(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: Compact air-filled ionization chambers or microchambers are commonly used for relative measurements during linac commissioning, especially under the conditions with high dose gradients such as small fields output and profile measurements. In this work, the clinical performance of two novel microchambers have been evaluated for a wide range of measurement conditions regarding their saturation behavior, polarity effect, volume effect and energy dependence.
Methods: Measurements were performed using the Razor Chamber CC01-G and Razor Nanochamber CC003-S (both from IBA Dosimetry, Schwarzenbrück, Germany). Jaffe-plots were obtained for different dose-per-pulse (DPP) values to determine the optimal operating voltage and the corresponding saturation correction factors. The polarity effect was evaluated for field side length from 0.5 cm to 40 cm. Using a narrow beam geometry, the lateral dose response functions K(x) were measured and approximated by single Gaussian functions for three chamber’s orientations. The energy dependent response of the chamber was evaluated by varying the field size and measurement depth.
Results: Due to their compact designs, the chambers should be operated at voltages between 100 – 200 V. At small field sizes (effective field side length < 15 mm), the chambers showed significant field size dependent polarity effect when positioned axially. The spherical Nanochamber exhibits symmetrical spatial response, where the lateral dose response functions show no chamber’s orientation dependence.
Conclusion: Correction factors have been determined for the Razor Chamber and Nanochamber for various measurement conditions encountered during linac commissioning. Due to the compact designs of the chambers, the polarity effect should be accounted for especially under the conditions where secondary electron equilibrium is not established such as at small field sizes and at the field borders during profile measurements.
Keywords
Polarity Effects, Volume Effects
Taxonomy
TH- External beam- photons: Small/nonstandard field experimental dosimetry
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