PREDICTION OF RADIATION-DOSES FROM THERAPY USING TRACER STUDIES WITH IODINE-131-LABELED ANTIBODIES

Tracer pharmacokinetic studies are often used in treatment planning fo r radionuclide therapy including radioimmunotherapy. This study evalua tes the validity of using tracer studies to predict radiation doses fr om therapy with the same radiolabeled antibody. Methods: Quantitative imaging and blood radioactivity were used to obtain the pharmacokineti cs and radiation doses that were delivered to the total body, blood, m arrow, lungs, liver, kidneys, thyroid, spleen and tumors. Tracer and t herapy data for eight patients with lymphoma and one patient with brea st cancer were compared using linear regression statistics. Doses of I -131-labeled antibody for the tracer studies ranged from 0.1 to 0.4 GB q (2 to 10 mCi), and therapy doses ranged from 0.7 to 5.6 GBq (20 to 1 50 mCi). Results: Radiation doses to tissues and, in particular, the b one marrow and tumors were reliably predicted from tracer studies. In this group of patients, median dose to marrow from marrow targeting, t otal body and blood was 9.2 cGy/GBq for tracer studies and 7.6 cGy/GBq for therapy studies with a median difference of 0.5 cGy/GBq. Median d ose to tumors was 81.1 cGy/GBq for tracer studies and 70.3 cGy/GBq for therapy studies with a median difference of 5.9 cGy/GBq. Conclusion: In these patients, tracer studies were predictive of the radiation dos es from therapy for total body, major organs and tumors. The radiation doses to marrow and tumors, which are the usual determinants of the t herapeutic index, correlated well between tracer and therapy studies ( r greater than or equal to 0.95).