Myeloid sarcoma: A 7-year retrospective study from a tertiary cancer care center, illustrating the clinical, morphological, and immunohistochemical features

Clinical features

The age range and gender ratio were in concordance with the reported literature.^[6,7] The most common site of presentation was lymph nodes and spine. However, rare sites like cervix, nasopharynx, and mediastinum were also noted in this study.

Isolated MS was noted in seven cases in the present study which is a rare finding. Six cases with concomitant AML showed french american british (FAB) M1, M2, and M4 morphologies in marrow. Secondary MS was noted in five cases [Table 1], two of which were MPN (PMF, CML) and one was MPN/MDS (JMML).

Extramedullary proliferation of blasts is a known occurrence in MPN and in transformation to leukemia this needs to be differentiated from MS. MS by definition shows effacement of the architecture of the organ involved by single lineage cells. Another differential diagnosis to be excluded is extramedullary hematopoesis (EMH) seen in cases of MPN.^[1,8,9] Though it is more common in hematopoietic organs like spleen and liver, other sites have also been described.^[8] Morphologically, proliferation of undifferentiated monomorphic cell population favors the diagnosis of MS over EMH. Also, the presence of cells of multiple lines of differentiation (myeloid/erythroid/megakaryocytic) would point toward EMH. The differentiated type of MS which shows admixed population of precursor blast cells with mature myeloid cells would be a challenging differential diagnosis. IHC can help in resolving this dilemma in some cases. The presence of markers of immaturity CD34, Tdt, and CD117 in the predominant population of cells favors the diagnosis of MS. Also, the presence of aberrant markers like CD19 and CD56 not commonly expressed in normal myeloid cells can be useful to confirm the neoplastic nature of cells. FISH workup for RUNXI::RUNX1T1 and CBFB::MYH11 can also help in these scenarios, as these are commonly associated recurrent genetic abnormalities in MS.

However, there would be a subset of cases, especially de novo MS and those associated with MPN/MDS syndromes which would require a multidisciplinary approach, correlating clinical, hematological, radiological, and at times molecular features in tandem. The presence of MS in MPN is associated with worse prognosis which might be due to the accumulation of various clonal abnormalities leading to complex karyotypes and thereby resistance to treatment.^[9,10]

Therapy-associated MS (t-MS) is a rare scenario, most commonly associated with chemotherapy or radiotherapy for solid/hematological malignancies. Carcinoma breast is the most common malignancy in which t-MS is seen and was seen in the present case (Case 15). The patient presented with spinal lesion thought to be metastasis of the epithelial malignancy; however, on HPE and IHC, it was proven to be MS. Further workup showed the presence of bone marrow involvement by AML. JAK2, BCR::ABL1, and KMT2A gene mutations are known to be common in secondary MS.^[10] However, BRAF and KMT2A associations are described in t-MS.^[11] Molecular workup was not available in the present case.

Pathological features

HPE showed diffuse effacement of architecture of the organ involved with the abnormal cell population. It is interesting to understand what causes the blasts to migrate from the bone marrow into the tissues. Though the exact reason is largely unknown, various theories have been proposed, the principal among them is the altered homing signals to blasts which leave the bone marrow niches and cause extramedullary infiltration.^[12,13] This is achieved by an unusual expression of CD56/NCAM1 which is cell adhesion molecule on blasts which dictate migration. CD56 expression is rich in tissues like breast, testis, gut, and ovary which is responsible in homing the blasts in these tissues.^[13] Also, deregulation of the CXCR4 expression facilitates accumulation of blasts in visceral organs. Another purported theory involves the role of matrix metalloproteinases (MMP9). High levels of expression of the enhancer of zeste2 (EZH2) attenuates expression of tissue inhibitor of metalloproteinases (TIMP) which in turn upregulates MMP. The uninhibited action of MMP degrades the extracellular matrix and facilitates escape of blasts in extramedullary spaces.^[12]

The involved organs showed sheets of neoplastic cells with the presence of residual normal structures in between [Figure 3a]. Lymph nodes showed loss of the follicular pattern, replaced with sheets of cells. There was obliteration of the subcapsular sinuses, and perinodal extension was noted [Figure 3b]. In spinal lesions, in addition to sheets of atypical cells, hyalinization and sclerosis were conspicuous [Figure 3c]. Indian file arrangement was seen in few cases, a finding also noted by Pileri et al.^[14] Skin lesions showed predominantly dermal and subcutaneous involvement [Figure 3d]. Periadnexal arrangement was seen in Case 4. Only one of the two cases showed monocytic morphology, an association commonly described in literature.^[14]

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Figure 3:

(a) Section of MS from cervix showing the neoplastic cells going around the normal cervical glands (H& E, 100x). (b) MS involving the lymph node showing effacement of nodal architecture with the peripheral localization of remnant follicles (thick arrow) and presence of neoplastic cells in the afferent vessels (thin arrows) (H&E, 40x). (c) Section from paraspinal mass from case of carcinoma breast showing sclerosis and Indian file arrangement of the neoplastic cells (H&E, 400x). (d) MS involving the skin with the presence of neoplastic cells along dermal vessels and periadnexally (H&E, 40x). MS: Myeloid sarcoma, H&E: Hematoxylin and eosin.

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Three morphological subtypes were noted in our series, and the most common morphology was undifferentiated morphology. It comprised monotonous population of medium-sized cells arranged in sheets, showing round nucleus, small inconspicuous nucleoli, and scant cytoplasm. The most common differential diagnosis for this subtype was non-hodgkin lymphoma. In children, round cell tumor, neuroblastoma, rhabdomyosarcoma, and ewing’s sarcoma also formed the differential as seen in Cases 6 and 7. The cells with differentiated patterns showed sheets of medium-sized cells, having round to indented nuclei with eosinophilic cytoplasm. The amount of cytoplasm in this subtype was more and eosinophilic compared to the undifferentiated subtype. Another clue to this subtype was the presence of granulocytes (band forms/neutrophils) and eosinophils admixed with the blastic population. The third subtype was with monocytic morphology, composed of large cells having vesicular nucleus, conspicuous nucleoli, and more amount of eosinophilic cytoplasm. Poorly differentiated carcinoma, large cell lymphoma, and melanoma form the differential diagnosis in this scenario.

IHC was done in 22 cases. The concise panel of IHC was positive for LCA (14/18), MPO (16/22), CD117 (20/22), CD34 (12/22), CD56 (5/10), and CD68 (4/11). All the cases were negative for CD20 except for one, CD3. Some other markers were added to the panel as per case requirements. Pancytokeratin was done to rule out epithelial malignancy. Desmin and CD99 were added to rule out round cell tumor. CD117 was the most sensitive marker positive in 90% [Figure 4]; two cases where it was negative showed monocytic and undifferentiated morphologies. CD117 is a very helpful marker in establishing a myeloid origin of neoplasm with immature phenotype.^[8] In monocytic morphology MPO, LCA, and CD117 can be negative; such cases present as a challenge in the diagnosis as hematolymphoid neoplasms. CD43 is a very sensitive marker and hence a useful marker, especially when it is expressed in CD20 and CD3 negative scenarios. In these circumstances, expression of CD43 points toward hematolymphoid malignancy with myeloid lineage differentiation.^[7,15] However, CD43 was not used as part of the panel; the absence of CD43 was compensated by use of markers for monocytic differentiation (CD68, CD163, and CD4). Many studies^[2,14] have found CD68 to be a very sensitive marker expressed in majority of the cases; our experience has been different with 36% of MS stained positive by this marker. It is a nonspecific marker seen in many hematological neoplasms,^[7,8] especially when the KP1 clone is used. In the present study, the PG-M1 clone was used which is more specific for monocytic differentiation,^[7,15] hence the lower expression percentage. Also, it was backed by a second monocytic marker in cases with monocytic differentiation. Many authors have also used HLA-DR, CD33, and lysozyme to establish myeloid differentiation. Other nonspecific markers like CD7 and PAX5 are common aberrant cross-lineage antigens that are expressed in AML and can sometimes support the diagnosis. CD56 expression has been postulated to be responsible for homing of the blast cells in extramedullary sites, especially in cases associated with RUNXI::RUNX1T1.^[14,16] In the present study, 50% of the cases showed CD56 expression, including two cases of RUNXI::RUNX1T1 which showed CD56 expression. Expression of CD56 in RUNXI::RUNX1T1-positive cases connotes a bad prognosis.^[16]

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Figure 4:

Common immunohistochemistry markers helpful in diagnosis (400x). MPO: Myeloperoxidase.

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In Cases 22 and 23, the diagnosis was done on cerebrospinal fluid (CSF) and pleural fluid, respectively. In both the scenarios, there was risk associated with biopsy procedure, and flow cytometry-based diagnosis alleviated the need for tissue biopsy. Case 22 was a known and treated case of acute promyelocytic leukemia, who presented with symptoms of headache and numbness in the left leg. Relapse with CNS involvement was suspected due to the presence of mass lesion in magnetic resonance imaging (MRI). CSF tapping was done for confirming the disease. The presence of promyelocytes on cytological evaluation confirmed the relapse of disease and the same sample was processed by flow cytometry. However, it failed due to low cell count and cell degeneration. In Case 23, the patient presented with retroperitoneal mass and ongoing acute pancreatitis, due to which biopsy was prohibited. Pleural effusion was present, and tapping was done to relieve respiratory distress. Cyto-morphology of the fluid showed the presence of blasts, which were confirmed of myeloid origin by flow cytometry. Both these scenarios prohibited biopsy and hence confirmatory tissue diagnosis was done by orthogonal modality of testing by cyto-morphology, and flow cytometry was useful. The factors pertinent for using flow cytometry would be the involvement by disease and availability of an ample number of viable cells, since the cells tend to degenerate faster in fluid than the blood sample.

Interesting case scenario

In present study, Case 20 was a diagnosed case of PMF (JAK2+), which progressed to AML. The patient completed induction chemotherapy elsewhere, was documented in remission, and was referred to our center for stem cell transplant. At our center, the patient presented with normal peripheral blood counts, and physical examination revealed a firm cervical lymph node. The node was biopsied. HPE showed involvement of the node by small round cells, and a possibility of MS was raised in lieu of history of AML. In a week’s time, the peripheral blood started to show the presence of blasts. Hence, bone marrow was performed to confirm relapse, which showed the presence of myeloid blasts with small clone (8%) of T lymphoid blasts. This led to reevaluation of the node with IHC, assumed to be MS on morphology. It showed predominant population of T lymphoblasts with admixed myeloid blasts [Figure 5]. Mixed phenotypic blastic transformation is known in MPN.^[1,17] This case is presented to emphasize the need for immunophenotyping by IHC in all cases, even those presenting concomitantly with AML.

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Figure 5:

Sections from the case of JAK2-positive MPN presenting as MS in the lymph node. Sections (a, b) show effacement of node with perinodal spread by neoplastic cells. The extranodal cluster is positive for MPO while the nodal neoplastic cells show CD3 expression. MPO: Myeloperoxidase, MPN: Myeloproliferative neoplasm, MS: Myeloid sarcoma, JAK 2: janus kinase 2.

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Molecular workup

The most common molecular abnormalities associated with de novo MS are RUNXI::RUNX1T1, CBFB::MYH11, and KMT2A mutations. The site of MS is shown to have specific molecular associations, for example, orbital MS is known to harbor RUNXI::RUNX1T1, while abdominal MS more commonly show CBFB::MYH11 abnormalities. In our case series, three out of eight cases tested positive for RUNXI::RUNX1T1. The details of cytogenetic abnormalities are shown in Table 2. NPM1 mutation was tested in two cases, and two showed positive results. These mutations are known to present in cases showing monocytic morphology. BCR::ABL1 was positive in two cases.

Treatment

There is no consensus on the treatment of MS; various modalities include chemotherapy, radiotherapy, targeted therapies, and stem cell transplant.^[2,18] Chemotherapy is the treatment of choice in predominant cases, especially when there is concomitant involvement of marrow or relapsed MS. Cytarabine- and anthracycline-based chemotherapy regimens as induction akin to those used in AML are used, followed by consolidation with high-dose cytarabine (HiDAC) or allogenic stem cell transplantation. In the present study, eight patients were treated at this center, of which all patients received 7 + 3 AML regimen induction. Six patients received consolidation with HiDAC and two patients in addition underwent allogenic stem cell transplant. Cranial irradiation with arsenic was given in the APML patient who developed central nervous system (CNS) relapse. Radiotherapy is included in the treatment of cases wherein vital structures are compromised, for example, spinal cord compression, superior vena cava obstruction, or as in the present case of isolated CNS involvement. Surgery alone does not have a role in treatment except in a scenario wherein excision biopsies are required to reach a diagnosis.^[2] Though at present the role of molecular aberrations in MS is limited in guiding therapy, targeted therapy can be used effectively. Allogenic stem cell transplant is considered in both de novo and concurrent setting, commonly applied post-induction therapy after achieving complete remission.^[2]

Limitation of the study

The study setting being a tertiary care center, many cases were received as slides for diagnosis and ancillary testing, and the patients left for treatment at the parent hospital. So, treatment and follow-up details are not available in these cases.

List of abbreviations

AML: Acute myeloid leukemia

APL: Acute promyelocytic leukemia

CNS: Central nervous system

CSF: Cerebrospinal fluid

EMH: Extramedullary hematopoiesis

EZH2: Enhancer of Zeste2

HiDAC: High-dose cytarabine

H & E: Hematoxylin and eosin

HPE: Histopathological examination

IHC: Immunohistochemistry

JMML: Juvenile myelomonocytic leukemia

MMP: Matrix metalloproteinases

MDS: Myelodyaplastic syndrome

MS: Myeloid sarcoma

MPO: Myeloperoxidase

MPN: Myeloproliferative neoplasm

TIMP: Tissue inhibitor of metalloproteinases

t-MS: Therapy-related myeloid sarcoma