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Monte-Carlo and Grid-Based-Deterministic Models for Patient Dose Computation
Published in W. P. M. Mayles, A. E. Nahum, J.-C. Rosenwald, Handbook of Radiotherapy Physics, 2021
Ray tracing (displacement and rotation) is carried out using elementary geometrical relationships. Given that a particle has a (vector) position r0 and (vector) direction u0 and distance to travel s, the new (vector) position, r, is given by:
Conceptual Overview of Algorithms
Published in Jerry J. Battista, Introduction to Megavoltage X-Ray Dose Computation Algorithms, 2019
This approach relies on the superposition of impulse-response functions, a technique often used to solve complex integral equations in many fields of physics and engineering. This has become the most prevalent algorithm in commercial software for clinical treatment planning (Ahnesjö et al. 1987). The required database is a set of energy deposition kernels that are pre-established by computer simulation of x-ray interactions forced to occur at an isolated point in a large homogeneous absorber. For a beam of x-rays, kernels are each intensity-modulated according to the local primary energy released (i.e. TERMA) and resultant patterns of energy propagation are overlapped to compose the overall dose distribution. In heterogeneous tissue, the situation is more complex. Ray tracing is most often used to determine radiological paths that predict not only the primary beam intensity throughout the patient, but also to reshape the kernel pattern for each pair of interaction and dose deposition sites. The convolution-superposition method offers a good compromise between the realism of radiation modelling and practical computation time.
EM behavior when the wavelength is much smaller than the object
Published in James R. Nagel, Cynthia M. Furse, Douglas A. Christensen, Carl H. Durney, Basic Introduction to Bioelectromagnetics, 2018
James R. Nagel, Cynthia M. Furse, Douglas A. Christensen, Carl H. Durney
In-between these extremes are the optical waves, which by most definitions encompass the infrared, visible, and UV regions. These waves are the main focus of this chapter. They share many common features and play an important role in human life. Since their wavelengths (from one-tenth to a few micrometers) are much smaller than the typical object, it is often convenient to describe them in two ways that are different from those used in the previous chapters. The first difference is that diffraction is often unnoticeable for these waves (unless purposely caused by specialized diffraction gratings or other very small structures). This is because, as covered in Section 3.8, the degree of diffraction is proportional to the ratio of wavelength to object size. Since the wavelength is short compared to most objects (for example, a mirror), diffraction is small and does not play a major role in the behavior of the propagating wave. The waves then can be conveniently described by straight-line ray propagation, and ray tracing is used to make optical propagation much easier to visualize and determine. This is the domain of geometrical optics, where the rays follow geometrical rules. Ray tracing is used extensively in the first half of this chapter.
Long‐term corneal multifocal stability following a presbyLASIK technique analysed by a light propagation algorithm
Published in Clinical and Experimental Optometry, 2019
Asier Villanueva, Veronica Vargas, David Mas, Magda Torky, Jorge L Alió
The outcomes of presbyLASIK are affected by the progressive decay in accommodation that occurs with ageing, and criticisms have been raised about this type of LASIK surgery, as limited evidence has been reported in this regard.2009 The subtle changes induced in the anterior corneal surface by the so‐called presbyLASIK techniques may be prone to regress due to epithelial compensation.2009 Analysis using objective methods, such as propagation of wavefronts, allows evaluation of the optical behaviour of the cornea via a single parameter. Light propagation facilitates analysis of the focality of the corneal surface in a given diameter, which is a more complete approach than the study of the means of the main meridians in different diameters.2005 The use of objective methods such as ray tracing has been reported as accurate and highly reproducible.2018 The software used in the present study has been used by Gharaibeh et al.2018 to set the stability of conventional LASIK treatments.
Survival and prognostic factors of pulmonary oligometastases treated with stereotactic body radiotherapy
Published in Acta Oncologica, 2019
Aman Sharma, Marloes Duijm, Esther Oomen-de Hoop, Joachim G. Aerts, Cornelis Verhoef, Mischa Hoogeman, Joost Jan Nuyttens
Details of SBRT procedure and dose fractionation schedules used for pulmonary oligometastases at the institution have been previously described in detail [21–24]. Briefly, the radiation schedule used did depend on the location of tumor. Peripheral lung metastases were treated to 51 Gy or 60 Gy in 3 fractions or to 30 Gy in a single fraction, whereas central tumors (located within 2 cm of trachea and main bronchus) received 50–60 Gy in 5 fractions, except when the tumor was located close to esophagus. In that case, metastases were treated with 6–7 fractions of 8 Gy or 7 fractions of 7 Gy. Dose to target volume was prescribed at the 70–90% isodose line (median 78%), covering at least 95% of the Planning Target Volume. Similar biological effect for different dose-fractionation schedules used in our study was assessed by converting the physical dose to the biological equivalent dose (BED10), assuming an α/β ratio of 10 Gy. To account for the dose calculation inaccuracy of the Ray Tracing as compared to Monte Carlo algorithm [25,26], the BED10 calculated with the Ray Tracing algorithm was corrected by reducing it by 12% for central tumors and 17% to 21% for peripheral tumors [25].
Experimental assessment of phase aberration correction for breast MRgFUS therapy
Published in International Journal of Hyperthermia, 2018
Christopher R. Dillon, Alexis Farrer, Hailey McLean, Scott Almquist, Douglas Christensen, Allison Payne
The results of this study are similar to those found by Mougenot et al. [32], who used ray-trace algorithms based on a MRI-derived model to correct for phase aberrations in both simulations (stochastic ray-tracing method [65]) and MRgFUS experiments (straight-line ray-tracing method [33]). Using a different MRgFUS system and a distinct heterogeneous breast mimicking-phantom, their alternate phase aberration correction technique led to increases in the maximum measured focus temperature rise of 23% and reductions to the FWHM of the focal temperatures of 22%, but the study did not assess focus accuracy.