DHS 157.85(1)(b)(b) Leakage radiation from contact therapy systems may not exceed one mGy (103 mR) per hour at 5 centimeters from the surface of the tube housing assembly. Contact therapy tube housing assemblies shall have a removable shield of material, equivalent in attenuation to 0.5 millimeters of lead at 100 kV, which may be positioned over the entire useful beam exit port during periods when the beam is not in use. DHS 157.85(1)(c)(c) Leakage radiation from systems operating at 150 kV or less may not exceed one mGy (103 mR) per hour at one meter from the tube housing. DHS 157.85(1)(d)(d) Leakage radiation from systems operating above 150 kV may not exceed 0.1% of the useful beam one meter from the source housing for any of its operating conditions. DHS 157.85(2)(2) Permanent beam-limiting devices. Permanent, non-adjustable collimators used for limiting the useful beam shall provide at least the same degree of attenuation as required for the tube housing assembly. DHS 157.85(3)(a)(a) All removable beam-limiting devices or diaphragms may not transmit more than one percent of the useful beam for the most penetrating beam used. This paragraph does not apply to beam shaping blocks or shaping materials. DHS 157.85(3)(b)(b) When adjustable beam-limiting devices are used, the position and shape of the useful beam shall be indicated by a light beam. These devices may transmit not more than 5% of the useful beam. DHS 157.85(4)(4) Filter systems. The filter system shall be designed to meet all of the following requirements: DHS 157.85(4)(a)(a) Accidental displacement of filters is not possible at any tube orientation. DHS 157.85(4)(b)(b) If the proper filter is not in place, an interlock system shall prevent irradiation. DHS 157.85(4)(c)(c) The air kerma rate escaping from the filter placement opening slot in the tube head may not exceed 100 mGy (one rad) per hour at one meter under any operating conditions. DHS 157.85(4)(d)(d) Each filter shall be marked as to its material of construction and its thickness. DHS 157.85(4)(e)(e) Each wedge filter that is removable from the system shall be clearly marked with an identification number. For removable wedge filters, the nominal wedge angle shall appear on the wedge or wedge tray. If the wedge or wedge tray is significantly damaged, the wedge transmission factor shall be reestablished. DHS 157.85(4)(f)(f) If the absorbed dose rate information relates exclusively to operation with a field flattening filter or beam scattering foil in place, that foil or filter shall be removable only by the use of tools. DHS 157.85(5)(a)(a) An x-ray tube shall be mounted so that it cannot accidentally turn or slide with respect to the opening in the tube housing through which radiation is emitted. DHS 157.85(5)(b)(b) The tube housing assembly shall be capable of being immobilized. DHS 157.85(6)(6) Emergency switches. At least one emergency power cutoff switch shall be present. If more than one emergency power cutoff switch is installed and not all switches are tested at once, each switch shall be tested on a rotating basis. Safety quality control checks of the emergency power cutoff switches may be conducted at the end of the treatment day to minimize possible stability problems with the therapeutic radiation machine. DHS 157.85(7)(7) Source marking. An x-ray tube housing assembly shall be marked so that it is possible to determine the location of the focal spot to within 5 millimeters and the marking shall be readily accessible for use during calibration procedures. DHS 157.85(8)(a)(a) A suitable irradiation control device shall be provided to terminate the irradiation after a preset time interval or after a preset radiation dose has been delivered. DHS 157.85(8)(b)(b) A timer with a display shall be provided at the treatment control panel. The timer shall have a pre-set time selector and an elapsed time or time remaining indicator. DHS 157.85(8)(c)(c) A timer shall be a cumulative timer that activates with an indication of “BEAM-ON” and retains its reading after irradiation is interrupted or terminated. After irradiation is terminated and before irradiation may be reinitiated, it shall be necessary to reset the elapsed time indicator. DHS 157.85(8)(d)(d) A timer shall terminate irradiation when a pre-selected time has elapsed, if any dose monitoring system present has not previously terminated irradiation. DHS 157.85(8)(f)(f) A timer may not activate until the shutter is opened when irradiation is controlled by a shutter mechanism unless calibration includes a timer end effect correction to compensate for mechanical lag. DHS 157.85(8)(g)(g) A timer shall be accurate to within one percent of the selected value or one second, whichever is greater. DHS 157.85(9)(9) Control panel indicators. An x-ray unit shall have all of the following: DHS 157.85(9)(a)(a) An indication at the control panel of whether electrical power is on and if activation of the x-ray tube is possible.