A modification of conventional PET/CT technique for radiotherapy planning

The definition of the tissue volume to be irradiated is the mos important task in radiotherapy planning. A PET/T fusion images allows the identification of the tissue with increased metabolic activity, thus enhancing the precise definition of the limits of the volume to be treated. To minimize errors that may occur by fusing the PET images with those of CT, some modifications were included to the original acquisition protocol which permitted the convenient utilization of PET/CT technique for the purposes of radiotherapy planning. The new protocol added one transmission tomographic sequence before conventional PET/CT, to check the position of the locators previously set for radiotherapy simulation, and a second multislice sequence after PET/CT for the purpose of reconstruction in the radiotherapy planning workstation. This second sequence is the one effectively used for the dosimetry plans. From August 2004 through November 2005 88 radiotherapy planning were performed at the PET/CT center according to the above described protocol. The mechanical axial alignment of the two imaging procedures this achieved, resulted in near perfect image fusion in 99% of the cases and improved the restriction of the treatment to the effective neoplasic tissue. (author)