Objective: A Monte Carlo (MC) model of a Halcyon and Ethos (Varian Medical Systems, a Siemens Healthineers Company) radiotherapy beam was validated and field-independent phase space (PHSP) files were recorded above the dual-layer multileaf collimators (MLC). Approach: The treatment head geometry was modeled according to engineering drawings and the dual-layer MLC was imported from CAD (computer-aided design) files. The information for the incident electron beam was achieved from an iterative electromagnetic solver. The validation of the model was performed by comparing the dose delivered by the square MLC fields as well as complex field measurements. Main results: An electron phase space was generated from linac simulations and achieved improved MC results. The output factors for square fields were within 1% and the largest differences of 5% were found in the build-up region of PDDs and the penumbra region of profiles. With the more complicated MLC-shaped field (Fishbone), the largest differences of up to 8% were found in the MLC leaf tip region due to the uncertainty of the MLC positioning and the mechanical leaf gap (MLG) value. The impact of the collimator rotation on the PHSP solution has been assessed with both small and large fields, confirming negligible effects on in-field and out-of-field dose distributions. Significance: A computational model of the Halcyon and Ethos radiotherapy beam with a high accuracy implementation of the MLC was shown to be able to reproduce the radiation beam characteristics with square fields and more complex MLC-shaped fields. The field-independent PHSP files that were produced can be used as an accurate treatment head model above the MLC, and reduce the time to simulate particle transport through treatment head components.
Uniform dose distribution with steep lateral gradient within depth range of 0-0.5 cm is crucial to be able to treat small skin lesions. The standard nominal 4 MeV electron beam from Elekta Versa HD linear accelerator was modified with degrading filter to remove the lateral scatter from treatment head and minimize the penumbra. The energy degrading method was verified based on dosimetric properties and output factors (OFs) with comparison of four types of measurement methods. The properties of degraded 4 MeV electron beam and developed electron applicators seem optimal for treating small targets near the skin surface.
Background With the ever‐increasing complexity of dynamic radiotherapy treatments, dose calculation algorithms are challenged to accurately calculate the dose resulting from small, on‐ and off‐axis multileaf collimator (MLC) aperture movements. Although the currently available Eclipse (Varian Medical Systems, Palo Alto) dose calculation algorithms still use a simplified, binary MLC model, a more advanced and detailed modeling of the MLC could be beneficial for the dose calculation precision of high‐end treatments. Purpose To improve the modeling of the MLC in the dose calculation algorithms of the Eclipse treatment planning system, an enhanced MLC attenuation model was constructed through ray tracing through the actual leaf designs for the most commonly used Varian MLC types. The enhanced leaf model (ELM) thus includes the rounded leaf tip shape, the drive screw cutout, and the leaf body thickness. The purpose of this work is to test out this new model and explore possible improvements compared to the previous model. Methods Dose calculations were performed in a research Eclipse environment equipped with the original and enhanced MLC model. Measurements were performed on TrueBeam and on Halcyon dual MLC treatment units. Dedicated static and dynamic MLC test plans were designed to challenge the dose calculation and highlight differences between both models while keeping the experimental setup simple in order to minimize measurement uncertainties. Measurements were performed with single ion chambers, 2D ion chamber arrays and film. Results The improved MLC model considerably improves the accuracy of the dose calculation for the test fields used in this study. For the TrueBeam MLC, improvements are most prominent for off‐axis dose delivery through narrow (static or dynamic) MLC gaps. For 3 mm narrow sweeping gap deliveries at 12 cm off‐axis, the advanced model agrees within 2% with the measurement, in contrast to the 12% deviation observed with the original MLC model. For the Halcyon MLC, improvements are especially prominent when the leaves of both MLC stacks are aligned, regardless of their position in the field. Sweeping gap measurements improve from a 7%–10% deviation with the original model to within 2% with the new model. Conclusions Although test fields designed in this study emphasize the flaws in the original MLC dose calculation model, the enhanced MLC model resolves all of the observed discrepancies, showing excellent on‐ and off‐axis agreements with all of the performed measurements.
The signal of the dosimetric detector is generally dependent on the shape and size of the sensitive volume of the detector. In order to optimize the performance of the detector and reliability of the output signal the effect of the detector size should be corrected or, at least, taken into account. The response of the detector can be modelled using the convolution theorem that connects the system input (actual dose), output (measured result) and the effect of the detector (response function) by a linear convolution operator. We have developed the super-resolution and non-parametric deconvolution method for determination of the cylinder symmetric ionization chamber radial response function. We have demonstrated that the presented deconvolution method is able to determine the radial response for the Roos parallel plate ionization chamber with a better than 0.5 mm correspondence with the physical measures of the chamber. In addition, the performance of the method was proved by the excellent agreement between the output factors of the stereotactic conical collimators (4-20 mm diameter) measured by the Roos chamber, where the detector size is larger than the measured field, and the reference detector (diode). The presented deconvolution method has a potential in providing reference data for more accurate physical models of the ionization chamber as well as for improving and enhancing the performance of the detectors in specific dosimetric problems.
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