AML Does Not "Metastasize" in the Traditional Sense—It Is Already a Systemic Disease at Presentation
Acute myeloid leukemia (AML) is fundamentally different from solid tumors: it is a disseminated hematologic malignancy from the outset, characterized by clonal expansion of myeloid blasts in the peripheral blood, bone marrow, and potentially other tissues simultaneously. 1 The concept of "metastasis" (spread from a primary site to distant sites) does not apply to AML because it originates in the bone marrow and circulates systemically through the bloodstream from diagnosis. 2, 3
Why AML Is Already Systemic at Diagnosis
AML involves the bone marrow, peripheral blood, and potentially extramedullary sites at presentation, making it a systemic disease rather than a localized one that spreads over time. 1
The malignant myeloid blasts circulate freely in the bloodstream, allowing them to infiltrate various organs without requiring a "metastatic" process. 3
Diagnosis requires ≥20% blasts in either bone marrow or peripheral blood, confirming that the disease is already widespread when detected. 4, 2
Extramedullary Disease (Myeloid Sarcoma)
While AML doesn't metastasize in the traditional sense, it can present with or develop extramedullary involvement (myeloid sarcoma):
Myeloid sarcoma represents tumor masses of myeloid blasts that can occur at any anatomical site, either with concurrent bone marrow involvement (synchronous) or without (isolated). 5
Common sites include skin, central nervous system, lymph nodes, and other organs, and this can occur at initial diagnosis or during disease course. 1, 5
The timeframe for extramedullary manifestations is variable: some patients present with extramedullary disease at diagnosis, while others may develop it during treatment or relapse. 5
If extramedullary disease is suspected, appropriate imaging (PET/CT) should be performed to assess extent and guide treatment planning. 4
Clinical Implications for Disease Progression
Without treatment, AML progresses rapidly and may be fatal within weeks to months due to bone marrow failure, not because of "metastatic spread." 3
The disease is already disseminated at diagnosis, so the focus is on systemic treatment (chemotherapy, targeted agents, or stem cell transplantation) rather than preventing spread. 2, 6
CNS involvement can occur and requires specific evaluation (CT or MRI) if signs or symptoms are present, with consideration of CNS prophylaxis in certain high-risk cases. 4, 2