Comparison of medium- and high-energy collimators for 131I-tositumomab dosimetry

Residence time measurements obtained by serial whole-body conjugate-view imaging are commonly used in patient-specific dosimetry for radioimmunotherapy applications. In order to determine the effect of collimator selection on residence time measurements for (131)I, the accuracies of (131)I half-life measurements obtained with multiple gamma-camera and collimator combinations were investigated.

Serial anterior and posterior whole-body images were acquired over a period of 15 d with 4 different gamma-cameras and medium- or high-energy collimators. Background-corrected geometric mean counts from the images were fitted to a monoexponential curve to determine the half-life of (131)I obtained with the different gamma-camera and collimator combinations.

An average half-life of 8.15 d (SD, 0.07 d) was obtained with all gamma-camera and collimator combinations. A half-life of 8.12 d (SD, 0.11 d) was obtained with the high-energy collimators, and a half-life of 8.18 d (SD, 0.04 d) was obtained with the medium-energy collimators. These values are all very close to the (131)I physical half-life of 8.02 d and were not found to be statistically significantly different (P = 0.44). Similar results were obtained for the half-life obtained with single-head gamma-camera configurations (mean half-life, 8.15 d; SD, 0.12 d). The therapeutic (131)I-tositumomab dose resulting from the differences in the measured half-life ranged from 2.58 to 2.6 GBq (69.8-70.4 mCi).

There is no significant difference in (131)I half-life and residence time measurements obtained with medium- or high-energy collimators in dual-head or single-head imaging configurations.