A comparison of 2D and 3D regions within the same patient to derive organ and tissue kinetics

Objectives Measurement of tissue and organ kinetics using scintigraphic image data following sub-therapeutic administration of a radiopharmaceutical is commonly used to derive estimates of radiation absorbed dose to critical organs at higher therapeutic levels. Underestimation of therapeutic doses could result in unexpected radiation toxicities while overestimation may lead to sub-therapeutic doses delivered to target lesions. The use of 2D Regions of Interest (ROIs) and 3D Volumes of Interest (VOIs) was investigated within the same subject to understand the differences between these two techniques and their influence on the calculated area under the curve for the lungs, liver, kidney and whole body.

Methods Serial PET/CT images were acquired approximately 1hr, 4hr, 20hr, 72hr and 93hr following the administration of 124I labeled MIP-1095, an investigational agent targeting PSMA in a patient with prostate cancer. For each time point, 2D ROIs were used to extract counts from summed coronal slices while 3D VOIs were created from summed ROIs within axial slices. Count data were generated for ROIs and VOIs of the whole-body, liver, kidneys and lungs and numerically integrated using a simple trapezoidal rule over the interval of 0-93 hrs post-injection.

Results The 2D technique demonstrated an increased area under the curve (AUC) for all organs when compared to the 3D method. The calculated 2D AUC for kidneys were greater by (77-222%), lungs (71-79%), liver (45%), and whole-body (12%).

Conclusions 2D ROIs produced a larger AUC than that of 3D VOI techniques. These data suggest that conventional whole-body planar conjugate view methods for measuring organ and tissue kinetics with 2D ROIs may lead to an overestimation of radiation dose to normal organs with overlapping structures. Overestimation could necessitate a reduction of the administered dose of a radiotherapeutic to be within tolerance limits such as those published by Emami (1991) and subsequently deliver a lower than expected dose to tumors