Time dependence of the activity concentration ratio of red marrow to blood and implications for red marrow dosimetry

Abstract BACKGROUND The method for red marrow dosimetry in radioimmunotherapy, in the absence of specific activity uptake in red marrow, is based on the activity measured in the blood or plasma. The activity concentration ratio of red marrow to blood is then assumed to be constant. The aim of the current study was to determine whether this ratio varies with time after injection. METHODS Measurements were carried out with both animals and patients.Tumor‐bearing rats were intravenously injected with iodine‐131–, iodine‐125–, indium‐111–, or rhenium‐188–labeled BR96, a chimeric immunoglobulin G1 monoclonal antibody. (All were chelate‐labeled, except for iodine‐131, which was iodogen‐labeled.) Measurements were made of the activity concentration in blood and bone marrow at different points in time after injection, and the ratio of activity concentration in red marrow and blood as a function of time postinjection (RMBLR[t)]) was calculated. For patients treated with iodine‐131–labeled monoclonal antibody (LL2, Immunomedics Inc., Morris Plains, NJ; anti‐CD22; immunoglobulin G2 isotype of mouse origin), blood samples were drawn and scintillation camera images taken at different times after injection. The red marrow activity concentration in the sacrum was determined by activity quantification from regions of interest. The activity concentration in blood was also measured. The RMBLR(t) was calculated based on these data. RESULTS For both patients and rats, the RMBLR(t) was increased 72 hours after injection. Furthermore, it was found that the use of a constant RMBLR can lead to an over‐ or underestimation of the absorbed dose in bone marrow. CONCLUSIONS These data demonstrate the difficulty in using fixed values of the activity concentration ratio of red marrow to blood for dosimetry. Cancer 2002;94:1235–9. © 2002 American Cancer Society. DOI 10.1002/cncr.10291