SU-E-T-457: Influence of Changing Magnetron and Injector Current on the Beam Characteristics of a Tomotherapy Hi-Art SYSTEM

Purpose: This study investigates the influence of the magnetron and injector current on the beam characteristics of a Tomotherapy system. Methods: Treatment on tomotherapy is time‐based : the gantry rotates while the beam is on and all beam modulation is performed by the MLC. This implies that maintaining output stability is crucial. However, daily changes in output have been observed of up to 2.5% depending on external (pressure, temperature) or internal (component wear or instability). The output is tuned by adjusting the magnetron and injector current. This however, has an influence on both beam energy and profile but to which extent or magnitude has not been clear. Therefor, 144 measurements were performed by altering the magnetron current from 86,75A to 101,7A in 6 equal steps and by altering the injector current from 277,5mA to 612mA in 8 equal steps. Beam profile was measured using Tomodose (Sun Nuclear corp.). The Quality index was calculated using the absorbed dose ratio at depth 20cm and 10cm in rectangular slabs of solid water phantom with an SSD of 85cm. To estimate the influence on the cone profile the ratio between two volumes (on‐axis and off‐axis) was calculated. Results: With a magnetron current change of 14A the average quality index difference was 1.63%. Varying the injector current with 334.5mA causes the average quality index to change with 1.43%. Dose variation at 10cm from the central axis in lateral direction was 3% when changing the injector current by 334.5mA and 1.9% when changing the magnetron current by 14A Conclusions: Modifying magnetron and injector current does not have a major effect on the quality index. The lateral dose profile sagging is the most important finding since this will have an effect on off‐axis located targets. Injector current variation has more influence on the profile than magnetron current variation.