Radiation Physics Pearls – Roach 2 – Flashcards
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There are three major types of brachytherapy implants:
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(1) Molds/plaques (2) Interstitial implants (3) Intracavitary implants
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Type of brachytherapy used for superficial lesions where radioactive sources are placed over skin or orbital lesions
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(1) Molds/plaques - used for superficial lesions where radioactive sources are placed over skin or orbital lesions,
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type of brachytherapy used w/radioactive sources are incased in wire or seeds and inserted in tumor (e.g., prostate),
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(2) Interstitial implants - radioactive sources are incased in wire or seeds and inserted in tumor (e.g., prostate),
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Type of brachytherapy used w/sealed radioactive sources are placed inside a body cavity (e.g., cervix).
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(3) Intracavitary implants - sealed radioactive sources are placed inside a body cavity (e.g., cervix). Temporary seed insertion and removal are now performed with computerized afterloaders.
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High-dose rate (HDR) implants use dose rates of ___ cGy/min. Lower than this is generally termed low dose rate (LDR).
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High-dose rate (HDR) implants use dose rates of >20 cGy/min. Lower than this is generally termed low dose rate (LDR).
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Major radionucleotides used in brachytherapy Radionuclide: I-125 Half-life Photon energy (MeV) HVL (mm Pb) Clinical use
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I-125 59.4 days 0.0028 avg 0.025 Permanent prostate implant
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Major radionucleotides used in brachytherapy Radionuclide Half-life Photon energy (MeV) HVL (mm Pb) Clinical use
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Pd-103 17.0 days 0.021 avg 0.008 Permanent prostate implant
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Major radionucleotides used in brachytherapy Radionuclide: Cs-131 Half-life Photon energy (MeV) HVL (mm Pb) Clinical use
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Cs-131 9.7 days 0.029-0.034 0.030 Permanent prostate implant
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Major radionucleotides used in brachytherapy Radionuclide: Au-198 Half-life Photon energy (MeV) HVL (mm Pb) Clinical use
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Au-198 2.7 days 0.412 2.5 Permanent head and neck implant
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Major radionucleotides used in brachytherapy Radionuclide: Cs-137 Half-life Photon energy (MeV) HVL (mm Pb) Clinical use
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Cs-137 30 years 0.662 5.5 Temporary intracavitary implants
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Major radionucleotides used in brachytherapy Radionuclide: Ir-192 Half-life Photon energy (MeV) HVL (mm Pb) Clinical use
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Ir-192 73.8 days 0.38 avg 2.5 Temporary intracavitary or interstitial implants (HDR) for prostate, breast, cervix. Also used for skin
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Major radionucleotides used in brachytherapy Radionuclide: Co-60 Half-life Photon energy (MeV) HVL (mm Pb) Clinical use
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Co-60 5.26 years 1.25 avg 13.07 Older source for teletherapy
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Major radionucleotides used in brachytherapy Radionuclide: Ra-226 Half-life Photon energy (MeV) HVL (mm Pb) Clinical use
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Ra-226 1,622 year 0.83 avg 12 Historical interest
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Major radionucleotides used in brachytherapy Radionuclide: Rn-222 Half-life Photon energy (MeV) HVL (mm Pb) Clinical use
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Rn-222 3.83 days 0.83 avg 12 Temporary implant
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What law relating distance to dose is of tantamount importance in brachytherapy?
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Briefly, this law states that the energy absorbed at a given distance from a point source is inversely proportional to the square of the distance of the source. This is denoted by 1/r^2.
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What are three ways of quantifying radioactivity:
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There are three ways of quantifying radioactivity: (1) mCi (see above), (2) mg-Ra (milligram equivalent of radium) (obsolete), or (3) air-kerma strength (the current standard). Air-kerma strength is the dose rate in air at a specified distance in units of (Gy)(m2)/h.
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Define Air Kerma Strength
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Air-kerma strength is the dose rate in air at a specified distance in units of (Gy)(m2)/h.
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What are the 3 systems for placing interstitial implants
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+Quimby system: radioactive sources are distributed uniformly over volume of tissue leading to nonuniform dose. +Manchester system: radioactive sources are distributed nonuniformly with the goal of ±10% dose uniformity. +Paris system: developed for linear sources of iridium wire; sources are distributed uniformly for a planar implant, but follow a particular pattern for volume implants.
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Radioactive sources are distributed uniformly over volume of tissue leading to nonuniform dose.
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+Quimby system: radioactive sources are distributed uniformly over volume of tissue leading to nonuniform dose. +Manchester system: radioactive sources are distributed nonuniformly with the goal of ±10% dose uniformity. +Paris system: developed for linear sources of iridium wire; sources are distributed uniformly for a planar implant, but follow a particular pattern for volume implants.
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radioactive sources are distributed nonuniformly with the goal of ±10% dose uniformity.
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+Manchester system: radioactive sources are distributed nonuniformly with the goal of ±10% dose uniformity.
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developed for linear sources of iridium wire; sources are distributed uniformly for a planar implant, but follow a particular pattern for volume implants.
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+Paris system: developed for linear sources of iridium wire; sources are distributed uniformly for a planar implant, but follow a particular pattern for volume implants.
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How is dose sculpted over an area using catheter based brachtherapy?
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By positioning sources at a given position for variable periods of time (called dwell times), one can produce conformal dose distributions.
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In order to perform photon dose calculations, three key variables are important:
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In order to perform photon dose calculations, three key variables are important: (1) attenuation (see above) in tissue, (2) inverse square law (see above) or the distance from the radiation source, and (3) photon scattering due to the Compton effect (see above).
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Generally radiation doses are given in the unit Gray (Gy), which represents ____
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Generally radiation doses are given in the unit Gray (Gy), which represents absorbed dose (specifically 1 J/kg of tissue).
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specific amount of charge collected in one of the beam monitoring ionization chambers =
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AMU represents a specific amount of charge collected in one of the beam monitoring ionization chambers.
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used to convert rectangular fields into square equivalents for ease of calculation;
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Equivalent square formula: used to convert rectangular fields into square equivalents for ease of calculation; E = 2XY/(X + Y), where E = equivalent square field size, and X and Y are the initial field dimensions.
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What is the Equivalent square formula:
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Equivalent square formula: E = 2XY/(X + Y), where E = equivalent square field size, and X and Y are the initial field dimensions.
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used to estimated necessary wedge angle when two beams are arranged at a particular hinge angle to each other in order to produce a more uniform dose distribution;
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Wedge/hinge angle formula: used to estimated necessary wedge angle when two beams are arranged at a particular hinge angle to each other in order to produce a more uniform dose distribution; wedge angle = 90° - (hinge angle/2).
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Wedge Angle Formula =
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wedge angle = 90° - (hinge angle/2).
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used to calculate the separation between two field edges (e.g., the gap) on the skin when they are matched at a given depth in tissue.
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Skin gap formula for matching fields: used to calculate the separation between two field edges (e.g., the gap) on the skin when they are matched at a given depth in tissue. Skin gap = (L1/2)*(d/SSD1) + (L2/2)*(d/SSD2). L = length of the field, d = depth of match, SSD = source to surface distance; for isocentric setups substitute SAD for SSD
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What is the formula for calculating the skin gap?
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Skin gap = (L1/2)*(d/SSD1) + (L2/2)*(d/SSD2). L = length of the field, d = depth of match, SSD = source to surface distance; for isocentric setups substitute SAD for SSD
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How do you calculate the Collimator angle of cranial field to match the inferior border with the superior border of spine field?
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Collimator angle of cranial field to match the inferior border with the superior border of spine field = atan [(1/2 spine field length)/SSD]
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How do you calculate the Couch angle to make superior edge of spine field parallel to inferior border of cranial field ?
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Couch angle to make superior edge of spine field parallel to inferior border of cranial field = atan [(1/2 cranial field length)/SAD]
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Unlike photons, electrons deposit most of their dose at the _____. Also unlike photons, as the energy of electrons >,<,= the percentage of dose deposited at the surface increases.
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Unlike photons, electrons deposit most of their dose at the surface. Also unlike photons, as the energy of electrons increases, the percentage of dose deposited at the surface increases.
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The 4:3:2 rule for electrons refers to _____
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The 4:3:2 rule for electrons refers to the fact that the: 90% isodose line for electrons is generally ~MeV/4, 80% isodose line is generally ~MeV/3, & the effective range of electrons is ~MeV/2.
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The amount of Pb shielding required for electrons may be estimated as _____
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The amount of Pb shielding required for electrons may be estimated as MeV/2 (in mm).
