67 Australasian Physical & Engineering Sciences in Medicine Volume 25 Number 2, 2002 TECHNICAL REPORT Display of positron emission tomography with Cadplan* T. Ackerly, J. Andrews, D. Ball, D. Binns, R. Clark, I. D’Costa, R.J. Hicks, M. Kenny, E. Lau, M. MacManus, and G. Song. Peter MacCallum Cancer Institute, Melbourne Abstract Recent clinical experience at Peter MacCallum Cancer Institute 1 (PMCI) with the use of unregistered Positron Emission Tomography (PET) images for radiotherapy target marking in the lung suggests that co-registered PET images would be invaluable 20 . PMCI has three radiotherapy treatment planning systems 2,3,4 but none of them currently is able to display or co-register PET images with Computed Tomography (CT) images. This paper details the approach taken to display co-registered PET images with the CADPLAN 3 treatment planning system. CT Image files are normally transferred to Cadplan by DICOM 5 transfer, but the Cadplan DICOM server will not receive (has no presentation context for) PET images. The fundamental design of the CADPLAN system envisages display of only a single image dataset, which must be a CT scan for planning reasons. The problem of data transfer is crudely solved by File Transfer Protocol 6 (FTP) over the network. Fortunately the multislice format of the PET image files 7 makes individual transfer manageable. A menu based C 8 program running at the same time as Cadplan is invoked to sample the DICOM PET Image and create multiple Cadplan CART 9 image format files that are co-registered with each existing transverse CT slice. With the Cadplan in contour mode, the program allows the co-registered PET images to be swapped in and out of the image section of the CART files promptly, while keeping the contour information. This allows radiotherapy target volumes to be marked using transverse PET emission images, and effectively circumvents the design constraints prohibiting the display of more than one image set. Contours can be over-laid for review on reconstructed sagittal or coronal views of CT or PET images constructed using the standard Cadplan tools. Co-registration is facilitated by identical positioning with the aid of lasers and 18 FDG loaded fiducial markers on the PET scanner and CT couch. A polyurethane cast fixed with EFFILOCK is used to ensure identical patient orientation on the CT and PET couches. Since both imaging modalities are without significant geometric distortion 10, 11 the co-registration is then simply a translation. PET transmission images can be used for co-registration verification. The practical implementation of display of PET images with CADPLAN has enabled us to begin a trial of 10 patients, the results of which will be reported separately. Key words PET, Co-registration, radiotherapy Introduction The Peter MacCallum Cancer Institute (PMCI) is a comprehensive cancer centre with a total of ten linear accelerators for radiotherapy treatment and treated 5,688 radiotherapy courses in the year 2000/2001 12 . Positron Emission Tomography (PET) is carried out at the on-site PET centre, which is equipped with 2 PET cameras 13 and a cyclotron. The main isotope in use for radiotherapy planning purposes is 18 FDG 14 . The radiotherapy treatment unit has a dedicated CT scanner 15 for radiotherapy treatment planning purposes. * Presented at EPSM 2001, Fremantle 30/9-4/10/2001. Corresponding author: T. Ackerly, Physical Sciences Department, Peter MacCallum Cancer Institute Locked Bag 1, A'Beckett Street, Melbourne 8006, Tel: (03) 9656 1260 Fax: (03) 9650 4870, Email: tackerly@petermac.unimelb.edu.au Received: 5 February, 2002; Accepted: 24 May, 2002 Motivation ICRU 62 16 identifies the marking of target contours from CT scans as the largest contributor to the global uncertainty of radiation therapy treatment planning, drawing primarily on the work of Leunens 17 with regard to neurologic cases. Difficulty marking target volumes from CT scans is also a problem in lung cancer. 18, 19 PET has been in use at PMCI since 1996. It has been used as a clinical staging tool, and patient management has been influenced by PET study results. In early 2001, the radiotherapy department held a meeting in the anticipation of the imminent release of a system to enable importation and co-registration of PET image data into a 3D radiotherapy treatment planning system. The methodology proposed was to use a simple system of fiducials. It was intended to duplicate the patient position at the CT scanner and the PET scanner. In this way the problem of co- registration would be reduced to selecting the appropriate translation from one coordinate system to another. There was a preference to use Cadplan, as the most likely site was lung 20 and the computer routinely used for