Subjects: Nuclear Science and Technology >> Particle Accelerator submitted time 2023-06-18 Cooperative journals: 《Nuclear Science and Techniques》
Abstract： A direct incident fluence measurement method based on amorphous silicon electronic portal imaging device (a-Si EPID) has been developed for pretreatment verification of intensity-modulated radiation therapy (IMRT). The EPID-based incident fluence conversion method deconvolves EPID images to the primary response distribution based on measured lateral scatter kernels in the EPID detector using Conjugate Gradient algorithm. The primary response is converted to the incident fluence based on measured fluence conversion matrix which corrects for off-axis position dependence of the a-Si EPID response and the "horn" beam profile caused by flatting filter. To verify feasibility and accuracy of this method, square fields of various sizes and two IMRT plans were delivered. The dose distributions computed based on EPID-derived incident fluence were compared with the measurement data. For all square field sizes except the smallest field (2 cm), the mean dose differences in cross-line dose profiles were within 1% excluding the penumbra region, and gamma passing percentages with a 2%/2 mm criterion were about 99%. For two IMRT plans, the least gamma passing percentage for all eight IMRT fields was 98.14% with 2%/3 mm criteria. It can be concluded that our direct EPID-based incident fluence conversion method is accurate and capable of being applied to pretreatment dose verification in clinical routines.
Subjects: Physics >> Nuclear Physics submitted time 2023-06-18 Cooperative journals: 《Nuclear Science and Techniques》
Abstract： Optimization of the inverse planning becomes critical because it follows the invention of intensity modulated radiotherapy (IMRT) to shorten the previous "trial-and-error" treatment process and increase efficiency. In this paper, the inverse planning is used to direct aperture optimization in the ARTS (Accurate/Advanced Radiotherapy System). The objective function was quadratic, both tolerance and dose-volume constraint types are supported. The memory efficient conjugate gradient algorithm is used to cope with its large data. Furthermore, to fully exploit the solution space, a shortest path sub-procedure is coupled into the whole algorithm, thus giving further possibility decreasing the objective function. Two clinical cases are tested, indicating that the applicability of this algorithm is promising to clinical usage.
Subjects: Nuclear Science and Technology >> Particle Accelerator submitted time 2023-06-18 Cooperative journals: 《Nuclear Science and Techniques》
Abstract： The absolute dose difference of the iso-center position at different times was used to evaluate the stability of medical linear accelerator (LINAC). In order to eliminate the drawback of point dose without statistical significance and to investigate the output stability of segment with small weight related to intensity-modulated radiotherapy (IMRT), a modified method of evaluating the stability of LINAC was proposed based on planar dose combined with gamma method. With a commercial available two-dimensional ionization chamber, a set of planar doses with dose gradient from 2 cGy to 100 cGy were obtained. Then the gamma method was adopted to analyze the dose difference between the reference data and the evaluated data at each dose level. The results showed that the improved method based on planar dose for analyzing the output stability of the medical LINAC was feasible and efficient, and suggested that the reverse optimization should be aborted in clinical when the segment weight related to IMRT was under 10 MU.
Subjects: Physics >> Nuclear Physics submitted time 2023-06-18 Cooperative journals: 《Nuclear Science and Techniques》
Abstract： The reliability of radiotherapy was evaluated and effective approaches were obtained in order to improve radiotherapy quality by using the Probabilistic Safety Assessment (PSA) method. This study investigated the feasibility of the PSA method being applied to radiotherapy through Image-guided Radiotherapy (IGRT) and chest tumor irradiation. A fault tree has been constructed after analyzing causal relationship of the events. After calculating RiskA, a total inaccuracy radiotherapy probability and the importance of all base events were obtained. The probability of inaccurate radiotherapy was 2.87%. Under the condition that the target delineation was perfectly right, the accuracy of radiotherapy significantly improved. With the calculation without Cone-beam Computed Tomography (CBCT) being corrected before irradiation, the accuracy significantly decreased. The most important events were connected with the human factor. Improving human technical level could enhance radiotherapy quality control efficiently.
Subjects: Physics >> Nuclear Physics submitted time 2023-06-18 Cooperative journals: 《Nuclear Science and Techniques》
Abstract： An accurate and fast sampling method was developed on the modeling of a voxel phantom called Rad-HUMAN for radiation protection of MC-based radiation transport and simulation. The segmented organ voxels, which were assigned with three dimensional (3D) coordinates, were simplified through a two-step hybrid sampling algorithm. Firstly, certain voxels were sampled into a coordinate matrix by the nearest neighbor sampling. Secondly, the coordinate matrix was renewed using a weighted sampling. To compare visualization with the sampling, the resultant matrix was used to extract the contour of organs/tissues for constructing polygon-surface phantom. The feasibility and effectiveness of the sampling method was verified through the modeling of large organs (e.g. skeleton system) and application of transformation to MC computational geometries.
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