Estimation of tumor activities from simulated I-123 SPECT studies

Dosimetry estimates for radioimmunotherapy can be improved with a better knowledge of the spatial distribution of radiolabeled antibody uptake within tumors. A method for estimating intratumor activity from quantitative filtered backprojection SPECT reconstructions has been developed and evaluated using Monte Carlo simulated I-123 studies. Tumor activity in a SPECT image is modeled as a blurred version of uniform activity in concentric spherical shells. The shell activities are estimated by a least squares procedure using an estimate of the blurring function. This method was evaluated for 1.25 and 2.9 cm radius tumors which were uniform or consisted of a lower activity core and a higher activity shell. The tumors were located in a water-filled cylinder with shell:background and shell:core activity ratios of 4:1. Projection data at clinical count levels were simulated and images were reconstructed by filtered backprojection with compensations for attenuation, scatter and detector response. For the 2.0 cm radius tumors, shell activity is recovered to within about 25% and core activity to within about 50%. For the 1.25 cm radius tumors, shell activity is estimated to within about 50% but core activity estimates are unreliable. Factors contributing to errors in intratumor activity estimates are limited SPECT resolution, image noise due to limited projection counts, and the anisotropic and spatially-varying character of the SPECT point source response function