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Output Dependence On Dose Rate for Uniform Scanning Proton Beams


E Ramirez

E Ramirez*, Y Zheng, ProCure Treatment Centers, OKLAHOMA CITY, OK

SU-E-T-18 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose:
Accurate determination of proton output is critical to ensure accurate proton dose delivery. It was found that the proton output factor was decreased when the dose rate was low. The purpose of the study is to investigate the dose rate effects on proton dose output factors.

Methods:
We systematically measured output (nC/MU) of the proton beam as a function of dose rate for various beam conditions. We first measured dose rate effects for proton beams of three range and modulation combinations: range 16 cm and modulation 10 cm, range 5 cm and modulation 3 cm, and range 30 and modulation 15 cm. We then investigated the dose rate effects for two scanning field sizes, the standard 18X18 cm^2 and the largest available 25x25 cm^2.

Results:
The output factor varied with dose rate, increasing more than 6% when the dose rate increased from 0.1 Gy /minutes to about 2.5 Gy/minute. The output variation on dose rate seems to depend on only the dose rate itself. The effect of proton energy and scanning field size on the dose rate dependence was minimal.

Conclusions:
The proton dose output factor depends on the dose rate at our facility, and can be large (>5%). This dependence needs to be taken account in output modeling for more accurate proton dose delivery.

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