A myeloid sarcoma is a solid tumor comprised of immature white blood cells that collect outside the bone marrow. It is a rare side condition where the patient suffers from acute myeloid leukemia. A leukemia specialist is unlikely to see one during a lifetime of practice. Even more rarely, the tumor can develop even when the underlying leukemia is in remission. They can occur anywhere, but the skin and the gums are the most common locations.
The appearance of a myeloid sarcoma in a patient suffering from other forms of leukemia usually indicates the leukemia has become acute and needs to be treated more aggressively. Even more rarely, a myeloid sarcoma can develop even if acute leukemia does not present.
This sort of tumor was first described in 1811, but its relationship to the related leukemia wasn't worked out until 1902.
The most common way to obtain a definitive diagnosis is with a biopsy. The condition should always be treated as if the patient had acute myeloid leukemia whether or not there are other clinical signs as treating the underlying leukemia will effectively treat the sarcoma.