Radiation Therapy: Treatment Planning II – Flashcards
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Body Contours
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Acquisition of body contours and internal structures is best accomplished by imaging (computed tomography [CT] and magnetic resonance imaging, etc.) with the patient positioned the same way as for actual treatment.
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body contour devices
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- A number of devices have been made to obtain patient contours - most common and the simplest of the devices is a solder wire or a lead wire embedded in plastic.
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Manual contour making
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- The patient contour must be obtained with the patient in the same position as used in the actual treatment - Important bony landmarks as well as beam entry points, if available, must be indicated on the contour. - Checks of body contour are recommended during the treatment course if the contour is expected to change due to a reduction of tumor volume or a change in patient weight.
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internal Structures
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Localization of internal structures for treatment planning should provide quantitative information in regard to the size and location of critical organs or inhomogeneities.
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Computed Tomography
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- By assigning different levels to different attenuation coefficients, an image can be reconstructed that represents various structures with different attenuation properties. Such a representation of attenuation coefficients constitutes a CT image. - Helical CTs are faster and provide better visualization of anatomy and target volumes.
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Hounsfield numbers
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Ct numbers H = utissue-uwater / uwater * 1000
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Radiographic Simulator
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an apparatus that uses a diagnostic x-ray tube but duplicates a radiation treatment unit in terms of its geometric, mechanical, and optical properties. The main function of a simulator is to display the treatment fields so that the target volume may be accurately encompassed without delivering excessive irradiation to surrounding normal tissues. - Specifications of a treatment simulator must closely match those of the treatment unit. - unforeseen problems with a patient setup or treatment technique can be solved during simulation, thus conserving time within the treatment room.
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CT Simulator
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- The nomenclature of virtual simulation arises out of the fact that both the patient and the treatment machine are virtual—the patient is represented by CT images and the treatment machine is modeled by its beam geometry and expected dose distribution. - The simulation film in this case is a reconstructed image called the DRR (digitally reconstructed radiograph), which has the appearance of a standard 2-D simulation radiograph but is actually generated from CT scan data by mapping average CT values computed along ray lines drawn from a "virtual source" of radiation to the location of a "virtual film." - DRR is essentially a calculated (i.e., computer-generated) port film that serves as a simulation film.
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Port Films
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- The primary purpose of port filming is to verify the treatment volume under actual conditions of treatment. - As a treatment record, a port film must be of sufficiently good quality so that the field boundaries can be described anatomically - for optimum resolution, one needs a single emulsion film with a front lead screen and no rear screen.
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Corrections for contour irregularities
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- the beam may be obliquely incident with respect to the surface and, in addition, the surface may be curved or irregular in shape. - Under such conditions, the standard dose distributions cannot be applied without proper modifications or corrections. - Contour corrections may be avoided by using a bolus or a compensator
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Effective Source to Surface Distance Method
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Suppose DA is the dose at point A. Assuming beam to be incident on a flat surface located at S′-S′: Da=D'max*P' where P′ is percent depth dose at A relative to D′max at point Q′. Suppose Pcorr is the correct percent depth dose at A relative to Dmax at point Q. Da=Dmax*Pcorr --> the SSD is increased by a distance h Pcorr = P'*(SSD+dm/SSD+h+dm)^2 (P' from SSD+h)
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Tissue-air (or Tissue-maximum) Ratio Method
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- depends on the principle that the tissue-air, or tissue-maximum, ratio does not depend on the SSD and is a function only of the depth and the field size at that depth - correction factor (CF) = T(d,r)/T(d+h,r) where T stands for tissue-air ratio or tissue-maximum ratio and rA is the field size projected at point A (i.e., at a distance of SSD + δ + h from the source). --> Pcorr = P(from SSD) * CF
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Corrections for Tissue Inhomogeneities
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(a) changes in the absorption of the primary beam and the associated pattern of scattered photons and (b) changes in the secondary electron fluence. - Changes in the associated photon scatter distribution alters the dose distribution more strongly near the inhomogeneity than farther beyond it. - The changes in the secondary electron fluence, on the other hand, affects the tissues within the inhomogeneity and at the boundaries.
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Corrections for Beam Attenuation and Scattering: Tissue-air Ratio Method
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- given electron density ρe - The following CF applies to the dose at P if the entire phantom was water equivalent: CF = T(d',rd)/T(d,rd) where d′ is the equivalent water depth (i.e., d′ = d1 + ρe d2 + d3) and d is the actual depth of P from the surface; rd is the field size projected at point P. - method only considers 1 dimension, and only corrects the area after the inhomogeneity (not before, through, or at the interface) - use electron density because main interaction is compton effect which varies by pe (if were using orthovoltage range then photoelectric effect will vary by z)
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Corrections for Beam Attenuation and Scattering: Power Law Tissue-air Ratio Method
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CF = [T(d2+d3,rd)/T(d3,rd)]^pe^-1 - the correction factor does depend on the location of the inhomogeneity relative to point P but not relative to the surface - allows for correction of the dose to points within an inhomogeneity as well as below it. This is given by: CF = T(d3,rd)^(p3-p2) / T(d2+d3,rd)^(1-p2) where ρ3 is the density of the material in which point P lies and d3 is its depth within this material. ρ2 is the density of the overlying material, and (d2 + d3) is the depth below the upper surface of it.
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Corrections for Beam Attenuation and Scattering: Equivalent Tissue-air Ratio Method
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CF = T(d',r')/T(d,r) where d′ is the water-equivalent depth, d is the actual depth, r is the beam dimension at depth d, r′ = r · p~ = scaled field size dimension, and p~ is the weighted density of the irradiated volume. --> no single equation is better than the other; it varies with energy, size, per case
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Corrections for Beam Attenuation and Scattering: Isodose Shift Method
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The isodose curves beyond the inhomogeneity are moved by an amount equal to n times the thickness of the inhomogeneity as measured along a line parallel to the central axis and passing through the point of interest.
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Typical corrections factors for beam attenuation and scattering
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(a) the TAR method overestimates the dose for all energies, (b) the ETAR is best suited for the lower-energy beams (≤6 MV), and (c) the generalized Batho method is the best in the high-energy range (≥10 MV). Thus, the accuracy of different methods depends on the irradiation conditions (e.g., energy, field size, location and extent of inhomogeneity, and location of point of calculation).
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bone attenuation correction in MV rang
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In the megavoltage range, the corrections for bone attenuation in most clinical situations are small and are usually neglected. However, as the x-ray energy increases beyond 10 MV, the shielding effect begins to increase because pair production becomes significant.
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Dose to tissues beyond lung
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- lung is low density —> there is an increase in dose to tissues beyond the lung (because less was attenuated along that path) (5cm of lung is like passing through only 2cm of water) - the higher the energy the less significant this effect will be (because beam will be more penetrative anyways)
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Dose to tissues beyond bone
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- compact bone (not spongy) is higher density than tissues --> decrease in dose to tissues beyond bone - higher energy beams will see less of a reduction in dose because they are more penetrative - At lower energies (orthovoltage), for bone there will be more photoelectric effect, so u/p will be more dependent on that factor (on z). - At 6-10mV there will be compton mainly, so bones will see more dose than tissue but not as dramatically as in lower energies. - above 10MV dose difference will grow again because of pair production beginning
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Dose to Soft Tissue in Bone
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- D(STB) = Db*(S/p)st,b where DB is the dose to the surrounding bone matrix and ( S/ρ)ST,B is the ratio of average mass collision stopping power of soft tissue to bone for the electrons. - the dose at a point in the bone mineral is related to the dose (DST) at the same point if the bone is replaced by a homogeneous medium of soft tissue: Db=Dst*(uen/p)b,st -> Db/Dst = (uen/p)b,st * (S/p)st,b
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Soft Tissue Surrounding Bone
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- On the entrance side of the photon beam, there is a dose enhancement in the soft tissue adjacent to the bone. - Because of the very short range of the backscattered electrons, the enhancement effect is limited only to a few millimeters - For energies up to 10 MV, the dose at the interface is initially less than the dose in a homogeneous soft tissue medium but then builds up to a dose that is slightly greater than that in the homogeneous case. - For higher energies, there is an enhancement of dose at the interface because of the increased electron fluence in bone due to pair production. The effect decreases with distance and lasts up to the range of the electrons
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bone/tissue interfaces
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- (uen/p)bone/tissue = 0.98 for E<10MV (due to compton) - (uen/p)bone/tissue= 1.02-1.04 for 10<E<25MV (due to pair production) - Lower energy sees more backscatter (because larger scattering angles occur at lower energies) - higher energies will have more forward scattering - for low energies, dose builds up faster than high energy (which diminishes the forward scatter effect with distance) - dose distributions are normally not corrected for the presence of bone when using megavoltage photon beams.
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Lung Tissue
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- loss of lateral electronic equilibrium when a high- energy photon beam traverses the lung - Because of the lower density of lung, an increasing number of electrons travel outside the geometric limits of the beam. This causes the dose profile to become less sharp. For the same reason there is a greater loss of laterally scattered electrons, causing a reduction in dose on the beam axis.
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Air Cavity
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- The most important effect of air cavities in megavoltage beam dosimetry is the partial loss of electronic equilibrium at the cavity surface. - not enough lateral scatter—> lower dose results - The actual dose to tissue beyond and in front of the cavity may be appreciably lower than expected. - The most significant decrease in dose occurs at the surface beyond the cavity, for large cavities (4 cm deep) and the smallest field (4 × 4 cm)
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Bolus
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a tissue-equivalent material placed directly on the skin surface to even out the irregular contours of a patient to present a flat surface normal to the beam.
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Buildup bolus
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a bolus layer, which is thick enough to provide adequate dose buildup over the skin surface.
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compensating filter
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- approximates the effect of the bolus as well as preserves the skin-sparing effect. - To preserve the skin-sparing properties of the megavoltage photon beams, the compensator is placed a suitable distance (≥20 cm) away from the patient's skin.
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Design of Compensators
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- the use of a compensator to provide the required beam attenuation that would otherwise occur in the "missing" tissue when the body surface is irregular or curved - the dimensions and shape of the compensator must be adjusted because of (a) the beam divergence, (b) the relative linear attenuation coefficients of the filter material and soft tissues, and (c) the reduction in scatter at various depths when the compensator is placed at a distance from the skin rather than in contact with it. - To compensate for this scatter, the compensator is designed such that the attenuation of the filter is less than that required for primary radiation only.
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density ratio or thickness ratio
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- The required thickness of a tissue-equivalent compensator along a ray divided by the missing tissue thickness along the same ray - The thickness ratio depends, in a complex way, on compensator to surface distance, thickness of missing tissue, field size, depth, and beam quality. - an average value of 0.7 for τ may be used for all irradiation conditions provided d greater than or equal to 20 cm (usually just need 70% of the thickness "missing")
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compensator thickness
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(τc)=TD*(τ/ρc) where TD is the tissue deficit at the point considered and ρc is the density of the compensator material. - use denser materials than water-equivalent materials to minimize space/size of compensator (can be smaller if its denser)
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Compensator ratio (CR)
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the ratio of the missing tissue thickness to the compensator thickness necessary to give the dose for a particular field size and depth. CR = TD/tc = ρc/τ
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Patient Positioning
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- CT simulation will be used for treatment planning—> need patient positioning to be identical during sim and during treatment for plan to be applied correctly - random uncertainty- day to day changes in setup that are inevitable (body motion, slight machine movement, matter in bowel/bladder) - in-room positioning lasers cross at isocenter of machine —> use laser positioning on patient surface to tattoo marks of where lines will be positioned when moving from sim to treatment - PTV considers uncertainty volume (so even with some movement we hit all of tumor)
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Treatment planning requirements
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Treatment planning requires accurate patient data acquisition such as external body contours and internal anatomy.
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devices for body contouring
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Manual devices for body contouring consist of solder wires, pantograph-type contour plotters, and electromechanical devices. Current methods primarily use CT scans for contour information.
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CT numbers
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- CT numbers bear a linear relationship with attenuation coefficients. - CT numbers depend on electron density (electrons/cm3) as well as atomic number, if the scanning beam used is kilovoltage x-rays as in conventional CT scanners. - Correlation between CT numbers and electron density of various tissues is established by scanning phantoms of known electron densities in the range that includes lung, muscle, and bone.
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Imaging Modalities
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- modalities such as ultrasound, MRI, and PET are useful in mapping out structural and/or functional anatomy, but their signal values are not correlated with electron density. - Fusion techniques are used to combine their images with the corresponding CT images to provide geometric accuracy of external contour and internal bony anatomy. - Radiographic and/or CT simulators are an essential part of the treatment-planning process.
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treatment verification
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Accelerator-mounted accessories such as EPID and cone-beam CT systems allow treatment verification before and during actual treatments. These capabilities are essential when using conformal radiation therapy techniques such as 3-D CRT, intensity-modulated radiation therapy (IMRT), and IGRT.
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correction-based algos
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Manual methods of contour and tissue heterogeneity corrections are semi-empirical and have given way to computer algorithms for treatment planning. These are collectively called correction-based algorithms. Currently, the most sophisticated algorithms are model-based (e.g., pencil beam, convolution/superposition, and semi- Monte Carlo techniques).
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Dose within bone
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- depends on beam energy. - (uen/p)bone/tissue = 0.98 for E<10MV (due to compton) - (uen/p)bone/tissue= 1.02-1.04 for 10<E<25MV (due to pair production)
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Dose to soft tissue within bone
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is two to five times higher in the orthovoltage and superficial range of beam energies. It is about 3% to 10% higher in the megavoltage range used clinically (Table 12.5).
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Dose at air cavity surfaces
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Dose reduction can occur at air cavity surfaces and lung due to a partial loss of electronic equilibrium. The effect is more pronounced for small fields (6 MV).
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Compensators
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- used to compensate for missing tissue at the surface or internal inhomogeneities such as lung. - Their design takes into account the extent of missing tissue, compensator to surface distance (or thickness ratio), and the density of the compensator material.
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requirements for precision radiotherapy
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Reproducible and stable patient positioning;; proper immobilization;; accurate measurements of external and internal bony landmarks;; precise X,Y,Z couch motions;; and isocentric accuracy are crucial requirements for precision radiotherapy.
