Patient-specific whole-body dosimetry : Principles and a simplified method for clinical implementation

The purpose of this article is to describe the methods used to determine the precise, patient-specific dose (in mCi) of 131I anti-B1 antibody needed to deliver a specified whole-body radiation dose (generally 75 cGy) to patients being treated for non-Hodgkin's lymphoma. The general principles, as well as simplified methods for dosimetry calculation, are described, and a "worked" example is provided. The approach described involves administration of a tracer dose that is used to determine the patient-specific whole-body residence time. The total-body radiation dose per unit of cumulated activity is calculated using an ellipsoid model of the human body, with an assumption of uniform distribution of radioactivity. Together, they determine the appropriate patient-specific therapeutic dose of radiolabeled antibody needed to deliver a specified total-body dose of radiation. Because a monoexponential whole-body effective half-time adequately describes the total-body kinetics, the patient's whole-body residence time is estimated from three whole-body radioactivity observations obtained over 6-7 days. Whole-body counts can be collected using either a thyroid probe system or a whole-body gamma camera. The residence time, in conjunction with the patient's weight, determines the desired therapeutic activity. Dose attenuations are instituted for obesity and reduced blood platelet counts. Methods for determining total-body dose of radiation, including a simplified dosimetric method based on just three data points, are described. Tables and graphs to allow such calculations are presented for radioimmunotherapy with 131I anti-B1 antibody, although they are likely to be useful in other forms of radiopharmaceutical therapy as well.