Radiation Therapy: Electron Beam Therapy – Flashcards
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most clinically useful energy range for electrons
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6-20 MeV
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Electron Interactions
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(a) inelastic collisions with atomic electrons (ionization and excitation, collision energy loss), (b) inelastic collisions with nuclei (bremsstrahlung, radiation loss: energy loss rate proportional to Z^2, probability proportional to EZ) (c) elastic collisions with atomic electrons, (no energy loss) and (d) elastic collisions with nuclei. (no energy loss)
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Inelastic collisions
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some of the kinetic energy is lost as it is used in producing ionization or converted to other forms of energy such as photon energy and excitation energy.
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Elastic collisions
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kinetic energy is not lost, although it may be redistributed among the particles emerging from the collision
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What happens as a beam of electrons travels through a medium
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the energy is continually degraded until the electrons reach thermal energies and are captured by the surrounding atoms.
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Collisional Losses (Ionization and Excitation)
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(a) The rate of energy loss depends on the electron density of the medium. (b) The rate of energy loss per gram per centimeter squared, which is called the mass stopping power, is greater for low-atomic-number (Z) materials than for high-Z materials c) the energy loss rate first decreases and then increases with increase in electron energy with a minimum occurring at about 1 MeV. Above 1 MeV, the variation with energy is very gradual. (d) The energy loss rate of electrons of energy 1 MeV and above in water is roughly 2 MeV/cm.
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Why stopping power is lower for high-Z materials
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- high-Z materials have fewer electrons per gram than low-Z materials have - high-Z materials have more tightly bound electrons, which are not as available for this type (collisional) of interaction.
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Radiation Losses (Bremsstrahlung)
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- The rate of energy loss per centimeter in a medium due to bremsstrahlung is approximately proportional to the electron energy and to the square of the atomic number (Z^2). - the probability of radiation loss relative to the collisional loss increases with the electron kinetic energy and with Z. - x-ray production is more efficient for higher-energy electrons and higher-atomic-number absorbers.
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Polarization
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- A high-energy electron loses more energy per gram per square centimeter in a gas than in traversing a more dense medium - The ratio of mass stopping power of water to air varies with electron energy, and consequently, the dose conversion factor for an air ionization chamber reading in water (or another condensed medium) varies with depth.
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total mass stopping power (S/ρ)tot
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- of a material for charged particles - defined as the quotient of dE by ρdl, where dE is the total energy lost by the particle in traversing a path length dl in the material of density ρ: (s/p)tot = (s/p)col + (s/p)rad
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Restricted collision stopping power
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- refers to the linear energy transfer (LET) concept - the rate of energy loss per unit path length in collisions in which energy is "locally" absorbed, rather than carried away by energetic secondary electrons. - dE is the energy lost by a charged particle in traversing a distance dl as a result of those collisions with atomic electrons in which the energy loss is less than Δ
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Electron Scattering
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- When a beam of electrons passes through a medium, the electrons suffer multiple scattering due to coulomb force interactions between the incident electrons and, predominantly, the nuclei of the medium. - the angular and spatial spread of a narrow, collimated beam of electrons can be approximated by a Gaussian distribution
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mass angular scattering power
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- the quotient ø^2/ρ, where ø^2 is the mean square scattering angle - ø^2/ρ is proportional to Z^2/E^2 - The scattering power varies approximately as the square of the atomic number and inversely as the square of the kinetic energy. - why high-Z materials are used in the construction of scattering foils. (more Bremms produced)
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Issue of high-Z scattering foils
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x-rays produced are an issue for high z materials when using electrons; neutrons produced are an issue for high z materials when using protons
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Electron Beam Energy Specification and Measurement
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- the random energy degradation that the electrons suffer as they pass through the exit window, scattering foil, monitor chambers, air, and other materials results in the beam taking on a spectrum of energies at the phantom surface. - there are several methods that can be used to determine this energy: measurement of threshold energy for nuclear reactions; range measurements; and the measurement of Cerenkov radiation threshold. Of these, the range method is the most practical and convenient for clinical use.
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Most Probable Energy
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- (Ep)o defined by the position of the spectral peak - (Ep)o = C1 + C2Rp + C3Rp^2 where Rp is the practical range in centimeters (C1 = 0.22 MeV, C2 = 1.98 MeV cm-1, and C3 = 0.0025 MeV cm-2)
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Practical range
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Rp, is the depth of the point where the tangent to the descending linear portion of the curve (at the point of inflection) intersects the extrapolated background
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beam divergence correction
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each point on the depth ionization curve should be corrected for beam divergence before the range is determined. The correction factor is (f+z / f)^2 , where f is the effective source to surface distance (SSD) and z is the depth
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Mean Energy
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- Eo, at the phantom surface - Eo = C4 * R50 where R50 is the depth at which the dose is 50% of the maximum dose (C4 = 2.33 MeV cm-1 for water)
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depth ionization curve
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- depth ionization curve is equal to depth dose curve for photons. - NOT equal for electrons, because ionization represents dose to air and when converted to dose to water multiplied by L/p ratio, which varies as electron spectrum changes. (energy changes, L is function of energy!) - for photons, dose to air is converted to dose to water by multiplying by u/p ratio, which is constant for a certain photon energy. - mistake in TG21: R50 should be D50, should come from depth dose curve (not depth ionization curve)
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Energy at Depth
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(Ep)z = (Ep)o(1-z/Rp) [most probably energy at depth z] Ez= Eo(1-z/Rp) [mean energy at depth z)
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Ionization Chambers
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- perturbation and displacement corrections are required for cylindrical chambers. - A general equation for obtaining percent depth dose in water (%DW) from ion chamber measurements made in any medium or phantom: %Dw= ({M * (L/p)W,air * (ø)W,med * Prepl}/{"}max) * 100 where the quantities in the numerator are determined at the effective depth of measurement and the denominator equals the value of the numerator at the depth of maximum dose.
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Ion chamber: Displacement correction
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- chamber should be positioned int he deepest point of the maximum to avoid e- contamination - the effective point of measurement is displaced upstream - 0.45r for 3 to 10MeV, 0.6r for 10 to 30MeV, 0.5r for all energy and front surface for thin plane-parallel chamber
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Ion Chamber: perturbation/replacement correction
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- caused by the air cavity in the ionization chamber - depending on e- E (significant perturbation at low energies) and size of the cavity - Prepl = PgradientPfluence - fluence is not changed significantly for photons -gradient varies more for electrons
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plane-parallel chambers
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for low energy, plane-parallel chamber is recommended because it's better at measuring dose at the surface (significant for low E e-s)
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Silicon Diodes
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- advantages in terms of small size and high sensitivity - diodes suffer from energy and temperature dependence and can be damaged by radiation - absolute dosimetry with diodes is not recommended. - the variation of silicon-to-water stopping power ratio with electron energy is quite minimal (∼5% between 1 and 20 MeV), measurements made with a diode may be used directly to give depth dose distributions
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Film
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- The energy independence of film may be explained by the fact that the ratio of collision stopping power in emulsion and in water varies slowly with electron energy - the optical density of the film can be taken as proportional to the dose with essentially no corrections. - Film is useful for a variety of dosimetry problems such as determining practical range, isodose curves, and beam flatness. - cannot be used reliably for absolute dosimetry because the optical density of a film exposed to electrons depends on many variables such as emulsion, processing conditions, magnitude of absorbed dose, and some measurement conditions, which can give rise to serious artifact - The use of film is, therefore, restricted to relative dosimetry
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Phantoms
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- Water is the standard phantom for the dosimetry of electron beams - For a phantom to be water equivalent for electron dosimetry it must have the same linear stopping power and the same linear angular scattering power. - This is approximately achieved if the phantom has the same electron density (number of electrons per cubic centimeter) and the same effective atomic number as water. - Of the commonly used materials for electron dosimetry, polystyrene and electron solid water - Dw(dw) = Dmed(dmed)*(S/p)w,med*(ø)w,med
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Effective density
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- because of differences in stopping power and scattering power among different phantoms, it is not possible to find corresponding depths at which the energy spectra are identical. - An effective density may be assigned to a medium to give water-equivalent depth dose distribution near the therapeutic range and along the descending portion of the depth dose curve - dw = dmed * peff = dmed (R50(water)/R50(med))
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high-energy electron energy loss rate
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high-energy electrons lose energy at the rate of about 2 MeV/cm of water or soft tissue.
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Central Axis Depth Dose Curves
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- The most useful treatment depth, or therapeutic range, of electrons is given by the depth of the 90% depth dose. - For modern accelerators with trimmer-type applicators this depth is approximately given by E/3.2 cm, where E is the most probable energy in MeV of the electron beam at the surface. - The depth of the 80% depth dose occurs approximately at E/2.8 cm. The depth of Dmax does not follow a linear relationship with energy but it covers a broad region and its value may be approximated by 0.46 E^0.67
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Choice of electron beam energy
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- the dose decreases abruptly beyond the 90% dose level - when in doubt, use a higher electron energy to make sure that the target volume is well within the specified isodose curve - skin-sparing effect with the clinical electron beams is only modest or nonexistent. - the percent surface dose for electrons increases with energy. - At the lower energies, the electrons are scattered more easily and through larger angles. - higher energy —> more forward scattering (lower angular scattering power)—> lateral dose constricted
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Field Flatness
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- The ICRU specifies beam flatness in terms of a uniformity index, defined in a reference plane and at a reference depth as the ratio of the area where the dose exceeds 90% of its value at the central axis to the geometric beam cross-sectional area at the phantom surface. - The uniformity index should exceed a given fraction (e.g., 0.80 for a 10 × 10-cm field size and at depth of maximum dose) - The AAPM (20) recommends that the flatness of an electron beam be specified in a reference plane perpendicular to the central axis, at the depth of the 95% isodose beyond the depth of dose maximum. - The variation in dose relative to the dose at central axis should not exceed ±5% (optimally to be within ±3%) over an area confined within lines 2 cm inside the geometric edge of fields equal to or larger than 10 × 10 cm.
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Beam symmetry
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- compares a dose profile on one side of the central axis to that on the other. - The AAPM recommends that the cross-beam profile in the reference plane should not differ more than 2% at any pair of points located symmetrically on opposite sides of the central axis.
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scattering foils
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- the first foil widens the beam by multiple scattering, - the second foil is designed to make the beam uniform in cross section
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beam collimation
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- The beam-defining collimators are designed to provide a variety of field sizes and to maintain or improve the flatness of the beam. - all collimators provide (a) primary collimation close to the source that defines the maximum field size and (b) secondary collimation close to the patient to define the treatment field. (can be in the form of trimmer bars or a series of cones)
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Field size dependence
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- The dose increases with field size because of the increased scatter from the collimator and the phantom. - the depth dmax shifts toward the surface for the smaller fields.
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lateral scatter equilibrium
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the minimum field radius for the establishment of lateral scatter equilibrium at all depths on central axis is given by the following approximate relationship: Req=~0.88sqrt(Ep,o) where Req is the field radius in cm and Ep,o is the most probable energy in MeV
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Field equivalence
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- means that for the same incident fluence and cross-sectional beam profile, the equivalent fields have the same depth dose distribution along the central ray. - all broad fields are equivalent because their depth dose distribution is the same irrespective of field size. (eg. 10 × 10, 10 × 15, 10 × 20, 20 × 20, etc., are all broad fields for energies up to 30 MeV) - for a square field of cross section (2a × 2a) the equivalent circular field has a radius Requiv, given by: Req=~1.116a - or a small rectangular or irregularly shaped fields, field equivalence is not as straightforward.
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virtual source
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- an intersection point of the backprojections along the most probable directions of electron motion at the patient surface - found by the backprojection of the 50% width of the beam profiles obtained at different distances. A broad beam (≥20 × 20 cm) is used for these measurements.
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virtual SSD
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- The use of virtual SSD does not give accurate inverse square law correction for output at extended SSDs under all clinical conditions. - Measurements have shown that the virtual SSD gives correct inverse square law factor only for large field sizes. - For small field sizes, the inverse square law correction underestimates the change in output with virtual SSD. - deviation from the inverse square law is caused by an additional decrease in output because of a loss of side-scatter equilibrium in air and in phantom that is significant for small field sizes and low electron energies.
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effective SSD
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- gives the correct inverse square law relationship for the change in output with distance - doses are measured in a phantom at the depth of maximum dose (dm), with the phantom first in contact with the cone or at the standard SSD (zero gap) and then at various distances, up to about 20 cm from the cone end - f = effective SSD, I0 = dose with zero gap, and Ig = dose with gap γ between the standard SSD point and the phantom surface. - Io/Ig=(f+dm+g / f+dm)^2 --> plot sqrt(Io/Ig) vs g to find slope=1/(f+dm) --> f=1/slope - dm - the effective SSD does change with energy and field size, especially for small field sizes and low energies.
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x-ray contamination
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- The x-ray contamination dose at the end of the electron range can be determined from the tail of the depth dose curve by reading off the dose value at the point where the tail becomes straight - depends very much on its collimation system - dose in a patient is contributed by bremsstrahlung interactions of electrons with the collimation system (scattering foils, chambers, collimator jaws, etc.) and the body tissues.
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e- treatment planning: choice of energy
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- when there is no danger of overdosing a critical structure beyond the target volume, the beam energy may be set so that the target volume lies entirely within the 90% isodose curve - Beyond the 80% depth dose, the dose falloff is characteristically rapid at these beam energies.
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e- treatment planning: choice of field size
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- should be strictly based on the isodose coverage of the target volume. - there is a significant tapering of the 80% isodose curve at energies above 7 MeV - with e-s, a larger field at the surface than one is usually accustomed to (in the case of photon beams) may be necessary to cover a target area adequately.
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Corrections for Air Gaps and Beam Obliquity
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- When the beam is incident obliquely on the patient surface, the point at the shallow depth receives greater side scatter from the adjacent pencil beams, which have traversed a greater amount of material, whereas the point at the greater depth receives less scatter. - because the beam is divergent, the dose will also decrease at all depths as a result of the inverse square law effect, as the air gap between the cone end and the surface increases with the increase in the angle of obliquity. - the depth dose at a point in an obliquely incident beam is affected both by the "pencil scatter effect" and the beam divergence.
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depth dose with obliquity/air gap
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Let D0(f, d) be the dose at a point at depth δ for a beam incident normally on a flat-surfaced phantom with an effective SSD = f. When the cone is placed on the chest wall, the depth dose D(f + g,d) will be given by: D(f+g,d) = Do(f,d)*(f+d / f+g+d)^2*OF(θ,d) where g is the air gap and OF(θ,d) is the obliquity factor for the pencil beam scatter effect discussed previously.
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Obliquity factor
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OF(θ,d) accounts for the change in depth dose at a point if the beam angle θ changes relative to the surface without change in the distance from the point to the effective source position.
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coefficient of equivalent thickness (CET)
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- the attenuation by a given thickness z of the inhomogeneity is equivalent to the attenuation (z × CET) of water. - The CET for a given material is approximately given by its electron density (electron/mL) relative to that of water. - deff = d-z(1-CET) where d is the actual depth of point P from the surface. [effective depth = depth - thickness + effective thickness] - the depth dose is read from dose distribution data for water at the effective depth. - additional inverse sq law correction: (f+deff / f+d)^2 where f is effective SSD
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Small inhomogeneities
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- causes cold spots and hot spots behind its edges.
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Bolus
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- often used in electron beam therapy to (a) flatten out an irregular surface, (b) reduce the penetration of the electrons in parts of the field, and (c) increase the surface dose. - Ideally, the bolus material should be equivalent to tissue in stopping power and scattering power.
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Adjacent Fields
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- When two adjacent electron fields are overlapping or abutting, there is a danger of delivering excessively high doses in the junction region. - On the other hand, separating the fields may seriously underdose parts of the tumor - the extent of hot and cold spots depends on the electron beam SSD
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Field Shaping
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- For lower-energy electrons (<10 MeV), less than 5-mm thickness of lead is required for adequate shielding (e.g., ≤5% - lead sheets of thick thickness can be molded to conform to surface contour and can be placed directly on the skin surface. - The alternative method is to support a lead cutout at the end of the treatment cone or the field trimmers - The thickness for shielding can be chosen on the basis of allowable transmission (e.g., 5%).
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Transmission curve
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- a plot of ionization current as a function of shield thickness.
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External shielding rule of thumb
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- The minimum thickness of lead required for blocking in millimeters is given by the electron energy in MeV incident on lead divided by 2. Another millimeter of lead may be added as a safety margin.
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Effect of blocking on dose rate
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- The ICRU suggested Rp as the lower limit for field diameter, above which the field size dependence of the depth dose is negligible. - That means that for a given point of interest in an irregularly shaped field, the field edges should be farther than Rp/2 for the lateral scatter equilibrium to be approximately achieved. - The minimum field diameter for approximate lateral scatter equilibrium (LSE) is given by E (MeV)/2.5 in centimeters of water
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Internal Shielding
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- enhancement in dose at the tissue-lead interface can be quite substantial (e.g., 30% to 70% in the range of 1 to 20 MeV), having a higher value for the lower-energy beams. - EBF is the electron backscatter factor, defined as the quotient of the dose at the interface with the lead present to that with a homogeneous polystyrene phantom at the same point. - Ēz is the average electron energy incident at the interface. - To dissipate the effect of electron backscatter, a suitable thickness of low-atomic-number absorber such as bolus may be placed between the lead shield and the preceding tissue surface
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Electron ARC Therapy
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- On the basis of isodose distribution, electron arc therapy is most suited for treating superficial volumes that follow curved surfaces such as the chest wall, ribs, and entire limbs. - Dose per arc can be determined in two ways: (a) integration of the stationary beam profiles and (b) direct measurement. - relative dose will be higher at the same depth using arc method, because deeper depths will be irradiated more times at different angles, while surface will be irradiated less times. - the larger the arc radius, the more the pdd will increase, because the treatment time is longer (the larger the portion in the middle will be irradiated more than the rest)
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ARC: Beam Energy
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- the depth dose curve shifts slightly and the beam appears to penetrate somewhat farther than for a stationary beam - The surface dose is reduced and the bremsstrahlung dose at the isocenter is increased (Bremss stronger along the central axis (x-ray production has more intensity that is forward-directed)—> Bremms will focus on center where central axis is focused during the whole treatment (from different angles) so it will be concentrated in center) - "velocity effect": A deeper point is exposed to the beam longer than a shallower point, resulting in apparent enhancement of beam penetration.
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ARC: Scanning Field Width
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- smaller scanning fields (e.g., width of 5 cm or less) give lower dose rate and greater x-ray contamination - small field widths allow almost normal incidence of the beam on the surface, thus simplifying dosimetry - the dose per arc is less dependent on the total arc angle - a geometric field width of 4 to 8 cm at the isocenter is recommended for most clinical situations.
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Total Skin Irradiation
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- Electrons in the energy range of 2 to 9 MeV have been found useful for treating superficial lesions covering large areas of the body, such as mycosis fungoides and other cutaneous lymphomas. - At these energies, electron beams are characterized by a rapid falloff in dose beyond a shallow depth and a minimal x-ray background (1% or less).
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ETSI: Translational Technique
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- the patient may be stationary and the radiation source translated horizontally - maximum energy of the β particles emitted by 90Sr is 2.25 MeV. - the effective depth of treatment in this case is only a fraction of a millimeter.
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ETSI: Large Field Technique
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- Large electron fields required for total body skin irradiation can be produced by scattering electrons through wide angles and using large treatment distances. - The field is made uniform over the height of the patient by vertically combining multiple fields or vertical arcing. - Low-energy electron beams are considerably widened by scattering in air. - For example, a 6-MeV narrow electron beam, after passing through 4 m of air, achieves a Gaussian intensity distribution with a 50% to 50% width of approximately 1 m - A proper combination of more such fields or a continuous arc can lead to a larger uniform field, sufficient to cover a patient from head to foot
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ETSI: X-ray contamination
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- The bremsstrahlung level can be minimized if the electron beam is scattered by air alone before incidence on the patient. - Stanford technique: the electron beam, after emerging from the accelerator window, is scattered by a mirror (0.028-inch Al), an aluminum scatterer located externally at the front of the collimator (0.037-inch Al), and about 3 m of air before incidence on the patient. The x-ray contamination incident on the patient is reduced by angling the beam 10 degrees to 15 degrees above and below the horizontal --> a large electron field with sufficient dose uniformity in the vertical dimensions of the patient. - field arrangement: the patient is treated with six fields (anterior, posterior, and four obliques) positioned 60 degrees apart around the circumference of the patient
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clinically useful energy range of electons
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- 6 to 20 MeV. - At these energies, electron beams can be used for treating superficial tumors (<5 cm deep).
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Electrons interact with matter by
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(a) ionization and excitation—the most predominant interaction in soft tissues, (b) bremsstrahlung—more significant at higher energies and in higher-atomic-number materials, (c) elastic scattering by atomic nuclei, and (d) elastic scattering by orbital electrons. - Collisions resulting in secondary electron production (δ rays) are possible but rare.
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electron beam energy specification
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Energy of clinical electron beams is specified by the most probable energy at the surface.
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rate of energy loss with depth in water
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(or soft tissue) approximately 2 MeV/cm.
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dose distribution
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- Electron beams have a modest skin-sparing effect, which gradually disappears with increasing energies. - The lower the energy, the sharper is the dose dropoff beyond the therapeutic range. - Depth dose distribution can be determined by ion chambers, diodes, and film. - Percent surface dose increases with increase in energy.
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collimation
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Electron applicators are necessary to collimate the beam close to the patient surface.
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dual scattering foils
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Acceptable beam flatness is achieved by dual scattering foils.
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PDD & field size
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Percent depth dose (PDD) and output vary with field size if the field size is smaller than that required for lateral scatter equilibrium.
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Virtual / Effective SSD
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Virtual SSD determines beam divergence. Effective SSD gives agreement with inverse square law.
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Beam obliquity
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Beam obliquity changes the PDD, giving rise to increase in dose at dmax and decrease in depth dose beyond.
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Changes in surface contour
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Steep changes in surface contour give rise to "hot" and "cold" spots.
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Large inhomogeneities
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For large slabs of inhomogeneities, PDD corrections can be made using CET or effective depth based on electron density (number of electrons/cm3 relative to water).
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Small Inhomogeneities and heterogeneities edges
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Around small inhomogeneities and at the steep edges of heterogeneities, electron scatter gives rise to "hot" and "cold" spots.
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adjacent beams
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Adjacent electron-electron or electron-photon fields give rise to "hot" and "cold" spots due to beam divergence and inter-field electron scattering.
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Bolus use
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Bolus may be used to build up surface dose or to decrease beam energy. In either case, bolus must be placed directly on the skin surface.
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Field shaping materials
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Field shaping can be done with lead or Cerrobend cutouts.
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Transmission vs lead thickness
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Minimum thickness of lead required for ∼5% transmission of dose is approximately E/2 in millimeters of lead, where E is the beam energy in MeV incident on lead.
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Issue of Internal shielding
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Internal shielding is possible in some cases, but dose enhancement due to backscatter from lead must be taken into account.
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Electron ARC Therapy
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Electron arc therapy is feasible for tumors along curved surfaces, but custom shielding (molded on to the surface) is required to define the treatment field and sharpen dose distribution at the field edges.
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Total skin electron irradiation
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Total skin electron irradiation is a useful technique for the treatment of mycosis fungoides. Considerable dosimetry is required before commissioning the procedure for actual treatments.
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Pencil beam algorithm
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Pencil beam algorithms, based on multiple scattering theory, are currently the algorithms of choice for electron beam treatment planning.
