ESTRO 2024 - Abstract Book

S3294

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

PFN’s charging cycles and vary the delay time (i.e. the time from the first detection of the PFN signal until the trigger pulses are allowed to pass the optocoupler). To investigate how the output in the first pulse depends on the PFN delay time, we measured the relative output for a 1-pulse delivery for different delay times between 200 ms and 720 ms. We also explored the ability to deliver a precise number of MUs ranging between 250 MU and 2500 MU, by synchronizing the trigger pulses with the PFN.

Results:

Improved beam tuning of the gun current and the magnetron frequency resulted in an output of >0.8 Gy/pulse or >1.1 Gy/pulse with or without the transmission chamber (equivalent to average dose rates >160 Gy/s or >220 Gy/s) at SSD=100 cm, compared to the previously reported 0.64 Gy/pulse at the cross-hair foil (1). The standard deviation in dose-per-pulse for 20 film measurements conducted every minute for 20 minutes was 2.2% (Figure 1A) and the standard deviation of 32 film measurements conducted on eight different days over a 3-month period was 3.0% (Figure 1B).

A linear relationship (R2>0.99) was observed between the film dose and the number of MUs calculated using the transmission chamber, as well as the number of pulses delivered. The deviation between the calculated MUs/100 and the film dose was ≤5% for 90% of the measurements in the total of 40 measurements. The output in the initial pulse depended on the PFN delay time (Figure 2), with the maximal output occurring at delay times of 440 ms and 700 ms. In a total of 50 measurements employing PFN synchronization to target the preset number of MUs, the absolute percentage error between the delivered number of MUs calculated by the transmission chamber and the preset number of MUs was 0.8±0.6% (mean±SD).