Dosimetric impact of correcting count losses due to deadtime in clinical radioimmunotherapy trials involving iodine-131 scintigraphy.

This study describes the use of a new method for correcting count losses due to deadtime in the context of quantitative imaging of patients undergoing scintigraphy after a 4 GBq therapeutic injection of iodine-131. This method, based on measuring the count rate observed throughout the spectrum detected (50-750 keV), had been validated in a previous study and was applied here to 10 patients. Imaging was performed 3, 6, 8 and 10 days after injection. Whole-body images were acquired in six steps in energy-indexed list mode. Before reconstruction of the whole-body image, each step was processed to obtain an appropriate correction. Three days after injection, corrective factors ranged between 1.01 (feet) and 1.20 (liver), and the increase in whole-body activity was estimated at around 10%. The difference between whole-body activities calculated from images corrected for deadtime and those estimated by urine collection was around 1% when urine collection was complete. Correction for count losses led to an 11% increase in whole-body cumulated activity. These results indicate that it is possible to integrate this correction into dosimetric studies in order to allow count rate variations to be taken into account as a function of the regions imaged. Although the complexity of acquisitions in energy-indexed list mode limits the systematic use of this method, it can be simplified if corrections are made only for those steps in which the correction factor exceeds a threshold value. However, this implies a selection of the regions to be corrected. Another possibility consists in acquiring spectrometric images in several windows, which also allows correction for count losses.