(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Micro-detectors have become popular for small field photon dosimetry. PTW has introduced a new diode unshielded detector called microsilicon (µSi) PTW-60023 that has disk diameter of 1.5 mm and thickness of 18 µm with minimum epoxy to eliminate perturbations. The characteristics of this detector have been published to be very favorable in photon and electron beams. However some characteristics such as temporal, thermal and angular dependence are not available which is presented here. Additionally output factor for small fields created with jaw and MLC are also investigated.
Methods: Measurements were performed on a Varian TrueBeam with scanning water phantom. The thermal response was carried out in a small phantom with variable temperature. Angular dependence was performed in air and in 5 cm water depth. The µSi measured output and beam profiles are compared with PSD-W2.
Results: Temporal response of µSi is constant. The thermal response is relatively constant with slight upward slope compared to PSD-W2 and SL-16 ion chambers that have negative slopes. Within operating range of water temperature (15-25 deg C) µSi is thermally independent. The angular dependence in air is very strong but when used in water scanning and computing the dose with beam angle, it is within <1%. The effect of MLC defined small field produces consistently <1.5% lower values compared to the jaw fields and the k(clin, msr) is nearly unity except for 0.6x0.6 cm² field where it deviates <5%.
Conclusion: µSi detector has optimum characteristics in photon beam with dose, dose rate, energy and thermal response. The temporal and angular dependence is also found to be minimum. It is concluded that µSi is one of the better detectors with k(clin, msr) values within ±1% except for very small fields. It also provides high resolution profile and accurate depth dose.
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