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 VIC yields an average 1.4% increase in the average dose rate. Additionally, a 0.02 V increase in VPFN yields an average 1.0% increase in the average dose rate. A 1.0% increase in the VIC value yielded an average 0.3% decrease in the energy ratio. Furthermore, changes in the energy ratio were more dependent on VIC than VPFN based on the fact that only a 0.5% variation in energy was noted when varying the VPFN from 4.00 to 4.10 V while a 2.0% change was noted when varying the VIC from 3.5 to 3.7 V. Conclusions: In this study, several working zones based on the VPFN and VIC parameter setting were found to exist for a HT unit. Inside the normal dose rate zone, the output and energy vary linearly with VIC and VPFN parameter values. The results of this study may provide a quick guide for physicists to adjust their HT unit VPFN and VIC values in order to reset the radiation beam output and energy back to within the tolerance of the commissioned baseline.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging