Myeloid sarcoma, also known as chloroma, granulocytic sarcoma and extramedullary myeloblastoma, is a solid tumor composed of myeloblasts or immature myeloid cells, i.e. a solid collection of leukemic cells in an extramedullary site. Thus, it is an extramedullary manifestation of acute myeloid leukemia. The tumor may precede or occur concurrently with acute or chronic myeloid leukaemia. Myeloid sarcoma is rare and remains an uncommon complication of acute myeloid leukemia. The presence of myeloid sarcoma is considered de facto evidence that these premalignant conditions have transformed into an acute leukemia requiring appropriate treatment. Although myeloid sarcoma can develop as the sole manifestation, it should always be considered manifestations of systemic disease, rather than isolated local phenomena. Although it commonly occurs in periosteum of the skull, paranasal sinuses, sternum, ribs, vertebrae, pelvis, lymph nodes and skin, it may also be seen in internal solid organs. The most common areas of involvement are the skin (also known as leukemia cutis) and the gums. The clinical presentation of myeloid sarcomas varies and is dependent on the site of involvement. About two thirds of the cases occur in patients younger than 20 years of age.
The WHO Classification based on the predominant cell type and their degree of maturation is found of no practical relevance while immunophenotyping seems of paramount importance for the diagnosis of the process. Grossly, these tumors often have a green tint due to the presence of myeloperoxidase, which gave rise to the name chloroma derived from the Greek word chloros meaning “green”. The neoplastic tissue usually appears firm with a fish-flesh appearance.
Definitive diagnosis of myeloid sarcoma usually requires histologic examination. Since myeloid sarcoma represents the tissue mass form of various subtypes of acute myeloid leukemia (AML), the diagnosis is equivalent to a diagnosis of AML. Microscopically, all tumors exhibit a diffuse and infiltrative growth pattern regardless of the site involved, but the general architecture of the involved tissue tends to be preserved. The cytological features (esp., cytoplasmical enzymetic granules and nuclear morphology) vary with the degree of myeloid differentiation. Most often there are medium-sized to large blastic cells with ovoid vesicular nuclei with centrally located prominent nucleoli and dispersed chromatin. The cytoplasm is scant to moderate. The mitotic count can be high. There may be apoptotic bodies phagocytosed by histiocytes (tingible body macrophages) that impart a starry sky appearance. Currently, the definitive diagnosis is usually based on immunohistochemistry. CD68/KP1 was the most commonly expressed marker. The best immunohistochemical stains used for this include myeloperoxidase (MPO) and lysozyme. Myeloperoxidase immunostain is positive in most myeloblastic variants while lysozyme is frequently expressed in monoblastic variants. Overall, tumor cells are positive for CD43 (100%), myeloperoxidase (97%); lysozyme (93%); CD117 (87%), Chloroacetate esterase (85%) CD68 (KP1) (>85%), CD33, CD34 (47%), but are negative for CD20, CD79a, CD3, S100 and CD30 by immunohistochemistry. Stain for CD15 and CD117 varies with status of tumor differentiation. The most frequent chromosomal abnormality associated with certain myeloid sarcomas is t(8;21)(q22;q22), also seen in some AMLs
The main differential is large cell non-Hodgkin's lymphoma: a panel including chloroacetate esterase, myeloperoxidase, lysozyme, CD43, together with CD79a and CD3 is particularly useful to confirm the diagnosis and exclude non-Hodgkin's lymphoma
Myeloid sarcomas are typically quite sensitive to standard anti-leukemic chemotherapy. Treatment is similar to that for AML, even in cases of isolated tumors with no blood or bone marrow involvement. Patient with newly diagnosed leukemia and an associated myeloid sarcoma is typically treated with systemic chemotherapy as the first-line treatment. Patients presenting with a primary myeloid sarcoma typically receive systemic chemotherapy.
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