Code of Maine Rules › 10 - DEPARTMENT OF HEALTH AND HUMAN SERVICES › 144 - DEPARTMENT OF HEALTH AND HUMAN SERVICES - GENERAL › Chapter 220 - Rules Relating to Radiation Protection › Part X - THERAPEUTIC RADIATION MACHINES › Section 144-220-X-2 - Definitions

Current through 2024-29, July 17, 2024
As used in this Part the following definitions
					 apply.
Absorbed dose rate
means absorbed dose per
					 unit time, for machines with timers, or dose monitor unit per unit time for
					 linear accelerators.
Accessible surface
means surface of equipment
					 or of an equipment part that can be easily or accidentally touched by persons
					 without the use of a tool.
Air kerma (K)
means the kinetic energy
					 released in air by ionizing radiation. Kerma is determined as the quotient of
					 dE by dM, where dE is the sum of the initial kinetic energies of all the
					 charged ionizing particles liberated by uncharged ionizing particles in air of
					 mass dM. The SI unit of air kerma is joule per kilogram and the special name
					 for the unit of kerma is the gray (Gy).
Authorized medical physicist
means an
					 individual as defined in G.2 and in compliance with the applicable provisions
					 of Part G of this rule .
Beam scattering foil
means a thin piece of
					 material (usually metallic) placed in the beam to scatter a beam of electrons
					 in order to provide a more uniform electron distribution in the useful
					 beam.
Bent beam linear accelerator
means a linear
					 accelerator geometry in which the accelerated electron beam must change
					 direction by passing through a bending magnet.
Contact therapy system
means a therapeutic
					 radiation machine with a short target to skin distance (TSD), usually less than
					 5 centimeters.
Dose monitor unit(DMU)
means a unit response
					 from the beam monitoring system from which the absorbed dose can be
					 calculated.
External beam radiation therapy
means
					 therapeutic irradiation in which the source of radiation is at a distance from
					 the body.
Field flattening filter
means a filter used to
					 homogenize the absorbed dose rate over the radiation field.
Filter
means material placed in the useful
					 beam to change beam quality in therapeutic radiation machines subject to
					 Subpart X.6.
Gantry
means that part of a radiation therapy
					 system supporting and allowing movements of the radiation head about a center
					 of rotation.
Interruption of irradiation
means the stopping
					 of irradiation with the possibility of continuing irradiation without resetting
					 of operating conditions at the control panel.
Isocenter
means the center of the sphere
					 through which the useful beam axis passes while the gantry moves through its
					 full range of motions.
Leakage radiation
means radiation emanating
					 from the radiation therapy system except for the useful beam.
Light field
means the area illuminated by
					 light, simulating the radiation field.
Megavolt (MV)
mega electron volt (MeV) means
					 the energy equal to that acquired by a particle with one electron charge in
					 passing through a potential difference of one million volts in a vacuum. Note:
					 current convention is to use MV for photons and MeV for electrons.
Moving beam radiation therapy
means radiation
					 therapy with any planned displacement of radiation field or patient relative to
					 each other, or with any planned change of absorbed dose distribution. It
					 includes arc, skip, conformal, intensity modulation and rotational
					 therapy.
Nominal treatment distance
means:
(a) For electron irradiation, the distance from the
					 scattering foil, virtual source, or exit window of the electron beam to the
					 entrance surface of the irradiated object along the central axis of the useful
					 beam.
(b) For x-ray irradiation, the virtual source or target to
					 isocenter distance along the central axis of the useful beam. For
					 non-isocentric equipment, this distance shall be that specified by the
					 manufacturer.
Periodic quality assurance check
means a
					 procedure, which is performed to ensure that a previous calibration continues
					 to be valid.
Practical range of electrons
corresponds to
					 classical electron range where the only remaining contribution to dose is from
					 bremsstrahlung x-rays.
Primary dose monitoring system
means a system
					 which will monitor the useful beam during irradiation and which will terminate
					 irradiation when a pre-selected number of dose monitor units have been
					 delivered.
Radiation field
(See Useful beam.)
Redundant beam monitoring system
means a
					 combination of two dose monitoring systems in which each system is designed to
					 terminate irradiation in accordance with a preselected number of dose monitor
					 units.
Secondary protective barrier
(SeeProtective
					 barrier.)
Shadow tray
means a device attached to the
					 radiation head to support auxiliary beam blocking material.
Simulator (radiation therapy simulation
					 system)
means any x-ray system intended for localizing the volume to be
					 exposed during radiation therapy and reproducing the position and size of the
					 therapeutic irradiation field.
Source skin distance (SSD)
(See Target skin
					 distance.)
Stationary beam radiation therapy
means
					 radiation therapy without displacement of one or more mechanical axes relative
					 to the patient during irradiation.
Target
means that part of an x-ray tube or
					 accelerator onto which a beam of accelerated particles is directed to produce
					 ionizing radiation or other particles.
Target skin distance (TSD)
means the distance
					 measured along the beam axis from the center of the front surface of the x-ray
					 target and/or electron virtual source to the surface of the irradiated object
					 or patient.
Tenth value layer (TVL)
means the thickness of
					 a specified material which attenuates X radiation or gamma radiation to an
					 extent such that the air kerma rate, exposure rate, or absorbed dose rate is
					 reduced to one tenth of the value measured without the material at the same
					 point.
Therapeutic radiation machine
means x-ray or
					 electron producing equipment designed and used for external beam radiation
					 therapy.