CMD flashcards
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32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Gray definition
- The SI unit of absorbed dose, equal to one joule of energy deposited per kilogram of matter; one gray equals 100 centigray and 100 rad.
- Inverse-square law
- Photon fluence and dose rate decrease with the square of distance from a point source, so doubling the distance reduces dose rate to one quarter.
- Percent depth dose
- The ratio of absorbed dose at a given depth to the dose at the depth of maximum buildup, expressed as a percentage, for a fixed source-to-surface distance.
- Tissue-phantom ratio
- Dose at depth in a phantom divided by dose at a fixed reference depth, measured with constant source-to-detector distance; independent of source-to-surface distance.
- Backscatter factor
- The ratio of dose at the depth of maximum buildup for a finite field to the dose at the same point for a zero-area field; also called peak scatter factor.
- Mayneord F factor
- A conversion factor used to transform percent depth dose values measured at one source-to-surface distance into values for a different source-to-surface distance.
- Equivalent square field
- A square field with the same scatter characteristics as a rectangular field, calculated as twice the product of the sides divided by their sum.
- Monitor unit definition
- A machine unit calibrated so that one monitor unit delivers one centigray at the depth of maximum dose in water under reference conditions.
- Collimator scatter factor
- The ratio of machine output for a given field size to output for the reference field size, measured in air with a buildup cap.
- Wedge factor
- The ratio of dose with the wedge in place to dose without it at the same depth and monitor units; depends on depth and field size.
- Electron practical range
- The depth at which the electron percent depth dose falls to approximately 90 percent, roughly equal to beam energy in MeV divided by four.
- Buildup region
- The shallow depth range where photon dose rises toward maximum as secondary electrons accumulate; skin sparing depends on this effect.
- Gamma index
- A composite metric combining dose difference and distance-to-agreement; a point passes when the minimum combined value is less than or equal to one.
- TG-218 passing criterion
- Recommended IMRT verification standard of 95 percent of points passing at 3 percent dose difference and 2 millimeter distance-to-agreement with a 10 percent threshold.
- Van Herk margin formula
- Clinical target volume to planning target volume margin equal to 2.5 times the systematic error plus 0.7 times the random error, in standard deviations.
- Patient-specific IMRT QA
- Measurement of a planned fluence on a phantom or detector array before treatment to confirm that the delivered dose matches the calculated dose.
- Stereotactic setup accuracy
- Intracranial stereotactic treatments require targeting accuracy of about one millimeter, verified with cone-beam CT or stereoscopic imaging and six-degree couch correction.
- Volumetric modulated arc therapy
- A rotational delivery technique in which gantry speed, dose rate, and multileaf collimator positions vary continuously during one or more arcs around the patient.
- Total body irradiation
- A technique using extended source-to-surface distance, large fields, and tissue compensators to deliver a uniform midline dose, often before bone marrow transplant.
- Dose-rate constant
- In TG-43 formalism, the dose rate in water at one centimeter along the transverse axis per unit air kerma strength, with units of centigray per hour per unit.
- Radial dose function
- A TG-43 factor describing attenuation and scatter in water along the transverse axis, normalized to unity at the reference distance.
- Anisotropy function
- A TG-43 factor accounting for source encapsulation and self-attenuation as a function of angle relative to the transverse axis.
- Air kerma strength
- The product of air kerma rate at one meter and the square of that distance, expressed in units of microgray square meters per hour.
- Iodine-125 half-life
- A permanent implant isotope with a half-life of about 59 days and an average photon energy near 28 kiloelectron volts.
- Iridium-192 HDR
- A high-dose-rate brachytherapy isotope with a half-life near 74 days and an average photon energy of about 380 kiloelectron volts.
- Palladium-103 properties
- A permanent implant isotope with a half-life of about 17 days and an average photon energy near 21 kiloelectron volts, used for prostate implants.
- ALARA principle
- The radiation protection principle that occupational and public exposures should be kept as low as reasonably achievable, accounting for economic and social factors.
- Half-value layer
- The thickness of a specified material that reduces the intensity of a narrow beam to one half of its original value; equals 0.693 divided by the attenuation coefficient.
- Primary barrier calculation
- Barrier transmission equals the weekly design limit times distance squared, divided by workload times use factor times occupancy factor.
- CT dose index volume
- A CT dose descriptor representing the average dose over a scan volume, calculated from the weighted CT dose index divided by pitch.
- Dose length product
- The product of volume CT dose index and scan length in centimeters, expressed in milligray-centimeters; used to estimate effective dose.
- Occupational dose limit
- The annual effective dose limit for radiation workers is 50 millisieverts, with a lifetime cumulative limit of ten millisieverts times age.