Room: AAPM ePoster Library
Purpose: The purpose of this study was to measure the magnitude and direction of patient shifts that occur from a mis-matched flat-top PET-CT simulation couch to first fraction of treatment. It is optimal for the CT simulation couch-top to match the couch-top used for radiation treatment. Understanding these innate errors will help improve tumor localization.
Methods: A flat-top PET-CT simulation couch must be manually placed on the curved CT couch prior to each CT simulation at a veterinary radiation oncology center; animals are subsequently treated on a Varian Exact Couch. Cone beam CTs are obtained prior to each treatment and positional shifts are recorded. Positional shifts on the first day of treatment were analyzed for oral (n=13) and anal/pelvic tumors (n=11). Average shifts on lateral, longitudinal, vertical, and coronal directions were calculated and compared.
Results: Patients with oral tumors required mean shifts of 2.31?0.57 mm laterally, 1.46?0.29 mm longitudinally, 1.00?0.61 mm vertically, and 1.15?0.33º coronally. Animals with anal/pelvic tumors required greater mean shifts of 2.18?0.58 mm laterally, 2.18?0.80 mm longitudinally, 3.00?0.86 mm vertically, and 0.93?0.33º coronally. Random errors were identified in all directions. Vertical shifts were significantly greater (p = 0.03) in the dorsal (posterior) direction between treatment sites. There may be a systematic vertical error for oral tumors (p=0.06).
Conclusion: Animals with oral tumors are more reliably positioned from CT simulation to first fraction treatment, likely due to a custom immobilization device secured to both flat-top couches. Wider range of shifts observed for anal/pelvic tumors may be due to larger field sizes, as well as lack of secured immobilization devices. Future work will expand tumor groups in order to identify treatment sites that may benefit from customized immobilization devices, thus improving reproducibility in target positioning between simulation and treatment.