Background and purpose: This work aims to present the strategy to simulate a clinical linear accelerator based on the geometry provided by the manufacturer and summarize the corresponding experimental validation. Simulations were performed with the Geant4 Monte Carlo code under a grid computing environment. The objective of this contribution is reproducing therapeutic dose distributions in a water phantom with an accuracy less than 2%. Materials and methods: A Geant4 Monte Carlo model of an Elekta Synergy linear accelerator has been established, the simulations were launched in a large grid computing platform. Dose distributions are calculated for a 6 MV photon beam with treatment fields ranging from 5 × 5 cm2 to 20 × 20 cm 2 at a source - surface distance of 100 cm. Results: A high degree of agreement is achieved between the simulation results and the measured data, with dose differences of about 1.03% and 1.96% for the percentage depth dose curves and lateral dose profiles, respectively. This agreement is evaluated by the gamma index comparisons. Over 98% of the points for all simulations meet the restrictive acceptability criteria of 2%/2 mm. Conclusion: We have demonstrated the possibility to establish an accurate linac head Monte Carlo model for dose distribution simulations and quality assurance. Percentage depth dose curves and beam quality indices are in perfect agreement with the measured data with an accuracy of better than 2%.

Introduction: Bolus material is frequently used on patient’s skin during radiation therapy to reduce or remove build-up effect for high-energy beams. However, the air-gaps formed between the bolus and the skin’s irregular surface reduce the accuracy of treatment planning. To achieve a good treatment outcome using bolus, experimental investigations are required to choose its thickness and to quantify the air-gap effect. Material and methods: Measurements for a 6 MV photon beam with a fixed source surface distance were carried out using the 31021 Semiflex 3D chamber into the water phantom. Firstly, the depth of maximum dose (R100) and the dose value at surface (Ds) were evaluated as a function of bolus thickness for some square fields. Secondly, to test the effect of the air-gaps ranged from 5 to 30 mm with a step of 5 mm between the bolus and the phantom surface, a water-equivalent RW3 (Goettingen White Water) slab form of 10 mm thickness was considered as a bolus. Results: We observed that the linear behaviour of R100 in terms of the bolus thickness makes the choice of this parameter more convenient depending on field size. In addition, increasing the air-gaps widens the penumbra and created electrons that have a greater probability to quit the radiation field borders before reaching the surface. The dose spread of the off-field area could have a significant influence on the patient treatment. Conclusion: Based on dose distribution comparisons between the measurements with and without air-gaps for the field size of 100 mm × 100 mm, it has been demonstrated that a maximum air-gap value lower than 5 mm would be desirable for an efficient use of the bolus technique.

In radiology, the photon fluence and the energy spectrum generated from an x-ray tube may depend on the anode tilt angle. In this contribution, a Monte Carlo investigation is performed to quantify this effect by modeling an x-ray tube based on published data [1]. The GATE simulation code is used for this purpose. The calculations have moreover confirmed this dependence; the tilt of the anode could be used to increase the photon fluence. The thermal analysis has shown that the hot spot size is dependent as well on the anode tilt angle. The thermal focus temperature (△T ) decreases when the anode tilt angle increases. Finally, by moving the acquisition angle from 293-337° to 248-292° and changing the anode tilt angle from 8° to 28°, the photon fluence can be increased by 55%.

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This study aims to optimize the iterative deconvolution correction of air-filled ionization chamber measurements with limited spatial resolution for photon beam dosimetry. The ionization chamber volume effect could be explained by the inhomogeneity of the absorbed dose in its sensitive volume, leading to a volume averaging effect acting as a convolution kernel  K(x). Therefore, the “true” dose profile Pt(x) can be deduced by deconvolving measured 1D profiles Pm(x). The Semiflex 3D ionization chamber type PTW 31021 was studied for beams with flattening filter (WFF), while the SNC125c ionization chamber was investigated for flattening filter free (FFF) beams. The deconvolution distributions were compared for the first ionization chamber with reference dose profiles calculated by Monte Carlo simulation and for the second ionization chamber with reference measurements obtained using a diode EDGE detector, suitable for small field applications. The convolution kernel was assumed to be a sum of normalized Gaussian and Lorentz distributions parametrized by the pair (σicλic ). Good results were obtained with respect to the γ index 2.0%/0.5 mm criterion for the field sizes 6 × 6 mm2, 10 × 10 mm2 and 20 × 20 mm2. The highest agreement between reference and corrected measurement data was obtained by using the Gauss-Lorentz distribution parameters (1.92 mm, 0.80 mm) and (1.90 mm, 0.90 mm) for the ionization chambers Semiflex 3D type PTW 31021 and SNC125c, respectively.