Photon Dosimetry concepts in addition to Calculations Dose Calculation Monitor Units Radiation Therapy Prescription Isodose Plan
Keller, Shelly, Food Editor has reference to this Academic Journal, PHwiki organized this Journal Photon Dosimetry concepts in addition to Calculations Chapter 21 W/L Dose Calculation The treatment planning team has to quantify the overall prescribed dose of radiation in addition to determine how much dose will be delivered over the time frame outlined. There are many parameters of photon beam calculation. Monitor Units Monitor Units (MU): a measure of output as long as linear accelerators. The dose rate varies slightly from one moment to the next Normally the dose rate as long as the linear accelerator is 1.0 cGy/MU as long as a 10 x 10 field size defined at the isocenter.
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Radiation Therapy Prescription Radiation Therapy Prescription: Legal document defines the treatment volume Intended tumor dose (TD) Number of treatments Dose per treatment Frequency of treatment States the type in addition to energy of radiation Beam-shaping devices such as wedges in addition to compensators Any other appropriate factors. Clear, precise in addition to complete Isodose Plan Isodose Plan: part of the prescription May show field sizes Machine angels Doses Beam weighting Wedges compensators or blocks Treatment Time Treatment time: length of time a unit is physically left on to deliver a measured dose. Factors considered: Beam energy Distance from the source of radiation Field size
Dose Dose (absorbed dose): measured at a specific point in a medium in addition to refers to the energy deposited at that point. Measured in gray (Gy), which is defined so that 1 Gy equals 1J/kg. Depth Depth: distance beneath the skin surface where the prescribed dose is to be delivered. Opposed fields: patients midplane is used Multiple field arrangements: isocenter used Depth affects measurements of dose attenuation. Separation Separation: a measurement of the patients thickness from the point of beam entry to the point of beam exit. Normally measured along the beams central axis. Calipers ODI readings
Source Distance Source to Skin Distance (SSD): the distance from the source or target of the treatment machine to the surface of the patient. Source-axis Distance: the distance from the source of photons to the isocenter. Setup SSD: Isocenter established at the patients skin surface When the gantry rotates around the patient, the SSD will continually change. Dose calculations often at DMAX. (Given dose) SAD: Isocenter established within the patient The SAD in addition to the isocenter are at a fixed distance in addition to there as long as e do not change. Isocenter Isocenter: the intersection of the axis of rotation of the gantry in addition to the axis of rotation of the collimator as long as the treatment unit. Cobalt 60: SAD of 80 cm Linear Accelerators: SAD of 100 cm
Field Size Field Size: the physical size set on the collimator of the therapy unit that determines the size of the treatment field at a reference distance (defined at the machines isocenter) SAD: the field size set inside the patient (size measured on patients skin will be smaller) SSD: field size set on the collimator will be the same measured at the patients skin Scatter Backscatter: radiation that is deflected back toward the patient Most of the absorbed dose received by the patient results from the collisions of the scattered electrons produced when the primary photon interacts with the collimator. DMAX DMAX: the depth at which electronic equilibrium occurs as long as photon beams; the point where dose reaches its maximum value. Mainly depends on the energy of the beam The depth of maximum ionization increases as the energy of the beam increases. Factors such as field size in addition to distance may also influence the depth
DMAX DMAX dose occurs at the same depth as long as a given energy regardless of field size or distance from the source. The actual reading differs as long as different field sizes. Output Output: the dose rate of the machine, the amount of radiation exposure produced by a treatment machine or source as specified at a reference field size in addition to at a specified reference distance. Changing the field size, distance, or attenuating medium will change the dose rate. Increases with field size: primary component the same, increased scatter adds to the output If the distance increases, dose rate decreases due to ISL
Cobalt-60 Output Dose rate as long as Co-60 machine in cGy/min Dose rate due to the radioactive decay of the isotope Co-60 Can be assumed constant over short periods of time The Time that a machine is left On is adjusted by 1% increase every month TO2 = TO1 x 1.01 Linac Output Dose rate as long as Linac in cGy/MU Dose rate varies from one moment to the next Ionization chamber shuts down machine after predetermined dose has been given Output Factor Output Factor: the ratio of the dose rate of a given field size to the dose rate of the reference field size Allows as long as the change in scatter as the collimator setting changes Relates the dose rate of a given collimator setting to the dose rate of the reference field size
Dose Rate Commonly measured at the isocenter of the treatment machine. The dose rate of the beam is inversely proportional to the square of the distance Dose rate(Given f.s.) = Dose rate(Reference f.s.) x Output factor(Given f.s.) Dose Rate: Output (of machine) Output factor (C.S. field size) Scatter Ratio BSF or PSF (EFS/CS) PDD(EFS)/100 Tray factor Inverse square correction Equivalent Squares Equivalent Squares: rectangular field size that demonstrate the same measurable scattering in addition to attenuation characteristics of a square field size. Used to find the output, output factor, in addition to tissue absorption factors. 4 (A/P) Effective Field Size Effective Field Size (EFS): the equivalent rectangular field dimension of the open or treated area within the collimator field dimensions when blocks are used to customize the shape of the treatment area. The EFS equivalent square is normally used to determine the tissue absorption factors.
Tissue Absorption Factors Tissue absorption factors: different methods as long as measuring the attenuation of the beam as it travels through matter. Percent Depth Dose (PDD): works best with SSD setups Tissue Air Ratio (TAR) Tissue Phantom Ratio (TPR) Tissue Maximum Ratio (TMR) Percent Depth Dose (PDD) Percent Depth Dose (PDD): the ratio expressed as a percentage, of the absorbed dose at a given depth to the absorbed dose at a fixed reference depth usually DMAX. Calculated from two measurements at two different points in space. Requires SSD be constant. Written as PDD(d,s,SSD)= Dependant on: Energy- more penetrating- PDD Depth- PDD due to attenuation through matter Field size- more scatter- PDD SSD- PDD- ISL Percent Depth Dose (PDD) PDD = Dose @ d . Dose @ Dmax
Tissue Air Ratio (TAR) Tissue Air Ratio (TAR): the ratio of the absorbed dose at a given depth in tissue to the absorbed dose at the same point in air. Dependant of: Energy- TAR Field size- TAR Depth- TAR Independent of SSD Tissue Air Ratio (TAR) Calculated using two measurements at the same point in space When the depth in tissue corresponds to the level of DMAX, the TAR is known as the backscatter factor. Build-up cap: device made of acrylic or other phantom material that is placed over an ionization chamber to produce conditions of electronic equilibrium. Tissue Air Ratio (TAR) TAR = Dose in tissue Dose in Air Normally used to per as long as m calcs as long as SAD treatments of low energy machines. There is no patient backscatter in the in Air measurements.
Mayneords Factor Mayneords Factor: a special application of the inverse square law. Does not account as long as changes in scatter because of a change in beam divergence. New PDD = PDD x (SSD1 + d / SSD1 + DMAX)2 x (SSD2 + DMAX / SSD1 + d)2 Inverse Square Factor: = (Reference source calibration distance Treatment SSD + DMAX)2 SSD: reference point typically at DMAX SAD: reference point at isocenter TMR/TPR SAD COF(C.S.) (collimator output factor): used to determine scatter, measured in air, from the collimators The increase in the collimator opening, the more collimator surface area the photons will have to interact with. PSF(EFS) (phantom scatter factor): used to determine scatter from the patient.
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