SU‐C‐201‐06: Utility of Quantitative 3D SPECT/CT Imaging in Patient Specific Internal Dosimetry of 153‐Samarium with GATE Monte Carlo Package

Purpose: Patient‐specific 3‐dimensional (3D) internal dosimetry in targeted radionuclide therapy is essential for efficient treatment. Two major steps to achieve reliable results are: 1) generating quantitative 3D images of radionuclide distribution and attenuation coefficients and 2) using a reliable method for dose calculation based on activity and attenuation map. In this research, internal dosimetry for 153‐Samarium (153‐Sm) was done by SPECT‐CT images coupled GATE Monte Carlo package for internal dosimetry. Methods: A 50 years old woman with bone metastases from breast cancer was prescribed 153‐Sm treatment (Gamma: 103keV and beta: 0.81MeV). A SPECT/CT scan was performed with the Siemens Simbia_T scanner. SPECT and CT images were registered using default registration software. SPECT quantification was achieved by compensating for all image degrading factors including body attenuation, Compton scattering and collimator‐detector response (CDR). Triple energy window method was used to estimate and eliminate the scattered photons. Iterative ordered‐subsets expectation maximization (OSEM) with correction for attenuation and distance‐dependent CDR was used for image reconstruction. Bilinear energy mapping is used to convert Hounsfield units in CT image to attenuation map. Organ borders were defined by the itk‐SNAP toolkit segmentation on CT image. GATE was then used for internal dose calculation. The Specific Absorbed Fractions (SAFs) and S‐values were reported as MIRD schema. Results: The results showed that the largest SAFs and S‐values are in osseous organs as expected. S‐value for lung is the highest after spine that can be important in 153‐Sm therapy. Conclusion: We presented the utility of SPECT‐CT images and Monte Carlo for patient‐specific dosimetry as a reliable and accurate method. It has several advantages over template‐based methods or simplified dose estimation methods. With advent of high speed computers, Monte Carlo can be used for treatment planning on a day to day basis.