The impact of the offset distance between the planning target volume and isocenter on irradiation time in TomoTherapy: A phantom study

Purpose: The influence of the offset distance from treatment target to gantry isocenter (GIC) on the dosimetric parameter and irradiation time was investigated using TomoTherapy Methods: The reference position was defined as the centers of both the I'mRT phantom and planning target volume (PTV) with a spherical of 4 cm diameter aligned with the GIC. The dose calculations were performed in two offset methods with 2 and 12 Gy/fr, Method 1. The PTV was moved from 0.0 to 12.5 cm along the RL direction and - 5.0 to 5.0 cm along the AP direction (PTV offset), Method 2. The phantom was moved from 0.0 to - 7.5 cm along the RL direction and - 5.0 to 5.0 cm along the AP direction (Phantom offset). The maximum, minimum and mean doses, homogeneity index, conformity index, irradiation time, and monitor unit were compared. Results: The irradiation times increased with increasing PTV offset. The increases in the irradiation time were 54.4% and 40.8% at PTV offsets of 12.5 cm along the RL direction for 2 and 12 Gy/fr, while the increases were 20.1% and 15.0% at a PTV offset of 5.0 cm along the AP direction. An increased irradiation time was not observed for the phantom offset. The offset didn't affect the other parameters. Conclusions: The PTV location offset of >= 5 cm from the GIC along the RL and AP axes increased the irradiation time; therefore, the PTV should be aligned with the GIC as much as possible to reduce the irradiation time on TomoTherapy.