Full Patient History:
Patient is a 25-year-old man with leukemia, status post chemotherapy. For two months, he noticed a firm soft tissue mass in the right upper pectoral region, without evidence of inflammation or pain.

General Discussion:
Granulocytic sarcoma, also known as chloroma or extramedullary myeloblastoma, is a rare solid tumor composed of primitive precursors of the granulocytic series of white blood cells that include myeloblasts, promyelocytes. It is associated with acute myeloblastic leukemia, chronic myelogenous leukemia, and other myeloproliferative disorders, such as myelofibrosis with myeloid metaplasia, hypereosinophilic syndrome, or polycythemia vera. They occur in 2.5-9.1% of patients with acute myelogenous leukemia and five times less frequently in patients with chronic myelogenous leukemia. This tumor was initially called chloroma, because typical forms have a green color caused by high levels of myeloperoxidase in these immature cells.

No prognostic significance exists between acute leukemic patients with granulocytic sarcomas and those without. However, patients with granulocytic sarcomas who have chronic leukemia or myeloproliferative disorders have a negative prognosis, because these tumors often occur during acute transformation. These sarcomas are very sensitive to focal irradiation or chemotherapy; they generally resolve completely in less than 3 months, although they recur in approximately 23% of patients

Patients with granulocytic sarcomas are frequently asymptomatic. These tumors can involve any part of the body, either concurrently or sequentially. They often occur in multiples and preferentially involve orbits and subcutaneous tissue, but they may also occur in paranasal sinuses, lymph nodes, bone, the spine, the brain, pleural and peritoneal cavities, the breasts, the thyroid, salivary glands, the small bowel, the lungs, or various pelvic organs

In soft-tissue, as in this case, granulocytic sarcomas are slightly hypodense to muscle at CT and isointense to bone marrow on both T1- and T2- weighted MR images; they enhance homogeneously after injection of contrast medium.

In this case, the pectoral subcutaneous nodule was confirmed to be granulocytic sarcoma on biopsy. The patient was treated with radiation. Repeat PET-CT 3 months later, after radiation therapy, showed resolution of this nodule, with mild residual skin thickening (not shown).