(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: To introduce a novel method to measure the transient time and dose directly using a high frequency electrometer in GK Icon
Methods: A high frequency electrometer was used to collect current every 0.05 sec. A sample differential data is in Figure 1. A Matlab algorithm was written to identify the transient period. Transient dose is either the “4mm dose� or the shutter dose. The first one is unique to the 16mm collimator. It is the dose from the 4mm when the sectors move to the treatment position. The second one is the shutter dose. The up and down shutter dose are the dose for the source sectors moving from the parking position to the treatment position and back, respectively.Transient time is the time corresponding to the transient dose. To our knowledge it is the first time that the “4mm time� is measured directly. The up and down shutter time can also be measured from graph. The parameter ‘timer error’ has been traditionally calculated for the 16mm only using the integral dose method. This could be conceptually misleading because the dose is from the combined “4mm dose� and shutter dose, while the timer error for 8mm and 4mm includes shutter dose only. A large chamber could introduce the partial volume error when the 4mm dose is measured. We think the term shutter time should be used instead of the timer error. There is no more "error" after Gamma Plan has considered the shutter dose. Table 1 in supplement page shows that treatment time is the sum of time of up and down shutter time and dwell time.
Results: See supplement page.
Conclusion: Transient time can be measured directly using a high frequency electrometer. The terms “4mm dose� and “4mm time� are introduced. Term timer error could be ignored.
Keywords
Taxonomy
TH- Radiation dose measurement devices: ion chamber
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