Purpose: To ascertain if PET image data of a positron tracer can be used for the quantitative description of dose distribution in support of direct prostate seed dosimetry. Materials and Methods: Simulated brachytherapy seeds were constructed containing trace amounts of a positron emitter, F-18, such that all annihilation events took place in the encapsulation wall. An acrylic prostate phantom containing these seeds was imaged with a GE Discovery ST PET/CT scanner in 2D and 3D acquisition modes and several image reconstruction methods. The PET scan data was used as the input for Monte Carlo calculation of dose distribution due to the F-18. This dose distribution was then compared to computations wherein the source was restricted to the encapsulation wall. This was done to determine if the measured data could be used to accurately compute the annihilation dose, which in turn would be used to compute the therapeutic dose due to known seed activity. Results: Examination of the dose distributions indicates a close agreement between the measured data and theoretical calculations for certain cases. We found that 2D acquisition with OSEM reconstruction resulted in a maximum difference in transaxial dose distribution of 15% in a single voxel, and a mean difference of 4% for the remaining voxels. However, the mean discrepancy between dose computations based on the ideal source versus PET based source is within or close to the Monte Carlo error of 2% to 4%. These results do not reflect any optimized acquisition protocol that may further reduce the observed differences. Conclusions: This work indicates there is potential for using PET data for the proposed link between the therapeutic brachytherapy dose and the dose due to a trace amount of encapsulated positron emitter, as developed by Sajo and Williams. Because this method does not require explicit information on seed locations, clinical implementation of this technique could significantly reduce the time needed for post-implant evaluation, and several of the uncertainties and limitations inherent in current prostate brachytherapy dosimetry
