Med Phys
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Asymmetric collimators are currently available in most of linear accelerators. They involve a lot of clinical improvements, such as the monoisocentric beam split technique that is more and more used in many external radiotherapy treatments. The tolerance established for each independent jaw positioning is 1 mm. Within this tolerance, a gap or overlap of the collimators up to 2 mm can occur in the half beams matching region, causing dose heterogeneities up to 40%. In order to solve this dosimetric problem, we propose an accurate jaw calibration method based on the Monte Carlo modeling of linac photon beams. ⋯ With this method, we are able to reduce the positioning accuracy to 0.2 mm. Consequently, the dose distribution in the junction of abutted fields is highly smoothed, achieving the maximum dose heterogeneity to be less than 3%.
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To introduce a correction for speed of sound (SOS) aberrations in three dimensional (3D) ultrasound (US) imaging systems for small but systematic positioning errors in image guided radiotherapy (IGRT) applications. US waves travel at different speeds in different human tissues. Conventional US-based imaging systems assume that SOS is constant in all tissues at 1540 m/s which is an accepted average value for soft tissues. This assumption leads to errors of up to a few millimeters when converting echo times into distances and is a source of systematic errors and image distortion in quantitative US imaging. ⋯ The SOS correction presented increases quantitative accuracy in US imaging which may lead to small but systematic improvements in patient positioning.
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Three-dimensional ultrasound (3D US) of the carotid artery provides measurements of arterial wall and plaque [vessel wall volume (VWV)] that are complementary to the one-dimensional measurement of the carotid artery intima-media thickness. 3D US VWV requires an observer to delineate the media-adventitia boundary (MAB) and lumen-intima boundary (LIB) of the carotid artery. The main purpose of this work was to develop and evaluate a semiautomated segmentation algorithm for delineating the MAB and LIB of the carotid artery from 3D US images. ⋯ The authors generated and tested a semiautomated carotid artery VWV measurement tool to provide measurements with reduced operator time and interaction, with high Dice coefficients, and with necessary required precision.
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The purpose of this paper is to suggest guidelines for target-dose conformity in gamma knife stereotactic radiosurgery (GKSRS) by taking into account factors that have been linked to GKSRS complications. We also suggest an explanation for the failure of previous studies to find a correlation between improved conformity index and reduced risk of GKSRS toxicity, where the conformity index, C(S), is defined as the ratio of the prescription volume, V(P), to the target volume, V(T). ⋯ Our model suggests that for target volumes > or = 3 cm3, high levels of target-dose conformity (C(S) < 1.3) are required for typical GKSRS prescription doses in order to limit VNTD to levels associated with either no toxicity or mild neurological symptoms in a previous investigation.
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Recently, a fast-kVp switching (FKS) dual-energy method has been presented with clinical and phantom results to demonstrate its efficacy. Patient dose concern has been raised on FKS dual-energy since it involves higher energy acquisition at 140 kVp and slower gantry rotation time (e.g., 0.9-1 s) as opposed to 0.5 s as used in routine single-energy exams. The purpose of our study was to quantitatively compare the CTDI(VOL) of FKS and routine CT exams under the body and head conditions. ⋯ For equal image quality as measured by low contrast detectability, the CTDI(VOL) of a FKS head and body exam is roughly 22% and 14% higher than that of a routine single-energy head and body exam, respectively, for the phantom measured.