Megavoltage computed tomography (MVCT) dose assessment at different depths

The aim of the study is to evaluate the point doses measured by different parameters at various depths with MVCT in the Tomotherapy Hi- Art (HT) treatment unit. HT works in two modes: visual modes and therapy modes. The MVCT images are taken in the visual mode. The user can choose the scan length and image pitch value in the visual mode. The system has pitch values called fine, normal and coarse. When the same volume is scanned during the gentry rotation, the scan times of fine, normal, and coarse modes are different from one another. Cheese Phantom is used to evaluate the point doses. The measured values ranged from 0.64 to 2.67 cGy with an average dose of 1.40cGy. The lowest MVCT dose is found when 7 slices are scanned with a depth of 20 cm for 51 seconds. The highest MVCT dose is found when 17 slices are scanned with a depth of 15 cm for 101 seconds. The measured values are the highest when the fine mode is selected with low depth and high slice. The IGRT method is used before each treatment and can be used more than once if necessary. Therefore, the right mode selection can prevent taking unnecessary doses during MVCT.