Estimating Tumor/Non-Tumor Uptake from Radiolabeled Monoclonal Antibodies using Scintigraphic Images and Dissecting the Animal Models

Introduction: Biodistribution study in animal models bearing tumors is one of the most important procedures in evaluation of fractional uptake of radiopharmaceuticals in the tumor and non-tumor organs. The aim of this study was to develop a new software-based method to determine activities that accumulate in the main organs as well as the tumor based on scintigraphy images, thereby obviating the need to kill the animals. Material and Methods: The 99mTc labeled MAb PR81 was injected into 30 BALB/c mice with xenograft breast tumors. Anterior and posterior images of the mice were taken 16 hours post-injection using a two-headed gamma camera. The images were transferred to a PC after converting them into the interfile format. Then the anterior and posterior images of each mouse were conjugated using the designed software. Then the counts were obtained within the regions of interest drawn around each organ and the tumor, considering the calibration and background contribution. Subsequently, the mice were killed and the organs were dissected, weighed and counted individually using a well counter. Finally, the measurements obtained using the two methods were compared. Results: The comparison of the results obtained by the two procedures showed that there was a significant difference between them. However, there was a linear relationship between the counts measured by both methods for the main organs. Discussion and Conclusion: The results showed that although the new method cannot replace the invasive one to estimate the absolute activity of each organ (mainly due to the overlapping of the organs), it can be used to compare the relative activities of tumor and main organs for initial evaluation of the quality of biologic radiophamaceuticals in diagnosing tumors.