(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: Recent reports have shown that very high dose rates of radiation can spare normal tissues while retaining the therapeutic effect on tumor. We converted a decommissioned clinical Varian linac IX to deliver dose at ultrahigh dose rate of >= 35Gy/sec from 6 Gy/min using 20 Mev electron beam.
Methods: We modified a decommissioned Varian IX linac to deliver 20MeV electrons ≥35Gy/s at the level of the mirror inside the collimator. We copied all beam parameters from the 18 X MV card to 20 MeV electron beam card. We tuned the 20 MeV gun current to 270 mA and the gun voltage to 14.25 kV, and finally, adjusted the bending magnet current to be correct for 20 MeV beam. We turned off the dose rate servos and ran beam at the maximal dose rate of 1000 MU/min with several interlocks disabled. A programmable Arduino board was used to generate the desired gating signal. We used Gafchromic film, TLDs, parallel plate and farmer chambers to confirm dose rates and dose delivery.
Results: We used an Arduino board to reproducibly deliver beam and verified the signal using an oscilloscope. The standard deviation of delivered dose for a gate of 1 second was about 1% open field and about 2% for a 2x2 field at the level of the mirror according to film and ion chamber measurements.
Conclusion: The FLASH dose rate can be achieved on a Klystron driven accelerator.. Gating the beam requires a workaround to initiate and terminate the beam with reasonable control. A built-in dose rate servo that monitors the dose pulse height level and an added transmission monitor chamber with faster electronics could be important in a FLASH-dedicated non-clinical linac.
Funding Support, Disclosures, and Conflict of Interest: All of this work was supported by an MD Anderson Cancer Center Radiation Oncology division's seed grant.
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