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Plastic deformation represents a fundamental concept in materials science, which explains the irreversible change in the shape of a material when it experiences stress beyond its elastic capability. This phenomenon is important in structural engineering, especially in designing and analyzing cantilever beams—structures that are securely fixed at one end and bear loads at the opposite end. When these beams are subjected to loads within their elastic range, they will return to their...
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It is essential to understand how structural members behave under plastic deformation when the bending stress exceeds the material's yield strength. This state of deformation permanently alters the shape of the member, in contrast to the linear elastic behavior observed before yielding. The strain at any point in the member is expressed in terms of maximum strain. Notably, the neutral axis, which coincides with the centroid during elastic bending, shifts away from the centroid under plastic...
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Accelerators in concrete serve as admixtures to speed up the hardening process, enabling the concrete to achieve early strength faster. Although accelerators do not necessarily impact the time it takes concrete to set, they reduce this time in practice. A common accelerator is calcium chloride, which is particularly useful for hastening early strength development in cold weather or for rapid repair jobs that require quick heat generation after mixing.
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An EPID-based method to determine mechanical deformations in a linear accelerator.

Daniel Gourdeau1,2, Luc Gingras1, Frédéric Beaulieu1

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Summary
This summary is machine-generated.

A new 14 degrees of freedom (DOF) method accurately measures medical linear accelerator (linac) mechanical deformations, including source movements. This advanced quality assurance (QA) ensures treatment precision by detecting errors with sub-millimeter accuracy.

Keywords:
geometric calibrationisocenter checklinear acceleratorradiation therapy

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Area of Science:

  • Medical Physics
  • Radiation Oncology
  • Quality Assurance

Background:

  • Modern radiation therapy employs complex treatments with medical linear accelerators (linacs).
  • Precise mechanical quality assurance (QA) is crucial to prevent deviations in gantry movement.
  • Existing QA methods often overlook critical degrees of freedom (DOF), such as in-plane rotations and internal source movements.

Purpose of the Study:

  • To introduce and characterize a comprehensive 14 DOF method for mechanical QA of linacs.
  • To measure all mechanical deformations, including source movements, and assess clinical parameters like mechanical and radiation isocenters.

Main Methods:

  • Utilized a commercial phantom and custom accessory imaged by an electronic portal imaging device (EPID) at various gantry angles.
  • Generated simulated images based on linac geometry and phantom models, modified by 14 DOF (translations and rotations for gantry/imager, source movements).
  • Employed a genetic algorithm coupled with gradient-descent optimization to minimize differences between simulated and measured images, identifying mechanical deformations.

Main Results:

  • Achieved an average distance of 0.086 mm between measured and simulated ball bearings.
  • Successfully measured sub-millimeter translations and sub-degree rotations across all 14 DOF during gantry rotation.
  • Demonstrated high precision in detecting phantom positioning errors (>0.1 mm), source movements, and gantry angular offsets, with an average absolute uncertainty of 0.15 mm/degree for the 14 DOF.

Conclusions:

  • Presented a novel, highly accurate 14 DOF method for linac mechanical QA.
  • The method offers a more complete and precise assessment of mechanical deformations compared to existing techniques.
  • Validated sub-millimeter accuracy through simulations and experiments, confirming its ability to detect introduced errors with high precision.