A method has been developed to measure a photon penumbra-generating kernel using dosimetry equipment available in most radiation therapy departments. The kernel is used in a convolution-adapted ratio-TAR algorithm in our three-dimensional treatment planning system. The kernel is assumed to be invariant with respect to off-axis position, axially symmetric, and is divided into short- and long-range components, with a different measurement technique for each. The data required to obtain the short-range component are measured by scanning across a split-field geometry incident on a water phantom. The derivative of the measured profile is proportional to one-dimensional projections across the kernel. Because the kernel is axially symmetric, only one profile measurement is required for each depth. A CT reconstruction technique is used to extract the radial dependence of the kernel from the strip integrals. Electronic noise in the acquisition system yields significant uncertainties in the kernel shape for distances beyond 3 cm. The long-range portion of the kernel is obtained by examining tissue-air ratios (TARs). The derivative of the TAR at the center of a circular field is proportional to the kernel value at the distance corresponding to the radius of the field. The kernel measurement method was tested by comparing measured and calculated square-field profiles at a variety of depths. Agreement was within 1% within the field boundary and 3% outside the field boundary for all depths.
A method has been developed to estimate potential dose errors due to linear accelerator angular setting misalignments of Intensity Modulated Radiation Therapy (IMRT) treatments. A first-order approximation to the dose error at a point is modeled as the dot product of the dose gradient and the shift vector of the point due to the rotational error. The analysis method is applied to a previously published set of optimized fluences for a 50 MV IMRT pelvis irradiation. Three of the published cases exhibiting a wide range of modulation are presented; a rectangular open field, a field optimized for a static multileaf collimator defining the portal outline coupled with a single broad bremsstrahlung profile modulation, and a fully modulated field using a physical modulator. To examine the energy dependence of angle setting errors, the study is repeated using the same fluence distributions, but with a dose-spread kernel appropriate for a 6 MV photon beam. The collimator angle error is set to 2 degree, and the dose error determined with both a centrally located isocenter and an isocenter chosen to model a split-field geometry. The dose error due to a 2 degree gantry setting error is assessed at a plane 10 cm distal to the isocenter. The mathematical form of the dose error due to couch motion is similar to the other two errors, so the dose error resulting from a couch angle missetting is not presented. The magnitude of the errors is largest for the 6 MV beam, while the volume encompassed by the errors is greater for the 50 MV beam. The gantry error yields the largest dose error values, with the 6 MV modulated case presenting dose errors of greater than 40%.
This paper addresses geometry design and operating mode optimum design of a new kind of 2-DOF parallel manipulator actuated horizontally by linear actuators. The forward and inverse kinematics of this manipulator are derived. The four groups of inverse solution correspond to four different operating modes which cannot transit to each other smoothly. The workspace and the singularity trajectory of each mode are discussed. Based on the desired workspace, the geometry of the mechanism is determined. The operating mode of the mechanism is optimized according to distributing of all global and local performance indices on the workspace. The results are very useful for the design and application of the new manipulator with multiple forward and inverse solutions.
In this paper, a 2-DOF planar parallel manipulator actuated horizontally by linear actuators is proposed. The inverse and forward kinematics can be described in closed form. The velocity equation, singularity of the manipulator and the conditioning index is investigated. In addition, the inverse dynamics problem of the device is investigated employing the Lagrange approach. The dynamic simulation is carried out. The results show that the kinematics performance and the force transmissibility are worse when the end-effecter moves near the singularity. The proposed manipulator can be applied to the field of machine tools or used as the mobile base for a spatial manipulator. The results of the paper are very useful for the design and application of the new manipulator.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.