Prognosis and Therapy When Acute Promyelocytic Leukemia and Other “Good Risk” Acute Myeloid Leukemias Occur as a Therapy-Related Myeloid Neoplasm

Treatment for a pre-existing condition using chemotherapy, radiation therapy, immunosuppressive therapy, or a combination of these modalities may lead to the devastating complication of therapy-related myelodysplastic syndrome or acute myeloid leukemia (t-MDS/t-AML), collectively known as therapy-related myeloid neoplasm (t-MN). This disorder arises as a direct consequence of mutational events induced by the primary treatment. The outcomes for these patients have been historically poor compared to people who develop AML de novo. Currently comprising 10–20% of all cases of AML, t-MN is relatively resistant to conventional leukemia therapies, and is associated with s ort survival times. Median life expectancy from diagnosis is about 8–10 months in most series. Although the spectrum of cytogenetic abnormalities in t-AML is similar to AML de novo, the frequency of unfavorable cytogenetics, such as a complex karyotype or deletion or loss of chromosomes 5 and/or 7, is considerably higher in t-MN. Two distinct groups of patients with t-MN have been described. The more common subtype, seen in about 75% of patients, typically occurs 5–7 years after first exposure to alkylating agents or radiation, is often preceded by a myelodysplastic syndrome (MDS), and is frequently accompanied by clonal cytogenetic abnormalities such as the loss of all or part of chromosomes 5 or 7. Mutations of the P53 tumor suppressor gene are also common. The risk is related to total cumulative exposure over time to alkylating agents. In contrast, among individuals who develop t-AML after treatment with topoisomerase II inhibitors, the latency period to the development of t-AML is often only 1–3 years, antecedent MDS is rare, and gene rearrangements involving MLL at 11q23 or RUNX1/AML1 at 21q22 are common. It is now well recognized that APL and other subtypes of AML with balanced translocations sometimes occur as therapy-related myeloid neoplasms (t-MN) in patients who have previously received cytotoxic therapy or ionizing radiation therapy (RT). The most of this review will focus on these “good risk” leukemias, i.e. those with APL or inv(16)/t(16;16) or t(8;21).