A myeloid sarcoma (chloroma, granulocytic sarcoma, extramedullary myeloid tumor) is a solid tumor composed of immature white blood cells called myeloblasts. A chloroma is an extramedullary manifestation of acute myeloid leukemia; in other words, it is a solid collection of leukemic cells occurring outside of the bone marrow. Types In acute leukemia Chloromas are rare; exact estimates of their prevalence are lacking, but they are uncommonly seen even by physicians specializing in the treatment of leukemia. Chloromas may be somewhat more common in patients with the following disease features: French–American–British (FAB) classification class M2 WHO Classification (2016 revision) is a separate entity under the "Acute myeloid leukemia (AML) and related neoplasms" those with specific cytogenetic abnormalities (e.g. t(8;21) or inv(16)) those whose myeloblasts express T-cell surface markers, CD13, or CD14 those with high peripheral white blood cell counts However, even in patients with the above risk factors, chloroma remains an uncommon complication of acute myeloid leukemia. Rarely, a chloroma can develop as the sole manifestation of relapse after apparently successful treatment of acute myeloid leukemia. In keeping with the general behavior of chloromas, such an event must be regarded as an early herald of a systemic relapse, rather than as a localized process. In one review of 24 patients who developed isolated chloromas after treatment for acute myeloid leukemia, the mean interval until bone marrow relapse was 7 months (range, 1 to 19 months). In myeloproliferative or myelodysplastic syndromes Chloromas may occur in patients with a diagnosis of myelodysplastic syndrome (MDS) or myeloproliferative syndromes (MPS) (e.g. chronic myelogenous leukemia (CML), polycythemia vera, essential thrombocytosis, or myelofibrosis). The detection of a chloroma is considered de facto evidence these premalignant conditions have transformed into an acute leukemia requiring appropriate treatment. For example, presence of a chloroma is sufficient to indicate chronic myelogenous leukemia has entered its 'blast crisis' phase. In Eosinophilic leukemia At least one case of FIP1L1-PDGFRA fusion gene-induced eosinophilic leukemia presenting with myeloid sarcoma and eosinophilia has been reported. This form of myeloid sarcoma is distinguished by its highly successful treatment with imatinib (the recommended treatment for FIP1L1-PDGRGA fusion gene-induced eosinophilic leukemia) rather than more aggressive and toxic therapy. Primary chloroma Very rarely, chloroma can occur without a known pre-existing or concomitant diagnosis of acute leukemia, acute promyelocytic leukemia or MDS/MPS; this is known as primary chloroma. Diagnosis is particularly challenging in this situation (see below). In almost all reported