Predicting hematologic toxicity in patients undergoing radioimmunotherapy with ⁹⁰Y-ibritumomab tiuxetan or ¹³¹I-tositumomab

This study aimed at identifying clinical factors for predicting hematologic toxicity after radioimmunotherapy with ⁹⁰Y-ibritumomab tiuxetan or ¹³¹I-tositumomab in clinical practice. Methods: Hematologic data were available from 14 non-Hodgkin lymphoma patients treated with ⁹⁰Y-ibritumomab tiuxetan and 18 who received ¹³¹I- tositumomab. The percentage baseline at nadir and 4 wk post nadir and the time to nadir were selected as the toxicity indicators for both platelets and neutrophils. Multiple linear regression analysis was performed to identify significant predictors (P < 0.05) of each indicator. Results: For both platelets and neutrophils, pooled and separate analyses of ⁹⁰Y- ibritumomab tiuxetan and ¹³¹I-tositumomab data yielded the time elapsed since the last chemotherapy as the only significant predictor of the percentage baseline at nadir. The extent of bone marrow involvement was not a significant factor in this study, possibly because of the short time elapsed since the last chemotherapy of the 7 patients with bone marrow involvement. Because both treatments were designed to deliver a comparable bone marrow dose, this factor also was not significant. None of the 14 factors considered was predictive of the time to nadir. The R² value for the model predicting percentage baseline at nadir was 0.60 for platelets and 0.40 for neutrophils. This model predicted the platelet and neutrophil toxicity grade to within ±1 for 28 and 30 of the 32 patients, respectively. For the 7 patients predicted with grade I thrombocytopenia, 6 of whom had actual grade I-II, dosing might be increased to improve treatment efficacy. Conclusion: The elapsed time since the last chemotherapy can be used to predict hematologic toxicity and customize the current dosing method in radioimmunotherapy.