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Semiconductor dosimeters are ubiquitous in modern external-beam radiation therapy. They possess key features. The response, electronically available in real time, is stable and linear with absorbed dose for given irradiation conditions; the radiation-sensitive volume can be rather small in size, while retaining mechanical strength and high sensitivity. We describe three common semiconductor dosimeters: diodes, metal-oxide-semiconductor field-effect transistors and diamonds. We discuss in detail their operation principles and applications in modern external-beam radiation therapy, primarily with megavoltage photon beams. We also explore their use in proton and heavy ion therapy, and in experimental radiotherapy techniques such as synchrotron-based micro-beam radiation therapy. [ABSTRACT FROM AUTHOR]

Quantum noise is common in CT images and is a persistent problem in accurate ventilation imaging using 4D-CT and deformable image registration (DIR). This study focuses on the effects of noise in 4D-CT on DIR and thereby derived ventilation data. A total of six sets of 4D-CT data with landmarks delineated in different phases, called point-validated pixel-based breathing thorax models (POPI), were used in this study. The DIR algorithms, including diffeomorphic morphons (DM), diffeomorphic demons (DD), optical flow and B-spline, were used to register the inspiration phase to the expiration phase. The DIR deformation matrices (DIRDM) were used to map the landmarks. Target registration errors (TRE) were calculated as the distance errors between the delineated and the mapped landmarks. Noise of Gaussian distribution with different standard deviations (SD), from 0 to 200 Hounsfield Units (HU) in amplitude, was added to the POPI models to simulate different levels of quantum noise. Ventilation data were calculated using the ΔV algorithm which calculates the volume change geometrically based on the DIRDM. The ventilation images with different added noise levels were compared using Dice similarity coefficient (DSC). The root mean square (RMS) values of the landmark TRE over the six POPI models for the four DIR algorithms were stable when the noise level was low (SD <150 HU) and increased with added noise when the level is higher. The most accurate DIR was DD with a mean RMS of 1.5 ± 0.5 mm with no added noise and 1.8 ± 0.5 mm with noise (SD = 200 HU). The DSC values between the ventilation images with and without added noise decreased with the noise level, even when the noise level was relatively low. The DIR algorithm most robust with respect to noise was DM, with mean DSC = 0.89 ± 0.01 and 0.66 ± 0.02 for the top 50% ventilation volumes, as compared between 0 added noise and SD = 30 and 200 HU, respectively. Although the landmark TRE were stable with low noise, the differences between ventilation images increased with noise level, even when the noise was low, indicating ventilation imaging from 4D-CT was sensitive to image noise. Therefore, high quality 4D-CT is essential for accurate ventilation images. [ABSTRACT FROM AUTHOR]

Objective . To reduce the magnetic isocenter position variation with gantry rotation on an 0.35 T MRI-guided linac to a practically negligible level. Approach . Central fRequency (CF) offset, eddy current calibration, cross-term calibration, gradient delay, and gradient offsets are tuned for each MR linac installation at every 30° of gantry rotation and stored in a look-up table (LUT). During treatment, the CF is tuned only once in the beginning at an arbitrary gantry angle. After that, imaging paramters are offset based on the stored LUT values for any given gantry angle. Main results . For the same hardware configuration, the implementation of the gantry-angle-specific parameter corrections reduced the total isocenters range of travel in the transverse plane from 1.1 to 0.3 mm and from 0.8 to 0.2 mm in horizontal and vertical directions, respectively. With the longitudinal shift always being negligible (≤0.2 mm), the radius of the sphere encompassing the isocenter locations was reduced from 0.6 to 0.2 mm. Geometric distortion improved as well; in particular, the gantry-angle-averaged maximum longitudinal distortion within a 35 cm diameter sphere was reduced from 1.4 to 0.8 mm. Since the CF is tuned only once during treatment, imaging may resume promptly after the gantry reaches the next target position. Significance . The MRI-guided linear accelerator was conceived primarily as an instrument for precision image-guided therapy. Thus, it is important to keep the treatment and imaging isocentres as close as possible while minimizing the geometric distortion. The described solution reduces the walkout of the imaging isocenter to a fraction of 1 mm, while keeping geometric distortion in a substantial volume below 1 mm. The approach is robust and does not increase the overall procedure time., (© 2023 Institute of Physics and Engineering in Medicine.)