Myeloid sarcomas, also called granulocytic sarcomas, chloromas, or extramedullary myeloid tumours, are rare extramedullary masses comprised of myeloid precursor cells. These tumours represent a unique presentation of acute myeloid leukaemia (AML), whether in isolation to, preceding, or simultaneous with bone marrow and peripheral blood disease.
Different from leukaemic infiltrates, myeloid sarcomas characteristically efface the involved tissue architecture, and are recognised as a distinct entity in the WHO classification of tumours of haematopoietic and lymphoid tissues.
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Epidemiology
It is more commonly seen in children, with ~60% occurring in individuals less than 15 years of age. There is no recognised gender predilection.
Myeloid sarcoma may develop during the course of a haematologic disorder but is seen to precede the disorder in 35% of cases.
Pathology
Myeloid sarcoma lesions are composed of immature myeloid elements and therefore represent focal accumulations of leukaemic cells. In contrast to its associated entities, the lesion takes the form of a solid soft tissue mass that effaces the underlying involved tissue architecture (cf. leukaemic infiltrates).
They can occur with or without relation to other forms of myeloid disorders 10:
de novo: with no prior history of myeloproliferative and myelodysplastic disorders
simultaneously with a myeloproliferative and myelodysplastic disorder: concomitant peripheral blood and marrow involvement
as a relapse of a treated myeloproliferative and myelodysplastic disorder
as a form of blast transformation from a prior myeloproliferative neoplasm
Location
Extramedullary involvement is a defining feature.
Almost any tissue can be affected, with skin and bone being the commonest. They have been reported in the skull, face, orbit, and paranasal sinuses. Lesions have been reported in the tonsils, oral and nasal cavities and within the lacrimal, thyroid and salivary glands. The central nervous system can also be affected.
Associations
As described in the World Health Organisation classification, myeloid sarcoma is considered a unique presentation, not a subtype, of acute myeloid leukaemia (AML) 9.
As with other forms of acute myeloid leukaemia, myeloid sarcoma can evolve from myelodysplastic syndromes as well as the myeloproliferative neoplasms:
Radiographic features
Exact imaging features will depend on the location and involved organ:
History and etymology
first described by A Burns in 1811 3
termed "chloroma" in 1853 by King due to typical forms have a green colour from high levels of myeloperoxidase in immature cells
renamed as "granulocytic sarcoma" in 1966 by Rappaport when it was demonstrated that an estimated 30% of cells do not contain myeloperoxidase, therefore, a significant proportion of cells are not green in colour
subsequently renamed again to "myeloid sarcoma" 9, as several associated types of myeloid leukaemia are defined by little to no granulocytic differentiation