Definitions

Definitions. As used in this section, the following definitions apply: 1. "Absorbed dose" is the energy imparted to matter by ionizing radiation per unit mass of irradiated material at the place of interest. The special unit of absorbed dose is the rad (see "Rad"). 2. "Absorbed dose rate" means absorbed dose per unit time, for machines with timers, or dose monitor unit per unit time for linear accelerators. 3. "Accessible surface" means the external surface of the enclosure or housing provided by the manufacturer. 4. "Added filtration" means any filtration which is in addition to the inherent filtration. 5. "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. Kerma is measured in the same unit as absorbed dose. 6. "Barrier" (See "Protective barrier"). 7. "Beam axis" means the central ray of the useful radiation beam that passes through the isocenter and the source of radiation. 8. "Beam-limiting device" means a field defining collimator which provides a means to restrict the dimensions of the useful beam. 9. "Beam monitoring system" means a system designed and installed in the radiation head to detect and measure the radiation present in the useful beam. 10. "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. 11. "Bent beam linear accelerator" means a linear accelerator geometry in which the accelerated electron beam must change direction by passing through a bending magnet. 12. "Changeable filters" means any filter, exclusive of inherent filtration, which can be removed from the useful beam through any electronic, mechanical, or physical process. 13. "Contact therapy system" means a therapeutic radiation machine with a short source to skin distance (SSD), usually less than 5 centimeters. 14. "Detector" (See "Radiation detector"). 15. "Dose monitor unit (DMU)" means a unit response from the beam monitoring system from which the absorbed dose can be calculated. 16. "External beam radiation therapy" means therapeutic irradiation in which the source of radiation is at a distance from the body. 17. "Field-flattening filter" means a filter used to homogenize the absorbed dose rate over the radiation field. 18. "Filter" means material placed in the useful beam to change beam quality in therapeutic radiation machines subject to1.15.6. 19. "Gantry" means that part of a system supporting and allowing movements of the radiation head about a center of rotation. 20. "Gray (Gy)" means the special name for the SI unit of absorbed dose, kerma, and specific energy imparted equal to 1 joule per kilogram. The previous unit of absorbed dose (rad) is being replaced by the gray. [1 Gy=100 rad]. 21. "Half-value layer (HVL)" means the thickness of a specified material which attenuates under narrow beam conditions, x-radiation or gamma radiation to an extent such that the air kerma rate, exposure rate or absorbed dose rate is reduced to one-half of the value measured without the material. 22. "Interlock" means a device preventing the start or continued operation of equipment unless certain predetermined conditions prevail. 23. "Interruption of irradiation" means the stopping of irradiation with the possibility of continuing irradiation without resetting of operating conditions at the control panel. 24. "Irradiation" means the exposure of a living being or matter to ionizing radiation. 25. "Isocenter" means the center of the smallest sphere through which the useful beam axis passes. 26. "Kilovolt (kV) [kilo electron volt (keV)]" means the energy equal to that acquired by a particle with one electron charge in passing through a potential difference of one thousand volts in a vacuum. [Note: current convention is to use kV for photons and keV for electrons.] 27. "Lead equivalent" means the thickness of the material in question affording the same attenuation, under specified conditions, as lead. 28. "Leakage radiation" means radiation emanating from the therapeutic source assembly except for the useful beam. 29. "Light field" means the area illuminated by light, being the locus of points at which the illumination exceeds a specific or specified level, simulating the radiation field. 30. "mA" means milliampere. 31. "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.] 32. "Monitor unit (MU)" (See "Dose monitor unit"). 33. "Moving beam radiation therapy" means radiation therapy with continuous displacement of the radiation source relative to the patient during irradiation. It includes arc therapy, skip therapy, conformal therapy and rotational therapy. 34. "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. 35. "Patient" means an individual subjected to machine produced external beam radiation for the purposes of medical therapy. 36. "Peak tube potential" means the maximum value of the potential difference across the x-ray tube during an exposure. 37. "Periodic quality assurance check" means a procedure which is performed to ensure that a previous calibration continues to be valid. 38. "Phantom" means an object behaving in essentially the same manner as tissue, with respect to absorption or scattering of the ionizing radiation in question. 39. "Practical range of electrons" corresponds to classical electron range where the only contribution to dose is from bremsstrahlung x-rays. Precise definition may be found in "Clinical Electron Beam Dosimetry: Report of AAPM Radiation Therapy Committee Task Group 25" [Medical Physics 18(1): 73-109, Jan/Feb 1991] and ICRU Report 35, "Radiation Dosimetry: Electron Beams with Energies Between 1 and 50 MeV", International Commission on Radiation Units and Measurements, September 15, 1984. 40. “Primary dose monitoring system" means a system which will monitor the useful beam during irradiation and which will terminate irradiation when a preselected number of dose monitor units have been acquired. 41. "Primary protective barrier" (See "Protective barrier"). 42. "Protective barrier" means a barrier of radiation absorbing material(s) used to reduce radiation exposure. The types of protective barriers are as follows: a. "Primary protective barrier" means the material, excluding filters, placed in the useful beam, to protect anyone other than the patient from radiation exposure. b. "Secondary protective barrier" means a barrier sufficient to attenuate the stray radiation to the required degree. 43. "Radiation detector" means a device which, in the presence of radiation provides, by either direct or indirect means a signal or other indication suitable for use in measuring one or more quantities of incident radiation. 44. "Radiation head" means the structure from which the useful beam emerges. 45. "Radiation Therapy Physicist" means an individual qualified in accordance with 1.15.3(4). 46. "Redundant dose monitoring combination" means a combination of two dose monitoring systems in which both systems are arranged to terminate irradiation in accordance with a preselected number of dose monitor units. 47. "Response time" means the time required for an instrument system to reach 90 percent of its final reading when the radiation-sensitive volume of the instrument system is exposed to a step change in radiation flux from zero sufficient to provide a steady state midscale reading. 48. "Scattered radiation" means ionizing radiation emitted by interaction of ionizing radiation with matter, the interaction being accompanied by a change in direction of the radiation. Scattered primary radiation means that scattered radiation which has been deviated in direction only by materials irradiated by the useful beam. 49. "Secondary dose monitoring system" means a system which will terminate irradiation in the event of failure of the primary dose monitoring system. 50. "Secondary protective barrier" (See "Protective barrier"). 51. "Shadow tray" means a device attached to the radiation head to support auxiliary beam blocking material. 52. "Shutter" means a device attached to the tube housing assembly which can totally intercept the useful beam and which has a lead equivalency not less than that of the tube housing assembly. 53. "Sievert (Sv)" means the special name for the SI unit of dose equivalent. The unit of dose equivalent is the joule per kilogram. The previous unit of dose equivalent (rem) is being replaced by the sievert [1 Sv=100 rem]. 54. "Simulator (radiation therapy simulation system)" means a radiographic or fluoroscopic x-ray system intended for localizing the volume to be exposed during radiation therapy and confirming the position and size of the therapeutic irradiation field. 55. "Source" means the region and/or material from which the radiation emanates. 56. "Source-skin distance (SSD)" means the distance measured along the central ray from the center of the front surface of the radiation source to the surface of the irradiated object or patient. [See also Target-skin distance] 57. "Stationary beam radiation therapy" means radiation therapy without displacement of the radiation source relative to the patient during irradiation. 58. "Stray radiation" means the sum of leakage and scattered radiation. 59. "Target" means that part of an x-ray tube or particle accelerator onto which is directed a beam of accelerated particles to produce ionizing radiation or other particles. 60. "Target-skin distance (TSD)" means the distance measured along the central ray from the center of the front surface of the x-ray target to the surface of the irradiated object or patient. [See also Source-skin distance] 61. "Tenth-value layer (TVL)" means the thickness of a specified material which attenuates under broad beam conditions, 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. 62. "Termination of irradiation" means the stopping of irradiation in a fashion which will not permit continuance of irradiation without the resetting of operating conditions at the control panel. 63. "Therapeutic radiation machine" means x-ray or electron-producing equipment designed and used for external beam radiation therapy. 64. "Tube" means an x-ray tube, unless otherwise specified. 65. "Tube housing assembly" means the tube housing with tube installed. It includes high-voltage and/or filament transformers and other appropriate elements when such are contained within the tube housing. 66. "Useful beam" means the radiation emanating from the tube housing port or the radiation head and passing through the aperture of the beam-limiting device when the exposure controls are in a mode to cause the therapeutic radiation machine to produce radiation. 67. "Virtual source" means a point from which radiation appears to originate. 68. "Wedge filter" means a filter which effects continuous change in transmission over all or a part of the radiation field. 69. "X-ray tube" means any electron tube which is designed to be used primarily for the production of x-rays.