Accuracy of I-131 tumor quantification in radioimmunotherapy using SPECT imaging with an ultra-high-energy collimator: Monte Carlo study

Accuracy of I-131 tumor quantification after radioimmunotherapy (RIT) was i nvestigated for SPECT imaging with an ultra-high-energy (UHE) collimator de signed for imaging 511-keV photons, Methods: First, measurements and Monte Carlo simulations were carried out to compare the UHE collimator with a con ventionally used, high-energy collimator. On the basis of this comparison. the UHE collimator was selected for this investigation, which was carried o ut by simulation of spherical tumors in a phantom. Reconstruction was by an expectation-maximization algorithm that included scatter and attenuation c orrection. Keeping the tumor activity constant. simulations were carried ou t to assess how volume-of-interest (VOI) counts vary with background activi ty, radius of rotation (ROR), tumor location, and size. The constant calibr ation factor for quantification was determined from VOI counts correspondin g to a 3.63-cm-radius sphere of known activity. Tight VOls corresponding to the physical size of the spheres or tumors were used. Results: Use of the UHE collimator resulted in a large reduction in I-131 penetration, which is especially significant in RIT where background uptake is high. With the UH E collimator, typical patient images showed an improvement in contrast. Con sidering the desired geometric events, sensitivity was reduced, but only by a factor of 1.6. Simulation results for a 3.63-cm-radius tumor showed that VOI counts vary with background, location, and ROR by less than 3.2%, 3%, and 5.3%, respectively. The variation with tumor size was more significant and was a function of the background. Good quantification accuracy (<6.5% e rror) was achieved when tumor size was the same as the sphere size used in the calibration, irrespective of the other parameters. For smaller tumors, activities were underestimated by up to -15% for the 2.88-cm-radius sphere, -23% for the 2.29-cm-radius sphere, and -47% for the 1.68-cm-radius sphere . Conclusion: Reasonable accuracy can be achieved for VOI quantification of I-131 using SPECT with an UHE collimator and a constant calibration factor . Difference in tumor size relative to the size of the calibration sphere h ad the biggest effect on accuracy, and recovery coefficients are needed to improve quantification of small tumors.