Preliminary evaluation of a reference detector measuring leakage radiation for use in small field beam data acquisition.

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Conference Proceeding

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Med Phys


Purpose: This work evaluates a novel ion chamber, designed to provide a reference signal based on leakage radiation from the linac head, for use in small field beam data acquisition. Methods: Beam profile measurements were acquired using a 3D Scanner (Sun Nuclear Corporation) for 6FFF and 10FFF beams of a Varian TrueBeam linear accelerator. A prototype 39 cc parallel plate ionization chamber (Model 1046 Reference Detector, Sun Nuclear Corporation) was paired with a CC-01 micro chamber (IBA Gmbh) as the field detector. Data were acquired at multiple scan speeds and dose rates for 3 field sizes: 2 cm92 cm, 1 cm91 cm and 0.5 cm90.5 cm. Comparisons were made to scans using a CC-13 chamber (IBA) as reference detector. While the CC-13 reference chamber was inserted in the primary beam (in the treatment head, above the secondary collimators), the model 1046 prototype was placed externally at approximately the highest leakage area on the linac head. Profiles were post-processed using a Savitzky-Golay filter, which reduced noise with negligible penumbra broadening. Results: The Model 1046 produced a useful reference signal for all dose rates and scan speeds evaluated. The noise in the data decreased with increasing dose rate, and decreasing scan speed. When compared to the CC-13, the raw data were noisier, partly due to the lower strength of the reference signal (about 1/ 6 that of CC-13 at 400 MU/min) and partly because the model 1046 is more sensitive than the CC-13 to noise spikes in the electrometer bias supply. With post-processing, the profiles obtained with the model 1046 at high dose rates were comparable to those obtained with a CC-13. Conclusion: A preliminary assessment of a novel prototype reference detector was performed. While more investigation is needed, it is expected that this detector will prove useful for small field reference dosimetry.





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