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Review of Radiation Therapy Physics: A syllabus for teachers ... - IRSN

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<strong>Review</strong> <strong>of</strong> <strong>Radiation</strong> Oncology <strong>Physics</strong>: A Handbook <strong>for</strong> Teachers and Students••One such model uses convolution methods whereby the dose at any point in themedium can be expressed as the sum <strong>of</strong> the primary and scatter components.These models use superposition principles to account <strong>for</strong> both local changes in theprimary fluence as well as changes in the spread <strong>of</strong> energy due to local scatteringcaused by patient and beam geometry. Under specific conditions <strong>of</strong> non-divergentsources and homogeneous phantoms, convolution type integrals can be used tosimplify and speed up these calculations.Monte Carlo or random sampling techniques are used to generate dosedistributions by following the histories <strong>of</strong> a large number <strong>of</strong> particles as theyemerge from the source <strong>of</strong> radiation and undergo multiple scattering interactionsboth inside and outside the patient.• Monte Carlo techniques are able to accurately model the physics <strong>of</strong> particleinteractions by accounting <strong>for</strong> the geometry <strong>of</strong> individual linear accelerators,beam shaping devices such as blocks and multileaf collimators (MLCs), andpatient surface and density irregularities. They allow a wide range <strong>of</strong> complexpatient treatment conditions to be considered. In order to achieve a statisticallyacceptable result, Monte Carlo techniques require the simulation <strong>of</strong> a largenumber <strong>of</strong> particle histories, and are only now becoming practical <strong>for</strong> routinetreatment planning as computing power reduces the calculation time to anacceptable level on the order <strong>of</strong> a few minutes <strong>for</strong> a given treatment plan.••Pencil beam algorithms are common <strong>for</strong> electron beam dose calculations. In thesetechniques the energy spread or dose kernel at a point is summed along a line inphantom to obtain a pencil-type beam or dose distribution. By integrating thepencil beam over the patient’s surface to account <strong>for</strong> the changes in primaryintensity and by modifying the shape <strong>of</strong> the pencil beam with depth and tissuedensity, a dose distribution can be generated.As pointed out by Cunningham, treatment planning algorithms have progressedchronologically to include analytical, matrix, semi-empirical and threedimensionalintegration methods.- The analytical technique as developed by Sterling calculated the dose in themedium as the product <strong>of</strong> two equations, one <strong>of</strong> which modeled the percentdepth dose, the other modeled the beam’s <strong>of</strong>f-axis component. The modelhas been extended to incorporate field shielding and wedge hardening.- Treatment planning computer systems developed in the 1970s began usingthe diverging matrix method <strong>of</strong> beam generation based on measured data.- The Milan-Bentley model was used to calculate diverging fan-lines thatradiate from a source and intersect depth lines located at selected distancesbelow the patient surface. Dose distributions are made by rapidlymanipulating measured data sets consisting <strong>of</strong> central axis percent depthdose and <strong>of</strong>f-axis ratio data sets stored as a function <strong>of</strong> field size. Thesetechniques continue to be used in treatment planning algorithms (Storchiand Woudstra), although they suffer from the perceived disadvantage <strong>of</strong>requiring large amounts <strong>of</strong> measured data, and their limited ability toproperly model scatter and electron transport conditions.321

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