Commissioning the CT part of a PET-CT scanner for use as a radiotherapy CT scanner

Abstract

The Siemens Biograph mCt PET-CT scanner in the Paul Strickland Scanner Centre, Mount Vernon Hospital is currently in use as a diagnostic scanner. This project involved commissioning the CT for use as a scanner for the adjoining radiotherapy department. External lasers and a carbon fibre flat couch top were procured, matching the set-up on the radiotherapy department. Couch deflection was tested. In conjunction with the Radiation Protection department it was decided that a new set of radiotherapy-specific scanning protocols needed to be created. The image quality, HU calibration and CTDIvol from the current radiotherapy scanners were used as guidelines. Specific parameters, which include kV and CARE kV, HD FoV, SAFIRE (Sinogram Affirmed Iterative Reconstruction), mAs and CARE Dose, patient size and position within the FoV, and reconstruction kernel, were examined using the CIRS CT Density phantom model 062, and image quality statistics were produced. The effect on the Hounsfield Units of the above parameters was analysed, and the consequential effects on MV dosimetry considered. From this a radiotherapy pelvis protocol was created that was then tested, in terms of patient size, SAFIRE, HD FoV and kV. Once this protocol was finalised, the same approach was rolled out to the rest of the radiotherapy protocols and a full list of radiotherapy protocols is now available on the Biograph. A HU-density curve was then created for the department's two treatment planning systems, ADAC Pinnacle (version 9.2) and Varian Eclipse (version 11). Finally a full system check was performed.