SU‐FF‐T‐250: Impact of Pulse Forming Network (PFN) and Injection Current (IC) Parameters On Output and Energy Variations of Helical TomoTherapy

Purpose: To quantity evaluate the impact of pulse forming network (PFN) and injection current (IC) parameters on the output and energy variations of a helical TomoTherapy (HT) unit. Method and Materials: Tomotherapy quality assurance (TQA™) platform, especially the step wedge phantom and step wedge static module, was used for the whole measurement. PFN and IC voltage values were coarsely sampled from 3.0 to 5.0 V in 0.2 V increments, and finely sampled ([formula omitted] and [formula omitted]) in 0.02 V increments. Results: Five working zones were found for different combination of PFN and IC voltage values: low dose rate zone, normal dose rate zone, dose rate failure during treatment zone, high dose rate zone, inoperable dose rate zone. It was noted that a 1.0% increase in V_IC yields an average 1.4% increase in the average dose rate. Additionally, a 0.02 V increase in V_PFN yields an average 1.0% increase in the average dose rate. A 1.0% increase in the V_IC value yielded an average 0.3% decrease in the energy ratio. Furthermore, changes in the energy ratio were more dependent on V_IC than V_PFN based on the fact that only a 0.5% variation in energy was noted when varying the V_PFN from 4.00 to 4.10 V while a 2.0% change was noted when varying the V_IC from 3.5 to 3.7 V. Conclusions: In this study, several working zones based on the V_PFN and V_IC parameter setting were found to exist for a HT unit. Inside the normal dose rate zone, the output and energy vary linearly with V_IC and V_PFN parameter values. 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.