocytes, and blasts is sometimes difficult,1 and distinguishing some cases of AML from CMML-2 can be challenging. When the number of blasts plus promonocytes is 20% or more in the blood or bone marrow, the diagnosis is AML rather than CMML. A more difficult issue is the finding of mutatedNPM1 in a case in which the diagnosis of CMML is being considered. This occasion is in the setting of CMML-2, and in such cases, close follow-up and aggressive clinical intervention are recommended as mutatedNPM1 is generally regarded as an AML-related mutation.39

XII Acute Myelomonocytic Leukemia [AML-M4]

Acute myelomonocytic leukemia shows two distinct neoplastic populations: blasts, with the phenotype similar to AML with/without maturation, and monocytic cells, which are positive for CD11b, CD11c, CD14, CD64, and HLA-DR.

Flow cytometry [Fig. 14] reveals two distinct populations: blasts [moderate CD45 and low side scatter] and monocytic cells [bright CD45 and slightly increased side scatter]. The monocytic component is positive for CD11b, CD11c, CD13, CD14, CD33, CD64, and HLA-DR and myeloblasts are positive for CD13, CD33, CD34, CD117, and HLA-DR. A subset of cases shows expression of CD2, CD7, CD34, and CD56 by atypical monocytes. The expression of CD56 is less common in neoplastic monocytes from acute myelomonocytic leukemia [22%] than in monocytes from either CMML [69%] or acute monoblastic leukemia [78%] [Gorczyca, 2004b]. Monocytic population from acute myelomonocytic leukemia less often displays aberrant lack of CD11b, CD14, and HLA-DR, or positivity for CD16, CD23 and CD117 compared to acute monoblastic leukemia or CMML [Gorczyca, 2004b]. Table IV presents immunophenotypic profile of acute myelomonocytic leukemia.

Table IV. Immunophenotypic profile of acute myelomonocytic leukemia [n = 40 cases]

Laboratory Tests

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Complete blood counts and blood smear evaluation. Note that morphologic evaluation of blasts may suggest myeloid or lymphoid origin, but flow cytometry or cytochemistries [often faster] are needed to confirm. Auer rods are seen in blasts of myeloid origin.

LDH is commonly elevated. Other biochemistries to assess organ function [creatinine, liver enzymes] and spontaneous tumor lysis syndrome [uric acid, potassium, phosphate, calcium].

Coagulation studies to assess DIC. DIC is always present in APML, but can be present inall forms of acute leukemia, especially acute monocytic leukemia.

HLA typing for possible bone marrow transplant and platelet support.

Cytochemical stains:

Myeloperoxidase can be performed in minutes, + in myeloid origin leukemia.

Alpha naphthyl acetate esterase [“nonspecific esterase”] stains mainly monocytic cells.

Flow cytometry on blood and/or bone marrow [seeTable E1].

Cytogenetic studies, ideally on bone marrow, but can be done on peripheral blood. Fluorescence in situ hybridization [FISH] is often used as an adjunct to conventional chromosome analysis.

Next Generation Sequencing [NGS] by PCR to detect specific prognostic gene mutations.

Molecular studies to further stratify risk and prognosis, which may affect treatment choices [seeTables E2,3, and 4]. Directing this workup should be done with combined hematology and laboratory expertise and typically will consist of studies for fms-related tyrosine kinase gene [FLT3] mutations, nucleophosmin gene [NPM] mutations, and CCAAT/enhancer binding protein α gene [CEBPA] mutations. Wider molecular panels are increasingly common because of the increasing numbers of potential markers and the potential availability of targeted therapies for FLT3 mutated disease and those with isocitrate dehydrogenase mutations, among others. TP53 mutation, mutated RUNX1, and mutated ASXL1 have recently been added to the NCCN poor-risk category for AML in 2018.Table E5 summarizes common recurrent mutations in adult acute myeloid leukemia.

Formal diagnosis of acute nonlymphocytic leukemia is established if the marrow or peripheral blood blast percentage is ≥20%, unless t[8;21], inv[16], t[16;16] or t[15;17] are present, in which case the percentage of blasts may be lower.

Myeloperoxidase [MPO] staining of 3% of blasts establishes myeloid lineage, but MPO may be negative in some AML cases diagnosed by flow cytometry.

Specific criteria exist for diagnosing other forms of ANLL, mainly to distinguish it from myelodysplasia. The WHO AML classification is outlined inTable 6.

Bone marrow findings are described inFig. E2.

Clinical Aspects

Acute myelomonocytic leukemia accounts for about 15–25% of all AMLs. The median age is around 50 years, but it may occur at any age. The incidence is slightly more in males than in females. Similar to other acute leukemias, clinical symptoms are the result of bone marrow involvement and extramedullary infiltration by the leukemic cells. Fatigue, fever, bleeding disorders, gingival hyperplasia, lymphadenopathy, hepatosplenomegaly, and skin involvement are among frequent clinical findings. As mentioned earlier in this chapter, patients with inv[16] have a favorable prognosis and those with translocation of 11q23 fall into the category of leukemias with intermediate survival rate [104]. Some studies show a correlation between the expression of CD56 by the leukemic cells and severe fatal hyperleukocytosis in patients with acute myelomonocytic leukemia [189]. Successful effect of treatment with NUP98–HOXD11 fusion transcripts and monitoring of minimal residual disease in patients with AML-M4 has been reported [198].

Acute myelomonocytic leukemia.

Acute myelomonocytic leukemia is characterized by a significantly elevated WBC count and the presence of myeloid and monocytoid cells in the peripheral blood and bone marrow [Figure 31.11]. Monocytic cells [monoblasts and promonocytes] constitute at least 20% of all marrow cells, as do neutrophils and their precursors. The monoblasts are large with abundant cytoplasm containing small granules and pseudopodia. The nucleus is large and immature and may contain multiple nucleoli. Promonocytes also are present and may have contorted nuclei. The cells are positive for the myeloid antigens CD13 and CD33 and the monocytic antigens CD14, CD4, CD11b, CD11c, and CD64. Nonspecific cytogenetic changes are found in most cases.6

Monocytic Sarcoma

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Many patients have history of acute monocytic or myelomonocytic leukemia

Bone marrow involvement by acute leukemia may be present simultaneously

Rarely, monocytic sarcoma precedes acute leukemia

Histologically, monocytic sarcoma can resemble LCS but

In some cases, monocytic sarcoma shows prominent single-file pattern of infiltration

Neoplastic cell nuclei can have prominent folds but lack twisted towel appearance or nuclear grooves

Immunohistochemistry

CD43[+], CD163[+]

CD68[+], lysozyme [+]

Expression can be focal or partial

CD4[+], CD45[+]

CD1a[-], S100[-], langerin [-]

Flow cytometry immunophenotypic analysis of fresh cells

Many more monocyte/histiocyte antigens available that can be analyzed

CD4[+], CD11b[+], CD11c[+], CD14[+], etc.

Electron microscopy

Myeloperoxidase [-/+]: No Birbeck granules

Leukemia

Early cutaneous involvement occurs more frequently in acute myelomonocytic and monocytic leukemia than in other leukemias and most systemic lymphomas. Infiltrated, hyperplastic, and friable gingival tissue strongly favors the diagnosis of acute myelomonocytic or monocytic leukemia, and biopsy specimens of such tissue should confirm the diagnosis. Oral involvement can occur rarely with other types of acute leukemia and, even more rarely, with chronic leukemias and lymphomas.

The syndrome of aleukemic leukemia cutis is the earliest cutaneous presentation possible. In this syndrome, lesions containing blast cells develop in the skin in the absence of peripheral blood and, occasionally, bone marrow evidence of leukemia. Such lesions may be present several months before diagnosis, but the full leukemic syndrome ultimately develops. Aleukemic leukemia cutis has been reported predominantly with acute myelomonocytic leukemia, much less often with acute monocytic leukemia, and rarely preceding acute lymphoblastic leukemia/lymphoma.

Granulocytic sarcoma is a rare tumor of malignant myeloid cells, which may present in the skin. It may precede the development of acute myelogenous leukemia [one form of aleukemic leukemia cutis], it may accompany acute myelogenous leukemia, or it may develop in myelodysplastic syndromes or chronic myelogenous leukemia at the time of leukemic transformation. Lesions are typically flesh-colored or erythematous papules or nodules in a child or young adult. Chloroma is an alternative name for granulocytic sarcoma when the solid mass of blast cells in the skin shows a diagnostic yellowish-green coloration following cutting of the lesion during pathologic examination. The greenish discoloration is attributed to the presence of high concentrations of myeloperoxidase in the myeloblasts.

Acute promyelocytic leukemia rarely involves sites other than blood and bone marrow, but skin is the most commonly affected extramedullary site, accounting for half the cases. The cutaneous presentations are clinically similar to those of acute myelogenous leukemia or granulocytic sarcoma. As noted previously, leukemia cutis may localize to sites of burns, trauma, injections, herpes, scars, or Hickman catheter use. Acute promyelocytic leukemia may be particularly prone to occur at sites of vascular puncture with blood draws or catheters, with a review suggesting that most reported cases of promyelocytic sarcoma occurred at puncture sites for venepuncture, central venous catheters, or bone marrow aspiration.

The term chronic myelogenous leukemia is now considered to encompass four syndromes. Classic chronic myelogenous leukemia has also been called chronic granulocytic leukemia and chronic myeloid leukemia, and is by far the most common subset. It is usually Philadelphia chromosome positive and presents with anemia, elevation of mature neutrophils and other granulated leukocytes, sometimes thrombocytosis, and frequently splenomegaly. This disease typically ends in an accelerated phase often evolving to acute leukemia, usually myelocytic but occasionally lymphocytic. Chronic myelogenous leukemia spares the skin until an accelerated blast phase or blast crisis develops, at which time multiple flesh-colored or erythematous papules and nodules may rapidly develop [Fig. 15–2]. An unusual manifestation of chronic myelogenous leukemia is a tender, edematous, purpuric area of induration on the lower leg that resembles stasis dermatitis but which is due to a perivascular and periappendigeal myeloid infiltration of the dermis. The three much less common subsets of chronic myelogenous leukemia include chronic myelomonocytic leukemia, juvenile myelomonocytic leukemia, and chronic neutrophilic leukemia. These are generally much more aggressive diseases when untreated, and much less treatment responsive than classic myelogenous leukemia. These are less likely than chronic myelogenous leukemia to have specific skin infiltrates, but are perhaps more likely to develop nonspecific lesions, either paraneoplastic [as in Sweet's syndrome] or cytopenic [infections or hemorrhage] manifestations of malignant disease.

Both Chediak–Higashi syndrome and Griscelli syndrome have autosomal recessive inheritance of pigmentary, hematologic, immunologic, and neurologic abnormalities. Dysfunction of lysosomes and melanosomes is common to both. Both are now known to have defects in the secretion of perforin-containing granules, essential for lymphocyte cytotoxicity; this may explain some of the nonphagocytic immune defects. Both may also have an ‘accelerated phase,’ with blood and systemic features similar to those of familial lymphohistiocytosis, and this may also be a consequence of perforin-related immune deficiency [see Chapter 17].

Chronic lymphocytic leukemia [CLL] is overwhelmingly B-cell derived in most countries where it is common [