# Medical Physics Dictionary

ADCL: See Accredited Dosimetry Calibration Laboratory

Accredited Dosimetry Calibration Laboratory (ADCL): A laboratory which provides calibration services for most radiation measurement instrumentation available in the medical and heal physics community. The ADCL’s equipment calibration is directly traceable to the National Institute of Stanards and Technology (NIST).

Air Kerma Rate: A quantity defined by ICRU 38 and ICRU 60 defining the rate of air kerma for a brachytherapy source. It is numerically identical to Air Kerma Strength.

Air Kerma Rate Constant ($$\Gamma_{AKR}$$): The rate of air kerma at a distance (usually 1cm) from a source. This unit is characteristic of a radionuclide.  Units: $$\frac{cGy \ cm^2}{hr \ MBq}$$

Air Kerma Strength (SK): The Air Kerma rate, in vacuo, due to photons of energy greater than δ at a distance d, multiplied by the square of this distance, d2This quantity was defined in AAPM TG-32 and subsequently used in AAPM TG-43 and AAPM TG-43U1. Units: $$1U = 1 \ \frac{\mu Gy \ m^2}{h} = 1\ \frac{cGy \ cm^2}{h}$$

$$\label{eq: air kerma strength} S_K = \dot{K}_{\delta}(d) \ d^2$$

ALARA (As Low As is Reasonably Achievable): means making every reasonable effort to maintain exposures to radiation as far below the dose limits as is practical.

ALI: See Annual Limit of Intake

Alpha-decay:

Alpha-Ray: See alpha particle.

Anisotropy: The property of being directionally dependent, as opposed to isotropy.

Anisotropy Factor: a factor used to account for the angular dependence of photon absorption and scatter in the encapsulation and medium for a brachytherapy source.

Annual Limit of Intake (ALI):

Annulus: a ring shaped object.

AAppSee Apparent Activity.

Apparent Activity (AApp): The activity of an unshielded source that would give the same exposure rate at a distance of 1m compared with the current filtered source. This quantity is used in TG-32 and TG-43 but was removed in TG-43U1.

Applicator [Brachytherapy]: A device used in brachytherapy which is inserted into the treatment region. The radioactive source is inserted into a channel in the applicator for treatment.

Average Life: See Mean Life

Beta Ray: See beta particle.

Bq: See becquerel (unit).

Brachytherapy:

Bragg-Gray Cavity Theory: This theory is used to determine the absorbed dose to a medium using an ion chamber. Click here for more information on Bragg-Gray Cavity Theory.

Charged Particle Equilibrium (CPE):

Ci: See Curie (Unit).

CPE: See Charged Particle Equilibrium

CNR: See Contrast to Noise Ratio

Contrast to Noise Ratio (CNR):

Controlled Area: Limited access areas where the occupational exposure of personnel to radiation is under supervision of a radiation protection program. These include treatment rooms, control areas, and other working areas for radiation workers where non-monitored persons are not able to enter. This term is defined in NCRP 151.

Decay Constant (λ): The decay constant (λ) is the fraction of atoms of a radionuclide which decay in a given time. Click here for more information on the Decay Constant.

Delta Rays: see Delta Particle.

Deterministic Effect: a somatic effect which increases in severity with increasing radiation dose above a threshold dose.

Dose Equivalent (H):  An earlier and similar quantity to Equivalent Dose  which is the product of absorbed dose and a quality factor which is similar to the radiation weighting factor.Click here for more information about Types of Dose.

dps: Disintegrations Per Second.

Dwell Times (Brachytherapy): The amount of time the source is left in a given treatment position during an high dose rate brachytherapy treatment.

e: See Elementary Charge (e)

Effective Dose:

Elementary Charge (e): The electric charge carried by a single proton.  1.602 176 487(40) E-19 C

Equivalent Energy: the energy which is equivalent to a given mass found by $$E = mc^2$$.

°F: See Fahrenheit

F fractor ($$f_{med}$$): This factor converts from Roentgens to rads (cGy) in a medium and is sometimes called simply the Roentgen-to-rad conversion factor. $$f_{med} = 0.876 \frac{(\frac{\mu_{en}}{\rho})_{med}}{(\frac{\mu_{en}}{\rho})_{air}}$$

Fahrenheit (°F): A temperature scale common in the USA in which the freezing point of water is set at 32.0 °F and the boiling point of water is set as 212 °F.

Geometric Penumbra: See Penumbra (Geometric)

Gray (Gy): A derived unit of ionizing radiation defined as the number of joules of radiation energy deposited per kilogram of matter. (\ref{eq:Gray})

\label{eq:Gray}

1Gy = 1 J/kg = 1 m^2 / s^2

Groundshine: Radiation reflected off of earth below the accelerator walls up to ground level outside of the vault.

Gy: See Gray

GYN: Gynecology

Gynecology (GYN): The branch of medicine dealing with functions and disease specific to women, especially those affecting the reproductive system.

H: See Dose Equivalent

Half Value Layer (HVL): Thickness of shielding which reduces the intensity of the transmitted beam to 50% of the incident intensity. 1TVL ≅3.32HVLs.

HDR: See High Dose Rate Brachytherapy.

High Dose Rate Brachytherapy:

High-Energy Accelerator: An accelerator with an accelerating voltage of >10MV corresponding to the energy in which neutron contamination must be considered. This term is defined in NCRP 151.

High Radiation Area: an area, accessible to individuals, in which radiation levels from radiation sources external to the body could result in an individual receiving a dose equivalent in excess of 0.1 rem (1 mSv) in 1 hour at 30 centimeters from the radiation source or 30 centimeters from any surface that the radiation penetrates. From NRC 10 CFR Part 20.

HVL: See Half Value Layer

IMRT: See Intensity Modulated Radiation Therapy

In Vacuo: measurements which are corrected for photon attenuation and scattering in air and in any other medium interposed between the source and the detector, as well as photon scattering from any nearby objects (i.e. walls, floors, and ceilings).

Intraluminal: the space inside a lumen (i.e. tubular structure)

Isocenter:

Isodose line/curve: Isodose lines are used in dose distribution maps to join points of equal dose.

Isotropy: The property of uniformity in all directions, as opposed to anisotropy.

Ka: See Kerma Measured in Air

KERMA: An acronym meaning Kinetic Energy Released Per Unit Mass

Kerma Measured in Air (Ka):

kVp: kilovoltage peak or peak kilovoltage. Used to specify the maximum voltage across an x-ray tube.

λ: Decay Constant

Law of Bergonie and Tribondeau: The radiosensitivity of a tissue is increased with greater number of undifferentiated cells, greater mitotic activity, and greater length of time they are actively proliferating.

LDR:

LET: See Linear-Energy-Transfer

Linear-Energy-Transfer (LET):

Low Dose Rate Brachytherapy:

Low-Energy Accelerator: An accelerator with an accelerating voltage of ≤10MV. Little neutron production occurs in this range. This term is defined in NCRP 151.

Lumen: The space inside a tubular structure such as an artery, intestine, or esophagus.

mA: The electron flow (number of electrons per second) from the cathode to the anode in an x-ray tube.

mAs: The product of tube current (mA) and time in seconds (s). Used to specify total energy in x-ray and CT imaging.

Microwave: The portion of the electromagnetic spectrum with wavelengths measuring between 1 and 100 centimeters. This term is often used loosely.

Modulator (pulse modulator): Provides a pulse or current waveform to power linear accelerator microwave sources (i.e. magnetron or klystron).

Modulation Transfer Function (MTF):

Monitor Unit:

MTF : See Modulation Transfer Function

National Institute of Stanards and Technology (NIST): a non-regulatory agency of the United States Department of Commerce which, among other activities, serves as the primary reference lab for dosimetry equipment in the United States.

National Regulatory Commission (NRC): an independent agency of the U.S. government tasked with protecting public health and safety related to nuclear energy.

Noise Power Spectrum (NPS):

NRC: See National Regulatory Commission

NIST: See National Institute of Stanards and Technology

NPS: See Noise Power Spectrum

OB/GYN: Obstetrics and Gynecology

Obstetrics: Branch of medicine concerned with childbirth and the care of women giving birth.

Occupancy Factor (T): The average fraction of time that the maximally exposed individual is present while the beam is on. This term is defined in NCRP 151.

P: See Shielding Design Goals

Palliative: Medical care focused on relieving pain and improving quality of life without dealing with the cause of the condition.

PDR: See Pulse Dose Rate Brachytherapy

Penumbra: a region at the edge of a radiation beam over which the dose rate changes rapidly as a function of distance from the central axis. Click here for more information on Penumbras.

Penumbra (Physical): The lateral distance between two specific isodose curves at a specific depth. Commonly taken as 90% to 10% or as 80% to 20%. Click here for more information on Penumbras.

Penumbra (Transmission): Penumbra arising from differences in transmission of a beam through parts of the collimator. Click here for more information on Penumbras.

Physical Penumbra: See Penumbra (Physical)

PFN: See Pulse Forming Network

Primary Barrier: A wall, ceiling, floor, or other structure that will directly intercept the primary radiation beam. This term is defined in NCRP 151.

Prone: A patient position where the patient lies horizontally with the face and torso facing down and the palms facing up.  Click here for more information on patient positions.

Pulse Forming Network (PFN): A linac component which both forms the pulse used to activate the microwave source (i.e. magnetron or klystron) and stores the energy to produce that pulse.

Pulsed Dose Rate Brachytherapy: A form of brachytherapy in which intermittent pulses of radiation (10-30 minutes/hours) are delivered using a machine similar to an HDR afterloader.

QA: See Quality Assurance

QC: See Quality Control

Quality Assurance:

Quality Control:

R: See Roentgen

Radiation Area: an area, accessible to individuals, in which radiation levels could result in an individual receiving a dose equivalent in excess of 0.005 rem (0.05 mSv) in 1 hour at 30 centimeters from the radiation source or from any surface that the radiation penetrates. From NRC 10 CFR Part 20.

RBE: See Relative Biological Effectiveness

Reference Man: A hypothetical aggregation of human physical and physiological characteristics arrived at by internal consensus. Reference man is used by researchers to standardize the biological impact of radiation.

Relative Biological Effectiveness: the ratio of biological effectiveness of one type of radiation to another given the same amount of energy deposited. Click here for more information on Relative Biological Effectiveness.

Roentgen (R):

Secondary Barrier: A wall, ceiling, floor, or other structure that will not intercept the primary beam but will receive radiation scattered by interactions within the patient or other object including accelerator leakage radiation. This term is defined in NCRP 151.

SF6See Sulfer Hexaflouride

Shall: Used in many AAPM and other professional reports to indicate that a recommendation is necessary to meet currently accepted standard of practice.

Shielding Design Goals (P): Maximum acceptable levels of Doses Equivalent used in computing the required shielding. These are typically defined at a reference point not less than 0.3m beyond a barrier. This term is defined in NCRP 151.

Should: Used in many AAPM and other professional reports to indicate that a recommendation is desirable only when practical. Recommendation should be disregarded when implementation costs are prohibitively high or when a superior alternative exists.

SIB: See Simultaneous Integrated Boost

Simultaneous Integrated Boost (SIB): A treatment technique in which a boosted region of the target is prescribed elevated dose per fraction relative to the rest of the target.

Skyshine: Radiation scattered by the atmosphere back to ground level outside the vault.

Solid Angle:

Specific Activity: the activity of a given radioisotope per gram.

Stereotactic Radio Surgery (SRS): use of external radiation in conjunction with a stereotactic guidance device to very precisely deliver a therapeutic dose to a tissue volume.

Stochastic Effects: health effects which occur randomly and for which the probability of the effect, rather than the effect’s severity, is related to dose.

Sulfur Hexaflouride (SF6): An insulating gas used in waveguide structures to prevent electrical arcing.

Supine: A patient position where the patient lies horizontally with the face and torso facing up and the palms facing down. Click here for more information on patient positions.

T: See Occupancy Factor.

T1/2Half-life

TaAverage or mean life

TMeanAverage or mean life

Teletherapy: a method of radiation therapy in which collimated radiaton is delivered at a distance from the patient.

iTenth Value Layer (TVL): Thickness of shielding which reduces the intensity of the transmitted beam to 10% of the incident intensity. 1TVL ≅3.32HVLs.

TVL: See Tenth Value Layer

Transmission Penumbra: See Penumbra (Transmission)

Trendelenburg (position): A position in which the patient body is supine with the feet 15-30 degrees above the head. This term may also be used as a synonym for pitch. Click here for more information on patient positions.

Two Source Rule: The two source rule treats the patient scatter and leakage components of secondary radiation as distinct sources. If the patient scatter and leakage transmission factors approximately equal, shielding thickness may be taken as the larger or the two barrier thicknesses plus 1 HVL. If the thickness of each source differs by 1 TVL or more, the larger barrier thickness may be used. Click here for more information on vault shielding.

U: See Use Factor.

Uncontrolled Area: All areas not considered controlled areas are considered uncontrolled areas. This term is defined in NCRP 151.

Use Factor (U):  The fraction of a primary-beam workload directed toward a given primary barrier. This term is defined in NCRP 151.

Very High Radiation Area: an area, accessible to individuals, in which radiation levels from radiation sources external to the body could result in an individual receiving an absorbed dose in excess of 500 rads (5 grays) in 1 hour at 1 meter from a radiation source or 1 meter from any surface that the radiation penetrates. From NRC 10 CFR Part 20.