Myeloid sarcoma is a rare cancer defined by the growth of a solid tumor composed of immature white blood cells outside of the bone marrow. This extramedullary manifestation signifies an aggressive form of blood cancer that has spread beyond its typical site of origin. Because the diagnosis is highly significant, the presence of this tumor mass often requires immediate, intensive systemic therapy.
The Nature of Myeloid Sarcoma
Myeloid sarcoma forms from the uncontrolled proliferation of myeloid precursor cells, primarily myeloblasts. These immature cells are destined to become various types of white blood cells. Unlike most blood cancers that circulate freely or reside in the bone marrow, this disease creates a distinct, infiltrating mass in soft tissues or organs. The World Health Organization (WHO) classifies myeloid sarcoma as a form of acute myeloid leukemia (AML) that has developed a tumor outside of the marrow.
Historically, this tumor was sometimes called “chloroma,” derived from the Greek word chloros, meaning green. This unusual color, observed in fresh tissue specimens, is caused by the high concentration of the enzyme myeloperoxidase (MPO) within the tumor cells. The older term, “granulocytic sarcoma,” refers to the specific cell line involved, but the current term, myeloid sarcoma, is more encompassing of all myeloid cell types that can form the mass.
The tumor masses can appear virtually anywhere in the body, frequently targeting soft tissues, bone, lymph nodes, and the skin. Other common sites include the gastrointestinal tract, testes, and the orbit, which is a frequent site of involvement in children. The specific location and size of the tumor dictate the symptoms experienced by the patient, making the clinical presentation highly variable.
Clinical Relationship to Systemic Blood Cancers
Myeloid sarcoma is rarely an isolated finding and almost always indicates the presence or future development of a systemic blood cancer. The majority of cases are associated with acute myeloid leukemia (AML) or, less commonly, myelodysplastic syndromes (MDS). The clinical presentation of myeloid sarcoma in relation to the systemic disease is categorized into three main scenarios.
The first is a de novo presentation, where the myeloid sarcoma is the initial finding, and the bone marrow shows no evidence of leukemia. If treated only locally, nearly all these isolated cases will progress rapidly to full-blown AML, demonstrating the systemic nature of the disease. The second presentation is concurrent disease, where the tumor is diagnosed at the same time as the underlying AML or MDS is discovered in the bone marrow.
The third scenario is a relapse, where the myeloid sarcoma appears after a patient has been treated for AML or MDS and was in remission. The appearance of the tumor often signifies that the systemic disease has returned, even if the bone marrow initially appears clear of cancer cells. Because of this intimate connection, the treatment of myeloid sarcoma must be viewed as the treatment of a systemic blood cancer, not merely a localized solid tumor.
Signs, Symptoms, and Diagnosis
The signs and symptoms of myeloid sarcoma are entirely dependent on the tumor’s location, leading to a wide range of clinical manifestations. A mass near the spine can cause neurological deficits or spinal cord compression. An orbital tumor may cause vision changes or bulging of the eye. Skin involvement often presents as firm, colored nodules or patches, sometimes referred to as leukemia cutis.
Beyond these specific, site-related signs, patients may experience non-specific symptoms associated with cancer, such as unexplained weight loss, persistent fatigue, and localized pain. Since the symptoms can mimic many other conditions, diagnosis requires a high index of suspicion from the physician. The definitive diagnosis relies on obtaining a tissue sample of the mass through a biopsy.
Imaging techniques, including Computed Tomography (CT), Magnetic Resonance Imaging (MRI), or Positron Emission Tomography (PET) scans, are used to locate the tumor and assess its size and extent. Following the biopsy, the tissue is examined using immunohistochemistry (IHC) and immunophenotyping to confirm the cells are of myeloid origin. Specific markers, such as myeloperoxidase (MPO), CD33, and CD117, are identified on the cell surface to differentiate the myeloid sarcoma from other tumors like non-Hodgkin lymphoma.
Current Therapeutic Approaches
The standard approach for treating myeloid sarcoma is systemic chemotherapy, mirroring the intensive regimens used for acute myeloid leukemia. This strategy is founded on the understanding that myeloid sarcoma is fundamentally a systemic disease, even when it presents as an isolated mass. Localized treatments, such as surgery or radiation alone, are insufficient and carry a high risk of subsequent bone marrow relapse.
For eligible patients, induction chemotherapy, often using a combination of drugs like an anthracycline and cytarabine, is initiated immediately to eradicate cancer cells throughout the body. Achieving a complete remission of the underlying systemic disease is paramount to a favorable outcome. Following remission, the most intensive consolidation therapy available is allogeneic hematopoietic stem cell transplantation (HSCT).
HSCT involves replacing the patient’s diseased blood-forming cells with healthy donor cells. This offers the best chance for a lasting cure, particularly for those with adverse genetic features. Radiation therapy plays a supportive role, reserved for localized tumors causing severe symptoms, such as pain or organ dysfunction, or when a mass persists after chemotherapy. Recent advancements in targeted therapies, including drugs like Venetoclax or hypomethylating agents, are being integrated into treatment protocols based on the specific genetic mutations found in the tumor cells.