Myeloid Sarcoma Pathology

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 (MDS), myeloproliferative neoplasm (MPN), or MDS/MPN.[2, 3] 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, testis, and peritoneum.[3, 4, 5] Rarely, multiple anatomic sites are involved (< 10% of cases).

Myeloid sarcoma has a slight predilection for males (male-to-female ratio: 1.2:1). It typically occurs in the later decades of life (median age: 56 years; range: 1 month to 89 years).[6, 7]

Myeloid sarcoma is an extramedullary tumor mass consisting of myeloid blasts that efface the tissue architecture.[8]  The latter feature is particularly important 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, chloroma), monoblastic/monocytic sarcoma, and erythroid sarcoma.[8]

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 the image below). Immunohistochemical stains are required to establish the diagnosis and determine lineage (see the section on Immunophenotypic Features and Methods).

Myeloid Sarcoma Pathology. Granulocytic sarcoma (hematoxylin and eosin).

If unstained touch imprints of involved tissue are available, enzyme cytochemistry may also help delineate lineage and aid in subclassification. Cytochemical stains for myeloperoxidase (MPO), Sudan Black B, and chloroacetate esterase (CAE) are positive in cases with myeloid or granulocytic differentiation, whereas nonspecific esterase (NSE) (also known as α-naphthyl acetate esterase or α-naphthyl butyrate esterase) is positive in cases with monocytic or monoblastic differentiation (see the following image).

Myeloid Sarcoma Pathology. Monocytic/monoblastic sarcoma (left to right: lysozyme stain, Wright stain of touch preparation, and neuron-specific enolase [NSE] stain of touch preparation).

Erythroid sarcoma is relatively rare and displays sheets of primarily immature erythroid precursors (ie, pronormoblasts) (see the image below). It represents the extramedullary mass-forming equivalent of a diagnosis of pure erythroid leukemia (PEL) involving the bone marrow, and it is more often seen in the setting of blastic transformation of an underlying myeloproliferative neoplasm (MPN).[6] Immunohistochemical stains for E-cadherin or glycophorin are useful to establish erythroid differentiation.

Myeloid Sarcoma Pathology. Erythroid sarcoma (hematoxylin and eosin).

Myeloid sarcomas have immunophenotypic features that are identical to acute myeloid leukemia (AML)  (see Acute Myeloid Leukemia, Not Otherwise Specified (NOS)). Fresh tissue suspensions may be analyzed by flow cytometric analysis to aid in characterization. However, if the diagnosis of myeloid sarcoma is not suspected, then only formalin-fixed, paraffin-embedded tissue may be available.

Multiparameter flow cytometry and/or a battery of immunohistochemical stains are often employed, including markers of immaturity (CD34 and CD117 [KIT]), as well as markers to establish myeloid/granulocytic (myeloperoxidase [MPO], CD15, CD33) or monocytic (lysozyme, CD14, CD64, CD11c, CD68 [KP1 clone], CD163, PU.1) differentiation (see the following images). Tumours with an underlying NPM1 mutation will show expression of NPM1 by immunohistochemistry in both the nucleus and cytoplasm, reflective of the aberrant cytoplasmic localization of this protein conferred by the mutation.[6] Staining for CD45 (leukocyte common antigen), terminal deoxynucleotidyl transferase (TdT), CD43, CD56, and CD99 is variable and not lineage defining. Stains for B- and T-cell markers are typically performed to exclude non-Hodgkin lymphoma and lymphoblastic lymphoma, which mimic myeloid sarcoma morphologically. 

Myeloid Sarcoma Pathology. Immunohistochemistry performed on this myeloid sarcoma involving the salivary gland showed the large neoplastic cells to be diffusely positive for CD34 (upper left), CD117 (upper right), and CD43 (lower left), as well as variably positive for myeloperoxidase (MPO) (middle left), lysozyme (middle right), and CD68 (lower right), indicative of both myeloid and monocytic (ie, myelomonocytic) differentiation.

Myeloid Sarcoma Pathology. In the same case of myeloid sarcoma involving the salivary gland as in the previous image, flow cytometry of the salivary gland mass detected a population of CD45 dimly positive cells (A, gold population) with increased forward and side scatter by light scatter analysis (B, gold population), indicative of primitive blasts with large cell size. Further immunophenotypic analysis showed the blasts to be positive for the myeloid antigens CD13 and CD33 (C, blue population), markers of immaturity CD34 and CD117 (D, blue population), and myeloperoxidase (MPO) (D, blue population), with expression of the mature monocytic marker CD14 (C, blue population) seen in a small subset.

Myeloid sarcomas are associated with chromosomal abnormalities (55% of cases) seen in acute myeloid leukemia (AML), including monosomy 7, trisomy 8, KMT2A (MLL) rearrangement, inversion 16, trisomy 4, monosomy 16, 16q-, 5q-, 20q-, and trisomy 11.[7, 9]  In a subset of cases, NPM1 mutations are identified and are frequently associated with myelomonocytic, monocytic or monoblastic differentiation.[6, 10, 11]  The t(8;21)(q22;q22) abnormality is more frequently seen in pediatric cases.[7, 12]

Detection of these genetic abnormalities requires advanced ancillary diagnostic techniques, such as conventional cytogenetic analysis (karyotype),  fluorescence in situ hybridization (FISH), and mutational analysis via next-generation sequencing (NGS) or single nucleotide polymorphism (SNP) array analysis to allow for identification of multiple genetic mutations that may impact prognosis or be targeted by specific therapies.[13] As with flow cytometry, fresh tissue may not be available for molecular and cytogenetic analyses if the diagnosis of myeloid sarcoma is initially unsuspected; therefore, cytogenetic analysis may be limited to FISH of paraffin-embedded tissue for recurrent genetic abnormalities in AML (see the image below). Most NGS or SNP array multigene panels can be performed on DNA extracted from fresh tissue or formalin-fixed, paraffin-embedded tissue.

Myeloid Sarcoma Pathology. In the same example of myeloid sarcoma involving the salivary gland discussed in the previous two images, there is complete effacement of the underlying tissue architecture by a diffuse infiltrate of large cells with folded nuclei, vesicular chromatin, and moderately abundant pink cytoplasm (left, hematoxylin and eosin). Also present were numerous admixed mature eosinophils and eosinophil precursors (left, hematoxylin and eosin). As myeloid sarcoma was unsuspected at surgery, no fresh tissue of the salivary gland was sent for karyotyping. Establishing the specific World Health Organization (WHO) Classification in this case required directed fluorescence in situ hybridization (FISH) of paraffin-embedded tissue. Based on the morphologic finding of eosinophilia, a characteristic association of acute myeloid leukemia (AML) with inv(16), FISH with a dual-color breakapart probe for the CBFB locus at 16q22 revealed the presence of a rearrangement (right, abnormal cells with separate red and green signals), indicative of an underlying inv(16)(p13.1q22) or CBFB-MYH11 rearrangement.

A 2020 case series of 11 cases combined with a literature review of myeloid sarcoma with CBFB-MYH11 fusion (inv(16) or t(16;16)) indicates an apparent marked predilection for abdominal sites.[14]  The investigators suggest features such as "the lack of obvious associated eosinophils, presence of pDC nodules, and lack of concurrent [bone marrow] involvement" indicate that "myeloid sarcoma with CBFB-MYH11 fusion may represent a unique phenomenon."

The overall survival of patients with extramedullary myeloid sarcoma does not appear to be influenced by age, sex, site(s) of involvement, history of prior therapy, or pathologic features including morphology, immunophenotype, or cytogenetics, although the probability of prolonged survival or cure seems higher for patients who undergo allogeneic or autologous bone marrow transplantation.[6, 7, 15]

The presence of myeloid sarcoma in pediatric patients with acute acute myeloid leukemia (AML) at diagnosis appears to be a poor prognostic indicator, particularly in association with KMT2A rearrangements.[16] Stem cell transplantation may also not be beneficial for children with AML and myeloid sarcoma.

The revised 4th edition (2016/2017 revision) of 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.[17]  This may necessitate cytogenetic analysis by conventional karyotype or fluorescence in situ hybridization (FISH), as well as mutational analysis to identify recurrent genetic abnormalities that may impact prognosis and choice of therapy (see the section on Molecular/Genetic Features and Methods). For example, in the case of AML with inv(16)(p13.1q22)/CBFB-MYH11 rearrangement illustrated previously, analysis for mutations involving the KIT gene is particularly important, because the presence of such mutations confers a worse prognosis in CBFB-rearranged AML.