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Calculation for TG-51's KQ for a 2.5MV Beam

S Grzetic

S Grzetic*, J Woollard , N Gupta , A Ayan , Ohio State Univ, Columbus, OH


SU-I-GPD-T-482 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To establish a value for KQ in order to calibrate a 2.5MV beam using the TG-51 protocol.

Methods: During annual QA machine output, measurements for absolute dose calibration were performed at 100cm SSD, 10cm depth, 10x10cm field size, delivering 100 Monitor Units to a waterproof Farmer Chamber (PTW TN30013) for a 2.5MV photon beam. Assuming the wall material was PMMA (the makeup of the wall was 74% PMMA and 26% graphite), a temperature and pressure-corrected Dwater/U (at dmax) was calculated using the TG-21 formalism. Using the machine output measurements, the TG-21 calculated output and the new TG-51 equation, a KQ value was determined.

Results: A KQ value of 1.000 ± 1% was calculated using the above methods. This value can be used in TG-51 when calculating the output of a 2.5MV imaging beam.

Conclusion: A KQ value of 1.000 may be used in TG-51 for calculating the output of a 2.5MV beam.

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