Home > Archives > Additional Archives > Yuni Dewaraja   Yuni Dewaraja Department of Nuclear Medicine Universiry of Michigan yuni@umich.edu http://www.rad.med.umich.edu/   Abstract: Monte Carlo Simulation in Nuclear Medicine Imaging Radioimmunotherapy (RIT) in the treatment of cancers involve the use of radiolabeled antibodies for the systemic delivery of targeted radiation to areas of disease while sparing normal tissue. Some therapeutic isotopes used in RIT such as I-131produce both the beta particles that sterilize the tumor as well as gamma photons that can be used to image the radioactivity distribution. I-131 labeled RIT is showing great promise in the treatment of non-Hodgkin's lymphoma, which is the fifth leading cause of cancer morbidity and mortality in the US. Here at the University of Michigan, a phase II I-131 RIT study of patients with malignant follicular lymphoma showed that, out of 76 patients with no previous treatment, 48 achieved a complete response and 26 achieved a partial response. Recently I-131 labeled MIBG has shown promise in the treatment of metastatic neuroblastoma in children who have a poor long-term survival. The success of RIT and MIBG therapy at our University as well as at other institutions has renewed interest in quantitative I-131 imaging for accurate internal radiation absorbed dose estimates to determine dose-response relationships. Benefits of early and reliable prediction of response from dosimetry eventually include 1) early planning of subsequent therapy when needed, and 2) ability to predict which patients to treat based on a diagnostic infusion. Additionally, accurate non-target dose estimation will lead to accurate determination of the maximum activity that can be administered while avoiding critical organ toxicity. Single Photon Emission Computed Tomography (SPECT) using a rotating gamma camera is the preferred imaging modality for activity quantification in I-131 radionuclide therapy. SPECT quantification accuracy is affected by many factors, including photon attenuation, photon scatter and finite spatial resolution of the system. Monte Carlo is a powerful tool for investigating imaging characteristics and limitations of SPECT because unlike in experimental measurement, in simulation the details of each photon history are known. For example, Monte Carlo is often used to investigate compensation methods for scattering of gamma rays within patients since such events can be tracked separately in the simulation, which is impossible to do experimentally. One limitation to using Monte Carlo for I-131 SPECT studies is the long computational times involved in carrying out accurate simulations. Therefore, recently we implemented the SIMIND Monte Carlo code for nuclear medicine imaging applications on the IBM SP2 distributed memory parallel computer. The demonstrated speed-up made it feasible for us to carry out accurate and clinically realistic SPECT simulations using anthropomorphic digital phantoms. In the future we expect to carry out patient-specific simulations on the new IBM SP at SDSC (Blue Horizon) utilizing hundreds of processors. We have used the parallel SIMIND code to evaluate correction techniques for photon attenuation and scatter in the patient as well as for evaluating a new reconstruction algorithm that compensates for the depth dependent camera response function. These Monte Carlo studies are allowing us to evaluate and improve the I-131 SPECT imaging and activity quantification techniques used with RIT patients at our clinic. This work is funded by the National Cancer Institute, DHHS.
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