Radioimmunotherapy (RIT) is a therapeutic modality indicated for the treatment of patients with relapsed or refractory low-grade, follicular, or transformed non-Hodgkin’s lymphoma (NHL), where the mechanism of action involves the intrinsic activity of the monoclonal antibody and the cytotoxic effects of radiation. The primary toxicity is a late-occurring, transient, and reversible myelosuppression. Because of concerns regarding the long-term effects of radiation on the bone marrow, we investigated the incidence of treatment-related myelodysplastic syndrome (t-MDS) and treatment-related acute myelogenous leukemia (t-AML) after ibritumomab tiuxetan RIT. A total of 746 patients with NHL were treated with the ibritumomab tiuxetan regimen in registration and compassionate-use trials between 1996 and 2002. Patients had a median age of 61 years (range, 24–87) and had received a median of 3 prior therapies (range, 0–9+). The crude incidence of t-MDS or t-AML was 2.3% (17/746), with an incidence of 4.7% (10/211) in patients enrolled in registration trials and of only 1.3% (7/535) in patients included in the compassionate-use trial, with a median follow-up of 5.7 and 3.5 years, respectively. These malignancies were documented at a median of 5.6 years (range, 1.2–13.9) after the diagnosis of NHL and 1.5 years (range, 0.1–5.8) after RIT. The annualized rates were 0.3% (95% CI, 0.2%–0.4%) a year after the diagnosis of NHL and 0.7% (95% CI, 0.4%–1.0%) a year after RIT. Cox multivariate regression analysis found that previous treatment with a purine nucleoside analog was a significant risk factor for t-MDS or t-AML (hazard ratio, 3.9 [95% CI, 1.5–10.4]; P = .006). All patients in whom t-MDS or t-AML developed and in whom cytogenetic data were available (n = 15) had multiple cytogenetic aberrations, commonly of chromosomes 5 and 7, suggesting an association with previous exposure to chemotherapy. There were documented bone marrow cytogenetic abnormalities before treatment in 2 patients who developed t-MDS or t-AML. These data suggest that the annualized incidence of t-MDS or t-AML following ibritumomab tiuxetan RIT is consistent with that expected on the basis of the patients’ history of treatment for NHL. Cytogenetic testing before administration of RIT may identify existing chromosomal abnormalities in previously treated patients, particularly in those who have been treated with alkylating agents and/or purine nucleoside analogs and who are thereby at a higher risk for t-MDS or t-AML.

Disclosures: MD, SR, KV, AM - Biogen Idec.; LG - Biogen Idec.; TW - Biogen Idec.; MSC, CE, TW - Speakers Bureau, Biogen Idec.

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