Objective:Estimation of activity accumulated in tumor and organs is very important in predicting the response of radiopharmaceuticals treatment.In this study,we synthesized ~(177)Lutetium(~(177)Lu)-trastuzumabiron oxide nanoparticles as a double radiopharmaceutical agent for treatment and better estimation of organ activity in a new way by magnetic resonance imaging(MRI).Methods:~(177)Lu-trastuzumab-iron oxide nanoparticles were synthesized and all the quality control tests such as labeling yield,nanoparticle size determination,stability in buffer and blood serum up to 4 d,immunoreactivity and biodistribution in normal mice were determined.In mice bearing breast tumor,liver and tumor activities were calculated with three methods:single photon emission computed tomography(SPECT),MRI and organ extraction,which were compared with each other.Results:The good results of quality control tests(labeling yield:61%±2%,mean nanoparticle hydrodynamic size:41±15 nm,stability in buffer:86%±5%,stability in blood serum:80%±3%,immunoreactivity:80%±2%) indicated that ~(177)Lu-trastuzumab-iron oxide nanoparticles could be used as a double radiopharmaceutical agent in mice bearing tumor.Results showed that ~(177)Lu-trastuzumab-iron oxide nanoparticles with MRI had the ability to measure organ activities more accurate than SPECT.Conclusions:Co-conjugating radiopharmaceutical to MRI contrast agents such as iron oxide nanoparticles may be a good way for better dosimetry in nuclear medicine treatment. Objective: Estimation of activity in tumor and organs is very important in predicting the response of radiopharmaceuticals treatment. In this study, we synthesized ~ (177) Lutetium (~ (177) Lu) -trastuzumabiron oxide nanoparticles as a double radiopharmaceutical agent for treatment and better estimation of organ activity in a new way by magnetic resonance imaging (MRI). Methods: ~ (177) Lu-trastuzumab-iron oxide nanoparticles were synthesized and all the quality control tests such as labeling yield, nanoparticle size determination, stability in buffer and blood serum up to 4 days, immunoreactivity and biodistribution in normal mice were determined.In mice bearing breast tumor, liver and tumor activities were calculated with three methods: single photon emission computed tomography (SPECT), MRI and organ extraction, which were compared with each other. Results: The good results of quality control tests (labeling yield: 61% ± 2%, mean nanoparticle hydrodynamic size: 41 ± 15 nm, stability in buffer: 86% ± 5%, sta (177) Lu-trastuzumab-iron oxide nanoparticles could be used as a double radiopharmaceutical agent in mice bearing tumor. Results showed that ~ ( 177) Lu-trastuzumab-iron oxide nanoparticles with MRI had the ability to measure organ activities more accurate than SPECT. Conclusions: Co-conjugating radiopharmaceutical to MRI contrast agents such as iron oxide nanoparticles may be a good way for better dosimetry in nuclear medicine treatment .