With radioimmunotherapy, as with other forms of radiation, there is a delicate balance between not enough (inadequate treatment) and too much (excessive toxicity). Complicating the issue are the differences in the metabolism of antibodies from one patient to another. As a consequence, determining the dose of some of these products is complicated.

The dose-finding procedure for ⁹⁰Y-ibritumomab is straightforward. The patient receives a tracer dose of the monoclonal antibody linked to 111In and then undergoes a series of gamma camera studies to confirm that both the distribution of the antibody and its half-life are as expected. The ⁹⁰Y-tagged antibody is then given in a dose based on the patient's body weight.

The procedure for some agents is more complex. For one ¹³¹I-tagged antibody, a tracer dose of the antibody with lower radioactivity is injected, and images are obtained. The rate of antibody clearance is then measured, and the effective half-life is determined. This value is used to calculate the amount of radioactivity that will be required to deliver the desired dose. n

BIBLIOGRAPHY

DeNardo GL, Juweid ME, White CA, Wiseman GA, DeNardo SJ. Role of radiation dosimetry in radioimmunotherapy planning and treatment dosing. Crit Rev Oncol Hematol. 2001;39:203-218.

Wahl RL, Zasadny KR. Method of establishing the optimal radiation dose. US Patent 6,251,362. Issued June 26, 2001.

Judith Gunn Bronson, MS, is a contributing writer for Decisions in Imaging Economics.