Medical Physics Glossary
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AstrocytomaBiologyA central nervous system tumor originating from the nonneuronal supporting cells. It can be low grade or anaplastic.
ContralateralBiologyRefers to a body component on the opposite side of the body.
Coronal PlaneBiologyPerpendicular (at right angles) to the sagittal plane and vertical divides the body into anterior and posterior sections.
DesquamationBiologyAn acute effect of irradiation characterized by shedding of the epidermis.
Dose EquivalentBiologyProduct of the absorbed dose and a quality factor (QF), which takes into account the biologic effects of different types of radiation on humans; units are the rem (1 rem = 1 rad x QF = 1 cGy x QF = 1 cSv). Photons and electrons have a QF of 1 for instance where as protons have a QF of 3 and neutron 5-20.
Effective Dose EquivalentBiologyThe dose equivalent weighted by the proportionate risk for various tissues. For example, the gonads are more sensitive to radiation than the fingers so there weighting of effective dose is higher. Thus for equal equivalent dose to the gonads and the fingers, the gonads would be more adversely affected.
AnaplasticBiologyA pathological description of cells, describing a loss of differentiation and more primitive appearance.
EpendymomaBiologyTumors arising from the ependymal cells lining the brain ventricles and central spinal canal. They may be low or high grade.
ErythemaBiologyThis acute radiation effect, manifested by redness and inflammation of the skin or mucous membranes, is caused by capillary congestion, caused by dilation of the superficial capillaries.
FibrosisBiologyThe abnormal formation of fibrous tissue caused by alteration in the structure and function of blood vessels.
Germ Cell TumorsBiologyTumors developing from embryologic nests of tissue located throughout the body, from the brain down to the ovaries and testes.
IpsilateralBiologyRefers to a body component on the same side of the body.
Active LengthBrachytherapyIn Brachytherapy, the length of the area in which the radioactivity lies in the source. This is usually a little shorter than the physical length of the source.
AfterloaderBrachytherapyA system in Brachytherapy that allows the applicators to be placed at the treatment site, then loaded remotely without personnel in the room. This reduces dose to the staff and since done via computers, should ensure a more accurate placement provided the applicators are correctly placed. Usually associated with High Dose Rate Brachytherapy (HDR)
BrachytherapyBrachytherapy(from the Greek "brachy", meaning "short") A special treatment procedure that utilizes the irradiation of a target with sealed radioactive sources placed at short distances from the target and inside the patient's body. Typically, the sources are implanted in the target tissue directly (interstitial brachytherapy) or are placed at distances of the order of a few mm from the target tissue, in body cavities such as the uterus, mouth, etc. (intracavitary brachytherapy)
High Dose Rate (HDR) BrachytherapyBrachytherapy
Interstitial BrachytherapyBrachytherapyTreatment technique that is characterized by the placement of radioactive sources directly into a tumor or tumor bed. Interstitial implants can be either permanent or temporary. They often involve needles.
Intracavitary BrachytherapyBrachytherapyIn this aspect of brachytherapy, radioactive sources are placed within a body cavity for treatment. This type or brachytherapy has been the mainstay in treatment of cervical cancer for more than 50 years.
Intravascular Brachytherapy (IVB)BrachytherapyDelivering radioactive sources through vascular routes.
Digital Imaging and Communications In Medicine (DICOM)ComputingA standard for handling, storing, printing, and transmitting information in medical imaging. This protocol was formed to provide communication standards for sharing image information regardless of manufacturer and has included radiation therapy treatment information. This facilitates the use of picture archival and communications systems (PACS) and allows diagnostic images to be widely distributed.
GrayDosimetryIt is the unit of absorbed dose. If one joule of energy is deposited in one kilogram of a medium, then absorbed dose is said to be 1 Gray
Image FusionImagingThe process of combining images from different modalities with a CT image. Properly fused images combine the enhanced contrast imaging capabilities of MRI and/or the enhanced functional/physiological capabilities of PET with the spatial accuracy of CT. Usually the anatomy or Tumor Volume is defined on either the MRI or PET and then placed onto the CT which is then used to plan the treatment.
Intensifying ScreenImagingUsed to convert the invisible energy of X-ray beam in visible light energy. This is used on equipment such as a fluoroscopy unit.
Tomo ImageImagingA megavoltage CT image created on the Tomotherapy machine immediately prior to treatment. Used to verify setup of the patient.MVCT
Bending MagnetMachineUsed in the gantry head of high-energy linear accelerators to bend the electrons that will either be used directly for electron radiation treatments or will hit a target and produce photons for photon radiation treatment.  Bending magnets are used along with an energy slit that physically limits the energy of the electron beam to within a desired range.  Two models are in primary use 270degree (Varian), slolom method (Seimens) 3 magnets used to do slights bends.
Central AxisMachineIt is the central portion of the beam emanating from the target in a linear accelerator. It is the only part of the beam that is non divergent.
CollimatorMachineAn arrangement of shielding material in the linear accelerator designed to define the dimension of the beam of radiation. The collimators are located in the treatment head and are usually made of tungsten alloy.MLC
CouchMachineA device used to support the patient in a position roughly parallel to the floor during delivery of a treatment procedure.
CT SimulatorMachineA computed tomography scanner equipped with software that can provide information needed to design the patient’s treatment parameters. CT based planning allows more spatial orientation information required for 3D treatment planning
CyclotronMachineA charged particle accelerator that is used to produce certain therapeutic and diagnostic radionucleotides such as F-18 for PET Scanners. They are also used in some proton radiation treatment centers to accelerate the particles up to therapeutic energies.
DivergenceMachineDivergence is the spreading out of the beam of radiation. The farther from the source, the more the beam has spread. We need to be aware of beam divergence when setting up adjacent fields or where field edges are near critical structures. The divergence of the beam is taken into account when performing field size calculation and many dose calculations.
Dose RateMachineAlso known as output, the dose rate of a treatment machine is the amount of radiation exposure produced by a treatment machine or source as specified at a reference field size and at a specified reference field. Most linear accelerators treat between 200-600 cGy/min.
Electron GunMachineThe electron gun is responsible for producing electrons and injecting them into the accelerator structure.
Electronic Portal Imaging Devices (EPID)MachineA system producing near real-time portal images on a computer screen for evaluation. This is attached to the linear accelerator and is composed of amorphous Silicon (aSi) crystals. They receive MV x-rays that images the patients setup and is used to compare to the DRR’s from the Treatment Planning System (TPS). These are done almost universally on all patients now.
Focal spotFocal spotThe section of the target at which radiation is produced. The smaller the focal spot, the sharper the field produced (very defined penumbra), however the smaller the focal spot also means the hotter the target material gets and can cause damage. Therefore, a compromise must be struck between sharper fields and adequate and reasonable heat loading of the target.
Focal spotFocal spotOn a simulator, it is a mechanical C-shaped device that supports the x-ray tube and collimator device at one end and an image system at the other and allows the duplication of treatment unit motions.
InterlocksMachineSafety switches blocking or terminating radiation production and thus treatment.
Inverse Square LawMachineA mathematical relationship that describes the change in beam intensity as the distance from the source changes. The change in intensity is primarily caused by the divergence of the beam. The mathematical formula states that the intensity decreases inversely proportional to the square of the distance. (Intensity = 1/r2). Example: Triple the Distance (r), the Intensity drops by a factor of 9.
isocenterMachinePoint in treatment vault that the linac, collimator, and couch rotate about.
jawsMachineLarge, pieces of metal that shape the beam into a rectangle. <0.2% leakage.
Kilovoltage UnitsMachineEquipment carry out external beam treatment by using x-rays generated at voltages up to 500 kVp. These are normally used for superficial treatments.
Klystron Machine– Equipment that converts kinetic energy to microwave energy in the linear accelerator. Klystrons are high-vacuum devices that use a well-focused pencil electron beam that directs the stream through a number of microwave cavities, which are tuned at or near operating frequency of the tube. Conversion takes place as a result of the amplified RF input signal, causing the electron beam to form “bunches”. These “bunches” give up their energy to the high-level induced RF fields at the output cavity. The simplified signal is extracted from the output cavity through a vacuum window.
Linear Accelerator (linac)MachineMachine that produces high energy photons and electrons for patient treatment by accelerating electrons to megavoltage energies.
MLC (Multileaf Collimator)MachineA series of 0.3-1.0 cm wide metal leaves each robotically controlled to shape the radiation beam precisely. <2% leakage.  The leaves are either singly focused, doubble focused, or rounded.  Doubly focused MLC's are curved to match beam divergence.
SADMachineSource to Axis Distance. For isocentric machines. The distance from the source or focus to the axis of rotation of the gantry. It is 100 cm for most linacs and 80 cm for most cobalt units. There are also 80 cm linacs and 100 cm cobalt units, but they are uncommon.
X-ray Targetmachinea material used for the generation of x-rays when electron is incident on it. the type of targets are different for therapeutic and
IsocenterMachine, TreatmentThe point of intersection of the three axes of rotation (gantry, collimator, and bade of couch) of the linear accelerator. Usually the location of this point is defined on the patient for setup purposes.
IsocenterMachine, treatment, simlation (inc CT)Point about which the apparent source of radiation rotates (x, y, z axes)
DosimetristPersonnelRadiation therapy practitioner responsible for production of the patient’s treatment plan and any associated quality assurance components. They also deal with certain dosimetric aspects of low dose rate (LDR) brachytherapy.
Aria (Varian Medical Systems, Inc.)ProductRecord and verify product (also EMR) produced by the vendor Varian. 
DoseLab ProProductSoftware used to compare doses measured with film to doses computed by a treatment planning system. These comparisons are typically performed for every IMRT plan, as required by billing.
EBT2 filmProduct
EDR2 filmProduct
MammositeProductApplicator for delivering partial breast irradiation using brachytherapy.Partial Breast Irradiation
MOSAIQ (Impac Medical Systems, Inc.)ProductAn integrated software package that contains radiation oncology billing, schedules, record and verify for linear accelerators, prescriptions, documentation, etc.
nanoDot OSLDProductsee OSLD
Pinnacle (Philips Medical Systems)ProductA treatment planning system for conventional and Varian IMRT treatments. It can perform conventional, dynamic conformal arc, IMRT, and brachytherapy plans.
RadCalc (LifeLine Software, Inc.)ProductSoftware that performs simplified MU calculations to double check MUs generated by treatment planning systems.
RapidArcProductVarian's ( implementation of IMAT.
SaviProductBrand name of an HDR-PBI applicator.  The applicator looks like an eggbeater where each rung is a catheter.  The source can move along each rung of the device to deliver dose.  The competing techology is a liquid filled balloon where the catheters are placed more centerally to the device and thus some distance away from the breast tissue.  The SAVI catheters are in direct contact with breast tissue.  The applicator can be expanded and collapsed.  Typically tissue will drop down inbetween the struts, this is called invagination, thus the distance between any two struts may change significantly from the date of simulation to any date of treatment.  The struts may become inverted due to the structural demands and may require the device be removed or the plan be changed.  The surgical cavity will fill partially with seroma and/or error.  Some data is available to show that the air heterogeneity may acocunt for as much as a 10% dose change.
SiemensProductA vendor which produces LINACS.
SmartArcProductPhilips / ADAC software tool used to plan modulated arc therapy.
TomoTherapyProductThe rotational delivery of modulated beams. This machine is similar in physical appearance to a CT Scanner, however instead of delivering keV X-rays to garner diagnostic information, it delivers MV (therapeutic) beam and is outfitted with a collimator with two sets of binary leaves that either open and close and deliver modulated dose patterns to the patient.
TumorLOC (Philips Medical Systems)ProductA program on newer Philips CT simulators that allows therapists to easily set an isocenter and move the room lasers to mark the patient.
VMATProductElekta's term for IMAT
X-liteProductUsed to check the optical and radiation field congruence in real time
BrainSCAN (BrainLAB)ProductsA treatment planning system for all patients on the Novalis Linear Accelerator. It can perform conventional, dynamic conformal arc, stereotactic cones, and IMRT plans.
Cheese PhantomProductsA cylindrical phantom, 30 cm in diameter and 18 cm long, made of Solid Water. For patient QA, it separates into two hemispheres to place film and can have an ion chamber within 5 mm or film.phantom
CORVUSProductsAn inverse treatment planning system for creating intensity modulate fields (IMRT). It was the first system used for IMRT in this department and has since been largely replaced by other TPS.
iPlan (BrainLAB)ProductsThe updated contouring and image fusion tool for BrainSCAN.
Bragg PeakProtonsA sharp increase in the dose distribution curve of a charged particle at a particular depth. It is this physical phenomenon that is exploited in proton radiation treatments.
Spread Out Bragg Peak (SOBP)ProtonsBragg Peak
Absorbed DoseRadiationA measure of the energy deposited in a medium by ionizing radiation. It is equal to the energy deposited per unit mass of medium, and so has the unit J/kg, which is given the special name gray (Gy). You will also hear the term rad used. 1 Gy = 100 cGy = 100 radKERMA
Alpha ParticleRadiationParticulate radiation, positively charged, which consists of two protons and two neutrons (effectively a He nucleus). It has a high Radiobiological Effectiveness (RBE) compared to photons, and thus will cause significant damage to tissue, but it has a short track length (a piece of paper will shield most of it).
AttenuationRadiationThe removal of photons and electrons from a radiation beam by scatter or absorption as it travels through a medium, typically tissue or tissue equivalent materials
Bequerel (Bq)RadiationAn SI unit of radioactivity that equals one disintegration per second. It takes 3.7 x 1010 Bq = 1 Curie.
Beta ParticleRadiationElectrons (B-, negatively charged) or Positrons (B+, positively charged)
BremsstrahlungRadiationGerman term for “braking” radiation. The principal interaction in x-ray production results in the output or bremsstrahlung radiation.
Cesium-131RadiationA recent addition to the stable of radionucleotides used for Prostate brachytherapy, it has a half life of 10 days and an average energy of 30.4 keV.
Cesium-137RadiationA radioactive isotope with a half life of 30.2 years and an average energy of 662 keV. It is a pure beta emitter and is used for Low Dose Rate (LDR) Brachytherapy especially for use in tandem and ovoid treatments for gynecological cancers.
Characteristic RadiationRadiationPhoton radiation that is emitted in discrete energies when an outer shell electron fills an inner shell position after the original inner shell electron is ionized. The binding energy difference between residing in the outer shell compared to the inner shell is the energy of the photon emitted.
Curie (Ci)RadiationA historical unit of radioactivity that equals 2.22 x 10^12 dpm or 3.7 x 10^10 Bq.
Electron Binding EnergyRadiationThe amount of energy required to remove an electron from its orbit in an atom. This amount is subtracted from the initial energy acquired by the photon or electron once it leaves the proximity of the atom.
Electron DensityRadiationNumber of electrons per unit mass.
ElectronsRadiationA form of beta particle. Negatively charged subatomic particles that can be accelerated by a variety of machines or are emitted from decaying isotopes and are used for external beam treatment and brachytherapy.
ExposureRadiationAmount of ionization produced by photons in air per unit mass of air. The SI unit is Coulombs / kilogram (C/kg). Another popular unit, although not recognized by SI is the Roentgen (R). 1 R = 2.58 x 10-4 C/kg.
Free RadicalRadiationAn atom or atom group in a highly reactive transient state that is carrying an unpaired electron with no charge. These are often created by radiation and are one of the main mechanisms in which radiation damages chromosomal DNA, thus giving radiation its therapeutic value, making it carcinogenic and teratogenic.
Gamma RaysRadiationElectromagnetic radiation emitted from decaying isotopes and used for external beam and brachytherapy treatments. Since it is electromagnetic radiation it is a photon and has no mass and no charge. Gamma Rays typically have similar energies to X-rays but are distinguished from these radiations by the origin of emanation. Gamma Rays are from the nucleus of the atom whereas X-Rays come from the electron shells outside of the nucleus.
Gray (Gy)RadiationThe special unit used to describe absorbed dose and KERMA. It has unit of Joules / kg (J/kg). 1 Gray = 100 rads.
Half-LifeRadiationThe time period in which the activity decays to one half of the original value. It is the essential value to employ the decay formula for a particular isotope.
Half-Value Layer (HVL)RadiationThe thickness of absorbing material necessary to reduce the x-ray intensity to half its original value. Depending on the material, this thickness varies. Lead, for example, has a much shorter HVL than aluminum due to its increased density.
Ionizing RadiationRadiationRadiation with sufficient energy to separate an electron from its atom.
Iridium-192RadiationA radioactive isotope with a half life of 74 days. It is used in wire form for interstitial brachytherapy and as the source for HDR brachytherapy.
Kinetic energy released in matter (KERMA)Radiation
photonsRadiationThe radiation used to treat most patients, 6 MV and 18 MV energies most common, 6 MV = 100% dose at 1.5 cm, 66% at 10 cm, 23 MV = 3.3 cm dmax and 80% at 10 cm
Cobalt-60Radiation, TreatmentA radioactive isotope with a half life of 5.26 years that was heavily used for external radiation therapy before the popularization of linear accelerators. It is a beta emitter that decays to Ni-60 and gives off two gamma rays of average energy 1.25 MeV in the process. This is the therapeutic part of the beam. One major disadvantage of Cobalt compared to Linear Accelerators is a wider penumbra at the field edge. It is seldom used for external beam treatments but is still employed in the Gamma Knife stereotactic system.
ALARASafetyRadiation Safety term that stands for As Low As Reasonably Achievable. It states that radiation exposure should be kept to a minimum within reasonable effort.
Film BadgeSafetyA device for measuring dose. It makes use of the following phenomenon: when film is exposed to radiation and subsequently developed, the amount of blackening (AKA Optical Density) is proportional to the dose delivered to the film. By measuring this darkening, it is possible to determine the amount of dose the badge received. These are worn by all radiation workers to document the dose received during their time at work.