Evaluation of quantitative imaging methods for ibritumomab tiuxetan dosimetry using a population of phantoms having realistic variations in anatomy and uptake

Abstract

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Objectives: Estimating the organ residence times is an essential part of patient-specific dosimetry in targeted radionuclide therapy (TRT). Quantitative imaging methods for TRT are often evaluated using a single physical or simulated phantom but are applied to population of patients with variability in anatomy, biodistribution, and biokinetics. To provide a more relevant evaluation, we have thus developed a population of phantoms with realistic variations and applied it to the evaluation of quantitative methods both to find the best method and to demonstrate the effects these variations.

Methods: Using whole-body scans and SPECT/CT images, we measured the organ shapes and time-activity curves of In-111 ibritumomab tiuxetan in dosimetrically important organs in 10 patients undergoing a high dose therapy regimen. Based on these measurements, we created a 3D NCAT-based phantom population. SPECT and planar data at realistic count levels were then simulated using previously validated Monte Carlo simulation tools. The projections from the population were used to evaluate the accuracy of residence time estimation methods that used time series of SPECT and planar scans. Quantitative SPECT (QSPECT) reconstruction methods were used that compensated for attenuation, scatter and the collimator-detector response. Planar images were processed with a conventional (CPlanar) method that used geometric mean attenuation and triple-energy window scatter compensation and a quantitative planar (QPlanar) processing method that used model-based compensation for image degrading effects. Residence times were estimated from activity estimates made at each of five time points. We also evaluated hybrid methods that used CPlanar or QPlanar time activity curves rescaled to the activity estimated from a single QSPECT image. The methods were evaluated in terms of mean and standard deviation of the errors in the residence time estimates taken over the phantom population.

Results: The mean errors in the residence time estimates for all the organs were <4.7%, <13.8%, <6.0%, <15.1%, and 5-122% for pure QSPECT, pure QPlanar, hybrid QPlanar/QSPECT, hybrid CPlanar/QSPECT, and pure CPlanar, respectively. The standard deviations of the errors for all the organs over all the phantoms were <5.5%, <2.4%, <5.3%, < 24.1%, <86.5%, respectively.

Conclusions: The processing methods differed both in terms of their average accuracy and the variation of the accuracy over the population of phantoms. This demonstrates the importance of using phantom population in evaluating image processing methods. QPlanar or hybrid QPlanar/QSPECT method had means and standard deviations of accuracy that approached those of pure QSPECT while providing simplified image acquisition protocols and thus maybe more clinically practical.