To determine whether the therapeutic effect of 90Y-ibritumomab might be enhanced by a full course of rituximab followed by single dose of 90Y-ibritumomab, the trial included pre- and post-rituximab treatment imaging with 111In-ibritumomab and blood pharmacokinetics. Comparison of the pre- and post-rituximab imaging and blood data allowed for the assessment of impact of rituximab on 90Y-ibritumomab dosimetry. Methods: Seventeen patients with relapsed B cell non-Hodgkin lymphoma first received 250 mg/m2 of rituximab plus 185 MBq of 111In- ibritumomab for initial dosimetry evaluation. In weeks 2-4, patients received 3 weekly 375 mg/m2 doses of rituximab. In week 6, patients received a 250 mg/m2 dose of rituximab plus 185 MBq of 111In- ibritumomab for a second dosimetry evaluation. Five sequential, whole-body g-camera images were acquired after the 111In-ibritumomab injection. Calculated radiation doses were based on patient-specific organ masses. For each patient, paired comparison of radiation doses before and after rituximab treatment was performed. Paired comparison of residence times for spleen and tumor was also performed. Results: Before rituximab treatment, the median radiation dose (mGy/MBq) was 0.48 (range, 0.24-0.86) for total body, 3.7 (range, 2.1-11.6) for liver, 6.1 (range, 1.8-17.8) for spleen, 3.3 (range, 2.0-4.7) for kidneys, 2.4 (range, 1.3-3.7) for heart wall, 1.1 (range, 0.4-2.3) for lungs, 0.79 (range, 0.32-1.22) for marrow from blood, and 18.1 (range, 4.7-98.9) for tumor. Paired comparisons were performed in 16 patients only because human antimurine antibody developed in 1 patient. The median change was 0.007 mGy/MBq for body, 20.14 mGy/MBq for liver, 20.31 mGy/MBq for kidneys, 0.38 mGy/MBq for heart wall, 20.17 mGy/MBq for lungs, and 0.046 mGy/MBq for marrow from blood. The median change in residence time was 20.92 h for spleen and 20.24 h for tumor. The changes were statistically insignificant for total body, liver, kidneys, lungs, and marrow from blood. The median residence times, or mGy/MBq if there were no volume changes, decreased 24% for spleen (P 5 0.0005) and 28% for tumor (P 5 0.005). The median radiation dose to heart wall increased 16%, which was statistically significant (P 5 0.002). Conclusion: Changes in 90Y-ibritumomab dosimetry after 4 wk of rituximab treatment were not significant for most organs, except for the heart wall. The reduction of spleen and tumor residence times is more likely to be due to the therapeutic effects of rituximab. Copyright © 2010 by the Society of Nuclear Medicine, Inc.