Tumor burden. (A) Sequential dosimetric studies illustrating the increase in effective half-life of ¹³¹I rituximab after prior rituximab and between the first and second dosimetric doses. All 5 patients who had not received prior rituximab demonstrate an increase in the mean effective half-life of ¹³¹I rituximab from 45 to 98 hours. After rituximab treatment, the majority of patients show no change in the effective half-life between the first and the second dosimetric study; however, 4 patients demonstrate a marked increase in the effective half-life between the first and the second infusion. (B) The effect of tumor burden on the effective half-life of ¹³¹I-rituximab. Patients with greater tumor burden demonstrate significant increases in effective half-life of ¹³¹I-rituximab between delivery of the first and the second fractions. Each data point represents values for an individual patient. As the splenic and lymph node volume increases, the relationship between the effective half-life of the first and second dosimetric doses of ¹³¹I-rituximab significantly alters. With volumes of tumor of less than 600 mL, the effective half-lives for the first and second tracer infusions of ¹³¹I-rituximab are similar. In contrast, for patients with larger volumes of tumor and those with volumes more than 600 mL, there appears to be a substantial increase in the effective half-life of ¹³¹I-rituximab between delivery of the first and second fractions (P = .007).