The dose-rate dependence of the detector was evaluated by either changing the repetition rate of the linac or changing the source to surface distance (SSD). The former case tests the mean dose-rate dependence, and the latter case tests the instantaneous dose-rate dependence (i.e., dose-rate within the pulse) of the detector. For Varian based linacs, the radiation beam is typically delivered at a repetition rate of 360 Hz (600 MU/min). To achieve lower average dose-rates, pulses are dropped by delaying the gun pulse relative to the accelerating klystron pulse. In this study, repetition rates of ~360 Hz, 240 Hz, 120 Hz and 60 Hz were used to test the mean-dose rate dependence. For the instantaneous dose-rate study, four different SSDs were used: 77 cm, 90 cm, 110 cm, and 130 cm. To note, no beam collimation was used for this study.
Linacs
Manufactured by Agilent Technologies
Linacs, short for Linear Accelerators, are specialized laboratory equipment used to accelerate charged particles, such as electrons, to high energies. The core function of Linacs is to generate and direct a beam of high-energy particles for various scientific and industrial applications.
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4 protocols using linacs
1
Dose-Rate Dependence of Detector
2
Phantom Scatter Correction for Radiotherapy
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The size of a phantom influences the scattering dose and the charged-particle equilibrium. Correcting for the scattering dose between these phantoms and a water phantom is necessary. Under the condition of fixed distances between the source and the chamber for the photon and electron beams, we used the ion chamber to measure the ionization ratio of these small acrylic phantoms and the full-scatter solid water (30 × 30 × 30 cm3) for high-energy beams: 6 and 10 MV photon beams and 6, 9, and 12 MeV electron beams. According to AAPM TG-21, the absorbed dose in medium is defined as
Because the same ionization chamber was used to measure the Dmed values of acrylic and water, the Ngas, Prepl, and Pwall parameters are the same in these two mediums. Therefore, the dose ratio for the solid water and postal phantoms was defined as the phantom scatter correction factor, calculated using the ionization ratio and the ratio of the mean restricted mass collision stopping power:
Considering the phantom stability, we also used three different types of LINACs (produced by Varian, Siemens, and Elekta) and irradiated the measuring point of each phantom at 100 MU as calculated in the reference condition.
Because the same ionization chamber was used to measure the Dmed values of acrylic and water, the Ngas, Prepl, and Pwall parameters are the same in these two mediums. Therefore, the dose ratio for the solid water and postal phantoms was defined as the phantom scatter correction factor, calculated using the ionization ratio and the ratio of the mean restricted mass collision stopping power:
Considering the phantom stability, we also used three different types of LINACs (produced by Varian, Siemens, and Elekta) and irradiated the measuring point of each phantom at 100 MU as calculated in the reference condition.
3
Validation of MLC Modelling for Varian LINACs
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For validation of our MLC modelling, data obtained in part A are compared with the measured values of PDD for Varian LINACs based on IAEA TRS 398 protocol [ 20
].
].
4
Multicenter Validation of TPS Output Factors
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The participants were asked to calculate the monitor units (MUs) necessary to deliver 10 Gy on the central axis at 10 cm depth, 100 cm SSD in water for 5 MLC shaped fields (10 × 10 cm2, 6 × 6 cm2, 4 × 4 cm2, 3 × 3 cm2 and 2 × 2 cm2) using their TPSs. For Varian linacs with a tertiary MLC, the field size was defined by the MLC while the secondary jaws remained at a 10 × 10 cm2 field size. The dose per MU for each field was calculated and normalized to the 10 × 10 cm2 field. These calculated OFs were compared to reference output factors published by IROC-Houston QA Centre for the same beam energy and linac manufacturer [29] , [30] . References to these publications were included in the instructions for the participating centres. For analysis, the ratio of each institution’s TPS calculated OFs to the reference OFs was determined for each field size and nominal beam energy. An action limit of ± 3% for the 2 × 2 cm2 field and ± 2% for fields larger than 2 × 2 cm2 was defined. These action limits were derived using four times the average standard deviation of the reference data. The results were grouped by nominal beam energy (≤6 MV and >6 MV) and by TPS – treatment machine combination. The multicentre run was performed at the end of 2013 and the national runs were initiated in 2014. Data collection was completed in 2016.
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