Monte Carlo evaluation of object shape effects in iodine-131 SPET tumor activity quantification

In our clinical iodine-131 single-photon emission tomography (SPET) quantification for radioimmunotherapy, calibration and partial volume correction are based on measurements with phantoms containing spheres to simulate patient tumors even though real tumors are frequently nonspherical. In this study, Monte Carlo simulation was used to evaluate how object shape influences "spill-out" and "spill-in", which are major sources of quantification error associated with the poor spatial resolution of 131I SPET. Objects that varied in shape (spheres, cylinders, and an irregular structure) but were identical in activity and volume were simulated. Iterative reconstruction employed both attenuation and triple-energy-window scatter compensation. VOIs were defined in the reconstructed images both using physical boundaries and using expanded boundaries to allow for the limited resolution. When physical boundaries were used, both spill-out and spill-in were more significant for nonspherical structures than for spherical structures. Over the range of object volumes (50-200 ml) and at all background levels, VOI counts in cylinders were lower than VOI counts in spheres. This underestimation increased with decrease in object size (for the cold background -18% at 200 ml and -39% at 50 ml). It also decreased with increase in background activity because spill-in partially compensated for spill-out. It was shown that with a VOI larger than physical size, the results are independent of object shape and size only in the case of cold background. Activity quantification was carried out using a procedure similar to that used in our clinic. Quantification of nonspherical objects was improved by simple sphere-based partial volume correction, but the error was still large in some cases (for example, -39% for a 50-ml cylinder in a cold background and -35% for a 200-ml irregular structure defined on the basis of a typical tumor outlined on an X-ray computed tomography scan of a patient with non-Hodgkin's lymphoma). Partial volume correction by patient-specific Monte Carlo simulation may provide better quantification accuracy.