Myeloid Sarcoma Pathology

Updated: Feb 26, 2018
  • Author: Amandeep Aneja, MD; Chief Editor: Christine G Roth, MD  more...
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Overview

Overview

Myeloid sarcoma represents the tissue mass form of acute myeloid leukemia (AML), thus, the diagnosis is equivalent to a diagnosis of AML. [1] Myeloid sarcoma may occur de novo, may precede or coincide with AML, or may represent a blastic transformation of a preceding myelodysplastic syndrome or chronic myeloproliferative neoplasm. [2] Myeloid sarcoma may also be the initial manifestation of relapse in a patient with previously diagnosed AML.

Myeloid sarcoma is composed of myeloid blasts, similar to AML—that is, immature granulocytic precursors, monocytic precursors, erythroid precursors, or even megakaryocytic precursors.

The more frequent sites of involvement are the skin, lymph nodes, gastrointestinal tract, bone, soft tissue, and testis. [3, 4] More rarely, multiple anatomic sites are involved (<10% of cases).

Myeloid sarcoma has a predilection for males (male-to-female ratio, 1:2). It typically occurs in the later decades of life (median age, 56 years; range, 1 month to 89 years). [5, 6]

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Definition

Myeloid sarcoma is an extramedullary tumor mass consisting of myeloid blasts which efface the tissue architecture. [7]  The latter feature is particularly important in order to distinguish myeloid sarcoma from tissue infiltration by leukemic blasts in acute myeloid leukemia (AML) patients; AML tissue infiltrates do not form tumoral masses that efface the underlying architecture.

Associated disorders include granulocytic sarcoma (ie, chordoma), monocytic sarcoma, and erythroid sarcoma. [7]

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Morphologic Features

The morphologic features of myeloid sarcoma are similar to those of the myeloid blasts comprising the various subtypes of acute myeloid leukemia (AML). Blasts generally have high nuclear-to-cytoplasmic ratios, fine chromatin, and occasionally prominent nucleoli (see below). Immunohistochemical stains are required in order to establish the diagnosis and determine lineage (see the section on Immunophenotypic Features and Methods)

Granulocytic sarcoma (hematoxylin-eosin). Granulocytic sarcoma (hematoxylin-eosin).

If unstained touch imprints are available, cytochemical stains may also be helpful to delinate lineage. For example, monocytic sarcoma may be composed of a uniform population of monoblasts with nonspecific esterase (NSE) positivity (see below). 

Monocytic sarcoma (left to right: lysozyme stain, Monocytic sarcoma (left to right: lysozyme stain, Wright stain of touch preparation, and nonspecific esterase stain of touch preparation).

Erythroid sarcoma is relatively rare and displays sheets of primarily immature erythroid precursors (ie, pronormoblasts) (see the image below). Immunostains for e-cadherin or glycophorin may be useful to establish erythroid differentiation. 

Erythroid sarcoma (hematoxylin-eosin). Erythroid sarcoma (hematoxylin-eosin).
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Immunophenotypic Features and Methods

Touch preparations of the tissue involved may be stained with enzyme cytochemical stains (ie, myeloperoxidase, Sudan black B, chloroacetate esterase, α-naphthyl acetate esterase, or α-naphthyl butyrate esterase).

Fresh tissue suspensions may be analyzed by flow cytometric analysis, and aid in characterization. However, if the diagnosis of myeloid sarcoma is not suspected, then only formalin-fixed, paraffin-embedded tissue may be available. A battery of immunohistochemical stains is often employed, including immature markers (such as CD34, CD117) as well as markers to establish myeloid or monocytic differentiation (myeloperoxidase [MPO], CD68-R [PGM1 clone], lysozyme, CD33, CD14, CD163, etc). Myeloid sarcomas have immunophenotypic features that are identical to acute myeloid leukemia (AML) (see Acute Myeloid Leukemia, Not Otherwise Categorized).

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Molecular/Genetic Features and Methods

Myeloid sarcomas are associated with chromosomal abnormalities (55% of cases) seen in acute myeloid leukemia (AML), including monosomy 7, trisomy 8, MLL-rearrangement, inversion(16), trisomy 4, monosomy 16, 16q-, 5q-, 20q-, and trisomy 11. [6, 8]  In a subset of cases, NPM1 mutations are identified. [5, 9]  The t(8;21) abnormality is more frequently seen in pediatric cases. [6, 10]

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Prognosis and Predictive Factors

There are no prognostically predictive clinical or pathologic features associated with myeloid sarcoma.

The probability of prolonged survival or cure seems higher for patients who undergo allogeneic or autologous bone marrow transplantation. [6, 11]  The 2016 revision to the World Health Organization (WHO) classification recommends comprehensive investigation in patients without evidence of marrow disease to classify these neoplasms into the most precise disease category possible. [12]

The overall survival of those with extramedullary myeloid sarcoma appears to vary significantly on the basis of the patient's age, sex, race, and sites of presentation. [13]

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