Dosimetric analysis on the effect of tumor motion in IMRT for liver cancer: comparison of TomoTherapy and VMAT using the Delta 4 Hexa-Motion system

The internal target volume (ITV)-based IMRT method can be used to reduce the effect of respiratory tumor motion. In this study, the effect of tumor motion on the dosimetric accuracy during ITV-based IMRT was investigated. ITV-based IMRT plans for a total of ten cases of liver cancer were prepared in TomoTherapy (Tomo) and volumetric modulated arc therapy (VMAT). Delivery quality assurance (DQA) plans were created using Delta 4 phantom for the verification of dosimetric accuracy of the established Tomo and VMAT plans. DQA measurements were performed in both the static mode and the Hexa-Motion mode by using the Delta 4 Hexa-Motion system in Tomo and VMAT. In the static mode, the average gamma passing rate (GPR) using a 3% dose difference/3 mm distance-to-agreement criteria was 99.61% in Tomo and 99.35% in VMAT, which confirmed sufficient dosimetric accuracy in both IMRT methods. For the Hexa-Motion measurement, the average GRP result of 90.93% in Tomo was higher than the result of 82.58% obtained in VMAT. In addition, the difference in the GPR results between the static and the Hexa-motion modes in Tomo was 8.68%, which is approximately 8% less than the 16.77% difference in VMAT. This study verifies that the dosimetric error of ITV-based IMRT due to the tumor motion was larger in VMAT than in Tomo. Also, Tomo can be considered to be more suitable than the conventional LINAC-based VMAT method in terms of the dosimetric accuracy of ITV-based IMRT for the treatment of a moving tumor.