This study investigated the dose difference (DD) in the surrounding dose area using the real-time motion tracking (RTMT) system in tomotherapy.

Seven stereotactic ablative body radiotherapy treatment plans with a single lesion were used for the investigations. Each treatment plan was evaluated for the Gamma passing rate (GPR) analysis in a static target motion using the ArcCHECK® phantom. Subsequently, each plan was matched with 8 clinical respiratory cycles to simulate moving target motion. The DD was calculated through point-to-point comparison and expressed as the frequency of the DD levels. The DD frequency was analysed for significant correlations with the target travelling distance, target size and respiratory frequency.

The GPR for criteria of 3%/2mm and 3%/3mm revealed values of 97·8 ± 1·9% and 99·5 ± 0·6%, respectively, for static motion. The highest frequency of DD was in the 5 – 10% range. A significant correlation was found between the target travelling distance and the frequency of percent DD at the 2·0 – 3·0% and 10·0 – 15·0% levels, as well as between target size and the frequency of percent DD at the 0·0 – 4·0% and 10·0 – 25·0% levels. Finally, no significant correlation was found between the frequency of percent DD and respiratory frequency.

RTMT introduced the DD in the surrounding treatment area. The DD was found to be up to 15·0% at 119·5 mm (water-equivalent distance) from the phantom centre. The DD was varied depending on the target travelling distance and size, but it did not depend on the respiratory frequency.