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Using Cerenkov Light Emission for the Measurement of Output Factors at Clinical Linear Accelerators

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J Burg

K Zink, University of Applied Sciences, Giessen, Germany J Burg, Philipps University of Marburg H Vorwerk, University Hospital Giessen-Marburg


SU-F-T-482 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

A potential method of quality assurance for clinical accelerators is using the signal from Cerenkov light generated during irradiation. The example of measuring output factors (OF) of a clinical accelerator was used to investigate the potential of Cerenkov light as a dosimetry tool.
OF as a function of field size were measured under reference conditions using ionization chambers at 18 MV-X. Furthermore, for selected field sizes, the produced Cerenkov light within the water Phantom was imaged by a CCD-Camera. In order to improve the optical signal, any light source in the treatment room was blocked. The measurement was repeated three times for better statistics. Images were median filtered to reduced errors due to direct ionization of the CCD-chip. Afterwards, raw images were averaged and evaluated at ROI in 10 cm depth. Considering cone shaped geometrical corrections, the measured Cerenkov signal in relation to the signal from 10x10 cm field was compared with OF measured under reference conditions.
Comparing the OF as a function of field size with the measured signal change in Cerenkov images, a comparable trend is observable. A limiting factor was the small signal to noise ratio caused by the insufficient sensitivity of the CCD-Camera which results in large error bars.
The correlation between Cerenkov light production and dose deposition has already been published. The geometrical correction of the optical system for the non-isotropic Cerenkov signal represents a big challenge of defining correction factors for the system. A highly developed image system is required to image and use Cerenkov signals as a dosimetry tool with an acceptable signal to noise ratio. Moreover, correction factors for the measurement system have to be established. In further work, Monte Carlo simulation should be used in order to define correction factors for the given measuring setup.

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