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107 Cards in this Set
- Front
- Back
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Factors that Affect Monitor Units
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-beam energy
-calculation depth -prescribed dose -tissue density -source to skin distance -source to axis distance -collimator field size -treatment field size -beam modifiers |
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Dmax
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-depth where electronic equilibrium occurs for photon beams
-depth of maximum dose or given dose -dependent on photon energy -provides skin sparing effect, which is diminished by any sort of bolus -point where %DD=100 -%DD falls less rapidly as the photon energy increases |
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Dmax of Co-60 beam
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0.5 cm
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Dmax of 4 MV beam
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1.0 cm
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Dmax of 6 MV beam
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1.5 cm
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Dmax of 10 MV beam
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2.5 cm
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Dmax of 15 MV beam
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3.0 cm
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Dmax of 18 MV beam
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3.5 cm
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Dmax of 24 MV beam
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4.0 cm
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Half-Value Thickness
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-thickness of a material that will reduce the beam's intensity or penetration to half it's original value
-used to describe orthovoltage beams -usually aluminum, copper, or tin -high energy beams are described in MV or the accelerating voltage |
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Dose Rate
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-measurement of dose of a treatment machine
-describes the amount of exposure produced and measured at a specific distance for a reference field size of 10 x 10 |
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Dose Rate & Field Size
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-as field size increases, dose rate increases
-due to increased scatter output -not recommended to use a field size less than 4 x 4 |
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Dose Rate & Distance
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-inverse square law represents the change in beam intensity caused by divergence of the beam
-as the beam diverges, the beam intensity decreases |
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Inverse Square Factor
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(SSD1/SSD2)2
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Distance Factor Correction
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-used for isocentric calculations
-(Dmax depth of beam + 100/100)2 |
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Percent Depth Dose
%DD PDD |
-ratio expressed as a percentage of the absorbed dose at a given depth to the absorbed dose at a fixed depth, usually Dmax
-determined using the field size treating the patient -used for SSD calculations -also called Depth Dose Ratio, or DDR |
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Factors That Affect PDD
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-energy
-SSD or depth -field size -tissue type -distance from source |
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PDD & Energy
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-PDD increases as energy increases
-higher energies are more penetrating, so a greater percentage is available |
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PDD & SSD
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-PDD increases as SSD increases due to the Mayneord F factor
-due to the inverse square law and increased scatter production |
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Mayneord F factor
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-inverse square correction for the PDD
-F Factor= (SSD2 + Dmax)2 (SSD1 + depth)2 ----------- x ------------ x (SSD1 + Dmax) (SSD2 + depth) PDD old or PDD of SSD2=F x PDD of SSD 1 |
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PDD & Field Size
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PDD increases as field size increases due to increased scatter production
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PDD & Depth
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-PDD decreases as depth increases
-dose is deposited in tissue as it traverses, thus a smaller percentage is available at greater depths |
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PDD & Medium
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PDD varies depending on the type of tissue, as some tissue attenuates more of the beam than others do
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Tissue Air Ratio
TAR |
-ratio of absorbed dose at a given depth in a phantom to the absorbed dose at the same point in free space
-same characteristics as PDD except TAR is independent of SSD |
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Characteristics of TAR
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-TAR increases as energy increases
-TAR increases as field size increases -TAR decreases as depth increases -TAR is independent of SSD |
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Tissue Phantom Ratio
TPR Tissue Maximum Ratio TMR |
-ratio of the absorbed dose at a given depth in a phantom to the same dose at the same point at a reference depth in a phantom
-TMR when the reference depth is Dmax -determined using the field size treating the patient -used for isocentric calculations |
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Characteristics of TMR
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-the same as TAR
-TMR increases as energy increases -TMR increases as field size increases -TMR decreases as depth increases -TMR is independent of SSD |
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SSD
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-SSD setups are when the isocentric machine distance is established on the patient's skin
-PDD is used for calculations |
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SAD
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-SAD setups are when the isocentric machine distance is established within the patient
-TMR is used for calculations |
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Isodose Distribution
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-dose distributions are usually measured in a phantom for different field sizes and then plotted in terms of isodose curves
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Geometric Field Size
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-usually defined at the intersection of the 50% PDD isodose line and the surface on the isodose curves
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Penumbra
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region near the edge of the margin between the 90% and 20% lines
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Factors Influencing Penumbra
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-size of radiation source, with lower energy having greater penumbra
-distance from the source to the distal part of the collimator -SSD |
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Flattening Filter
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flattens the beam by reducing the dose along the CAX, producing a flat beam at a specified depth, usually 10 cm
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Dose Normalization
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isodose distribution concept used when doing external beam planning
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Dose Profile
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used for beam flatness and symmetry measurements, shows the variation of dose across the field at selected depths
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Collimator Scatter Factor
Sc |
-determined by field size as defined by collimator opening
-also known as output factor -ratio of the beam output in air to a reference field size of 10 x 10 cm |
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Sc & Field Size
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-as field size increases, the beam output increases due to the increase in scatter
-collimator scatter is added to the primary beam |
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Scatter Phantom
Sp |
-the ratio of a dose in a phantom for a given field at Dmax to the dose at the same point for a 10 x 10 cm field, with the same collimator opening
-determined by field size -takes all blocking into account |
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Tray Factors
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-trays are made with acrylic, either solid or slotted, and may vary in thickness
-useful for supporting cerrobend blocks, compensators, or total body irradiation screens -set factors in calculation book based on beam energy |
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Wedges
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-beam modifying devices that are inserted into the pathway of the beam which attenuate the beam
-attenuation is greater at the thicker end (heel) than at the thinner end (toe) of the wedge, resulting in a tilt of the isodose curve (if the beam profile is for treating a flat surface) |
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Wedge Angle
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-the angle through which an isodose line is tilted at the CAX of the beam and the 50% isodose curve
-not the actual angle of the wedge |
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Wedge Factor
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-included in the dose calculation to compensate for the dose at CAX
-set values in the calculation book |
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Universal Wedge System
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-system that is fixed in the beam for all field sizes
-center of the wedge is aligned to the CAX of the beam |
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Dynamic Wedge System
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-dynamic motion of independent jaws moving within the treatment beam
-dynamic MLCs are in use with IMRT, replacing the need for wedges |
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Monitor Unit
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number set on the console to deliver a prescribed dose of radiation
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Equivalent Square Equation
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Equivalent Square= (area/perimeter) x 4
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Tolerance for Kidney
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23 Gy to 1 kidney
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Tolerance for Intestine
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40 Gy
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Tolerance for Lens of the Eye
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10 Gy
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Tolerance for Liver
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30 Gy
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Tolerance for Spinal Cord
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45 Gy
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Given Dose
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-point where PDD is 100%
-applied dose -entrance dose -peak dose |
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Calculations for SSD Treatments
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dose at Dmax, Dmax is given dose
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Calculations for Points Other Than Dmax
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given dose (Dmax) will be greater than prescribed dose, as in the case of a direct field
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Calculating a Dose Delivered to a Depth Different than Dmax
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Direct Proportion
Tumor Dose Will Be Lower Than Given Dose |
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Exit Dose
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-dose absorbed by a point located at the depth of Dmax at the exit of the beam
-used when calculating parallel opposed SSD fields -must know Dmax of the beam and patient separation through region of interest |
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How to Determine total Dmax Dose
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-calculate anterior dose contribution to Dmax (entrance dose)
-calculate posterior dose contribution to Dmax (exit dose) -add anterior and posterior dose contributions for total Dmax dose |
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Half-Life of Co-60
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5.3 years
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Treatment Delivery on a Co-60 machine
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timer is used instead of MU
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Equation for Cobalt Treatment Adjustments
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new minute setting=old minute setting x 1.01
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Extended Distance Calculations
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-allow larger field areas to be treated by extending the distance from the source to the patient
-due to divergence -always set up SSD -PDD used for calculation |
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SSD for Co-60 Machines
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usually 80 cm
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Monitor Unit Equation
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MU=Dose/ [Sc x Sp x DF x TF x OAF x NF x WF x (PDD {also called DDR} or TMR)]
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Calculating Monitor Units for SSD Treatments
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-no distance factor is used
-PDD used |
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Calculating Monitor Units for Isocentric Treatments
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-distance factor correction
-TMR used |
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Determining Sp for SSD Treatments
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defined at patient's skin
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Determining Sp for Isocentric Treatments
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defined at isocenter
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Distance Factor
DF |
-determined using the inverse square law
-in unity when SSD=100 cm -distance factor listed on bottom of linac calculation sheets |
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Calculating Distance Factor for Extended SSD Treatments
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-DDR is used
-DF=(100 + d/SSD + d)2 where d is treatment depth -distance factor also includes Mayneard's F factor |
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Calculating Distance Factor Using TMR When Calculation Point is Not At the Isocenter
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-DF=(100 + Dmax/SSD + d)2 or DF=(Dmax + 100/100)2
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Off Axis Factor
OAF |
-factor used to correct for dose differences between the central axis and the calculation point
-in unity when and only when the dose calculations are performed on the central axis |
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Normalization Factor
NF |
corrects for any normalization that occurs in a patient plan
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Reasons SAD Treatments are Used Preferentially To SSD Treatments
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-usually no reason to move patient after isocenter has been established
-reduces risk of errors due to positioning variations that occur during patient movement |
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Calculating Dose To a Point Other Than Isocenter Using SAD Technique
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-field size defined at isocenter, not skin surface
-inverse square correction or distance factor correction needed |
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Standard Factors Used To Calculate SAD Techniques
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-TAR
-TMR -TPR -factor used determined when initial beam outputs are measured on the machine |
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When TAR Is Used
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usually for low energy beams like 4 MV
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Energies For Which TMR and TPR Are Used
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usually for high energy beams like 10 MV
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Substitution For TMR Values
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can be broken down into Collimator Scatter Factor (Sc) and Phantom Scatter Factor (Sp) multiplied together to give one output factor
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Source to Calculation Point Distance
SCPD |
prescribed dose for the field at some point
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Determining Total Dmax Dose for Single Field, SAD setup
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-must know SCPD for each point, TMR at each SCPD, and equivalent square at each SCPD
-Dose A= Dose B x (SCPD@B)2 x TMR@A ------ -------- TMR@B (SCPD@A) |
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Field Weighting
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-weighting sum is sum of the ratio (3:1 ratio has weighting sum of 4)
-divide total dose by weighting sum -multiply by any part of the ratio greater than 1 -divide if multiple fields share any part of the ratio |
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November 8, 1895
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William Conrad Roentgen discovers the first x-ray
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January 29, 1896
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first therapeutic x-ray used to treat breast cancer
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1908
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-radiation therapy first used in the United States
-used by surgeons and dermatologists -dosed by erythema dose -x-rays in kilovoltage range until 1950s |
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Forms of Radiation Under 300 kVp
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-Grenz Rays: less than 20 kV
-Contact Therapy: 40-50 kV -Superficial Therapy: 50-150 kV -Orthovoltage Therapy: 200-300 kV |
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1928
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Roentgen is introduced as unit of measurement for x-rays and gamma rays
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1934
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combination external and internal 'brachytherapy' given
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1953
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-Rad is introduced as unit of measurement for absorbed dose
-recommended by the IRCU |
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1980
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-Gray replaces the Rad
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Converting Gray to Rads
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1 Gy=100 rads
1 cGy=1 rad |
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1940-1960's
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-supervoltage machines used with energies greater than 1 MV
-Co-60 treatment machines used -treatment planning computers introduced |
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1960-1990
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major advancements in technology benefit treatment machines and planning computers
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Treatment SSDs
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4 MV with 80 SSD
6 MV with 100 SSD |
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1990's to present
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-IMRT
-IGRT -CBCT -Gamma Knife -Cyberknife -Tomotherapy -more focus on restricting beam to spare tissue |
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Varian Trilogy Accelerator Component
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-all capabilities of 21 EX or 23 EX
-dose rate of up to 1000 MU/min -tighter radius isocenter for gantry and collimator axes -smaller couch rotation motions of under 2 cm and under 2" |
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IMRT
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-delivery of radiation fields designed to reproduce the shape of the tumor precisely
-computer planning allows radiation to be delivered heterogeneously within a treatment field, fields to conform to complicated shapes, and concave structures to be treated |
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IGRT
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-delivery of radiation using on-board imaging to localize treatment volumes daily at the time of treatment
-3-D scans allow us to minimize dose to healthy tissue and accurately treat areas of the body that are subject to organ motion |
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CBCT
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on-board imaging that uses a cone-shaped x-ray beam (as opposed to a fan-shaped beam) to create a three-dimensional image of the patient just before they are treated, allowing us to verify patient alignment and dose during daily treatments
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Gamma Knife
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-uses gamma radiation from Co-60 sources to irradiate tumors, predominantly in the brain
-houses source in a heavily shielded assembly, retracting shielding from the appropriate sources to expose the target volume through stereotactic radiosurgery, using radiation from many paths to deliver high doses of radiation to the tumor at one time without causing excessive damage to the normal tissues |
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Cyber Knife
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-uses x-rays to irradiate through radiosurgery
-images region of interest during treatment -can track movement and treat the patient while tissue is moving |
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Tomotherapy
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-delivery of radiation using spiral CT to verify the location of the treatment volume prior to daily treatments
-uses IMRT and daily verification to acheive greater precision when irradiating structures that have the potential to move -capable of treating using stereotactic radiosurgery and stereotactic body radiotherapy |
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Problem Solving MUs
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-find equivalent square of the collimator setting
-find equivalent square of the blocked field size -look up dose rate factors -determine the prescribed dose |
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Usual Dose Rate Factors
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-Sc or Sp
-TMR or PDD |
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Problem Solving for Extended Distance
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-find equivalent square of the collimator setting to be used for output factor
-find equivalent square of the blocked field size to be used for the PDD -look up the dose rate factors -determine the prescribed dose -calculate the PDDs using Mayneord's factor -determine the time setting using the appropriate equation |
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Isocentric Calculation Process
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-find the equivalent square of the collimator setting to be used for output factor
-find the equivalent square for the blocked field size to be used for TAR, TMR, or TPR -look up the dose rate factors -determine the prescribed dose -use the appropriate equation to determine the time setting |
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Geometric Matching
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Gap=
(Length1 x depth of abutment) -------- ----------------- + (2 SSD1) (Length2 x depth of abutment) -------- ------------------ (2 SSD2) |
