(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose:
To investigate the in-field dosimetric fluctuation in the 45MV photon beam generated from a scanning beam accelerator (LA45 Racetrack Microtron).
Methods:
Two ion chambers were used to simultaneously measure the in-air and in-water (10cm depth) ionization along central axis in the 45MV photon beam (SSD=100cm, field size 20x20cm², dose rate 300MU/min and scanning period 0.2s/cycle). The chamber readings were taken with three different integration periods (1s, 4s and 8s) and were each repeated by 20 times during continuous beam delivery. The ratios of the two chamber readings were calculated, as well as the relative standard deviations for each group. The above measurements and analyses were repeated with the chamber in water shifted to different off-axis distances (OAD) at 2.5cm, 5cm, 7.5cm and 10cm, respectively. In addition, a 2D array detector (MatriXX) was used to measure the spatial dose fluctuation across the entire field and the measurements were repeated with different sampling rates (1s, 4s and 8s per snap).
Results:
With the OAD of the chamber in water increasing from 0 to 10cm, the variation of the two chamber reading ratio increased from 0.3% to 1.1%, 0.2% to 0.9% and 0.1% to 0.4% for chamber measurement integration period of 1s, 4s and 8s, respectively. For the MatriXX measurements, the variation among snaps also increased with increasing OAD and decreased with increasing sampling intervals. The average variations across the 20x20cm² field were 1.4%, 1.0% and 0.6% at sampling rates of 1s, 4s and 8s, respectively.
Conclusion:
When measuring the depth dose and beam profiles using a scanning chamber in water, a long chamber measurement period (>= 4s) is necessary to reduce the temporal and spatial fluctuation of the scanning beam accelerator. Alternative methods using films or MatriXX with sufficient MU may be preferred.
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