Impact of pulse forming network and injection current parameters on output and energy variations of helical tomotherapy

Purpose: In this study, an experiment was devised to establish the dependency of the impact ofpuheforming network (PFN) and injection current (IC) parameters on output and energy variations of helical tomotherapy (HT) on the radiation beam output and energy. Methods: HT has unique radiation beam characteristics due to the absence of a flattening filter. As with conventional linear accelerators, the machine output and energy should be within a ±2% tolerance according to published studies. However, because a dose servo is not utilized in a HT unit, these parameters may drift out of the ±2% tolerance due to various reasons such as high machine temperatures. With this in mind, physicists and engineers must adjust certain machine parameters to reset the output and energy to within the tolerance of the commissioned baseline. Two parameters commonly adjusted are: PFN voltage (V_PFN) and IC voltage (V_IC). Results: Results showed that the HT unit possesses different working zones defined by the V_PFN and V_IC parameter settings. The working zones were classified into 5 zones: 1) low dose rate zone - radiation dose rate much lower than nominal dose rate and machine cannot run due to low dose rate fault; 2) normal dose rate zone - dose rate is within tolerance of nominal dose rate and machine can run without dose rate fault; 3) dose rate failure during treatment zone - dose rate within the tolerance of the nominal dose rate however machine interrupts during treatment due to dose rate fault; 4) high dose rate zone - dose rate is higher than nominal dose rate and machine cannot run due to high dose rate fault; and 5) inoperable dose rate zone - dose rate is much higher than the nominal dose rate and machine cannot run. Conclusion: The results of this study may provide a quick guide for physicists to adjust their HT unit V_PFN and V_IC values in order to reset the radiation beam output and energy back to within the tolerance of the commissioned baseline.