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BackgroundThe beam model of medical linac is one of the bases of dose calculation. It is of great significance to study the beam model of medical linear accelerator in clinical application of radiation dose calculation.PurposeThis study aims to provide a virtual point source model of medical linear accelerator X-ray beam, and to discuss the applicability of the model by fitting the measured dose distribution.MethodsThe X-ray beam was divided into primary ray and scattered ray. The primary ray was modeled as an anisotropic point source whilst the scattered ray was modeled as a point source whose intensity was related to the radiation field. The model parameters were obtained by fitting the measured absorbed dose distribution in water phantom.ResultsFor Varian Trilogy 6 MV linear accelerator, the dose distribution given by the fitting model is in good agreement with the measured dose distribution, and the difference between the measured value and the fitting value of the total scattering factor (SCP) is less than 1.5%. For 3 cm × 3 cm, 10 cm× 10 cm and 40 cm× 40 cm radiation fields, from 1.4 cm (maximum dose depth) to 20 cm underwater depth, the average differences between the fitting and the measured value are 1.3%, 1.3% and 1.1% respectively. The average differences between the fitting and measured values of off-axis dose in the area with off axis dose ratio (OAR) greater than 90% are 2.35%, 0.75% and 0.53% respectively at the depth of 10 cm.ConclusionsThe virtual point source model of X-ray beam can well reconstruct the absorbed dose in phantom.

The purpose of this study is to investigate whether deep learning-based sCT images enable accurate dose calculation in CK robotic stereotactic radiosurgery. A U-net convolutional neural network was trained using 2446 MR-CT pairs and used it to translate 551 MR images to sCT images for testing. The sCT of CK patient was encapsulated into a quality assurance (QA) validation phantom for dose verification. The CT value difference between CT and sCT was evaluated using mean absolute error (MAE) and the statistical significance of dose differences between CT and sCT was tested using the Wilcoxon signed rank test. For all CK patients, the MAE value of the whole brain region did not exceed 25 HU. The percentage dose difference between CT and sCT was less than ±0.4% on GTV (D 2 (Gy), -0.29%, D 95 (Gy), -0.09%), PTV (D2(Gy), -0.25%, D95(Gy), -0.10%), and brainstem (max dose(Gy), 0.31%). The percentage dose difference between CT and sCT for most regions of interest (ROIs) was no more than ±0.04%. This study extended MR-based sCT prediction to CK robotic stereotactic radiosurgery, expanding the application scenarios of MR-only radiation therapy. The results demonstrated the remarkable accuracy of dose calculation on sCT for patients treated with CK robotic stereotactic radiosurgery., (© 2024 IOP Publishing Ltd.)