Radiation Protection in Mammography: Recommended Safety Procedures for the Use of Mammographic X-Ray Equipment - Safety Code 33

4. Facility, equipment and installation requirements

In the planning of any mammographic X-ray facility, account must be taken of the operating X-ray tube voltage, the expected maximum workload of the equipment, and the occupancy factors for areas adjacent to the facility. Allowance should be made for possible changes in any one or all of these parameters, such as increases in operating tube voltage andworkload, changes in equipment position, modification in techniques that may require ancillary equipment and an increase in the degree of occupancy of surrounding areas.

4.1 Facility design criteria

Certain basic principles must be observed when determining the shielding requirements for a room used routinely for diagnostic radiology. These are as follows:

1. the radiation levels in controlled areas that are occupied routinely by radiation workers must be such that no radiation worker is occupationally exposed to more than 20 mSv per year; and
2. the radiation levels in uncontrolled areas must be such that no person can receive more than 1 mSv per year.

In general, radiation levels directly beside the image receptor of mammographic X-ray equipment are such that the above limits could be exceeded. However, because mammographic X-ray equipment uses low X-ray tube voltage, reduction in radiation intensity can be easily accomplished with the presence of a suitable shielding barrier between the patient and the technologist, a suitable combination of distance from the sources of radiation and shielding barriers, and restriction of persons from all areas in which the respective recommended dose limit could be exceeded.

The radiation shielding barriers required to reduce radiation levels below maximum permitted limits may be determined on the basis of distance, maximum expected X-ray tube voltage, workload, and occupancy factor. To ensure that the radiation levels are always below maximum permitted limits, the maximum possible workload should be used to calculate shielding requirements during the planning stage of the facility. Also, due consideration should be given to possible future increases in occupancy factors.

Shielding calculations should be performed only by individuals with an in-depth knowledge of radiation protection requirements and radiation shielding barriers. When such calculations are required, contact the appropriate government agency. For installations under federal jurisdiction the responsible agency is the Radiation Protection Bureau, Health Canada, Ottawa, Ontario, K1A 1C1. Mammographic X-ray facilities that fall under provincial jurisdiction must meet the requirements of the responsible agency in their respective provinces. These requirements can be obtained by contacting the appropriate agency listed in Appendix IV.

4.1.1 General recommendations

Protection of the technologist and others near mammographic X-ray equipment should be achieved by:

1. operating mammographic X-ray equipment in a dedicated room designed for the purpose of mammography. The doors leading to the mammographic room from public areas should be equipped with self-closing devices;
2. ensuring that mammographic rooms are designed to provide adequate working space to allow for ease of patient movement, and ensuring that there is always an adequate shield between the patient and the technologist. If the radiation shield is not part of the mammographic X-ray equipment, an appropriate barrier allowing visibility of the patient must be provided;
3. shielding the floor, walls, ceiling and doors on the basis of distance, maximum expected X-ray tube voltage, and workload. The occupancy factors for the adjacent areas must be considered when calculations are made. Shielding must be constructed to form an unbroken barrier and shielding materials must be adequately supported;
4. positioning the control booth or mammographic X-ray equipment so that, during an exposure, no one can enter the room without the knowledge of the technologist;
5. using appropriate warning signs, which must be posted on the outside of all doors leading to each mammographic room. The warning signs must incorporate the X-radiation warning symbol specified in Appendix VII and should incorporate the words "Unauthorized Entry Prohibited";
6. arranging for the final plans of the installation to be reviewed by the appropriate responsible government agency when a new facility is constructed or modification to an existing one is made.

The plans and accompanying documents must show:

• the dimensions and shape of the room where the mammographic X-ray equipment is operated;
• the materials used to construct the walls, floor, ceiling, and the control booth, and their thicknesses including additional materials used in radiation shielding barriers;
• the positions of all windows, doors, louvres, etc., that may affect radiation protection requirements;
• the location and orientation of mammographic X-ray equipment;
• the location, use and accessibility of adjacent rooms, as well as rooms above and below the facility;
• the expected maximum workload;
• the brief description of the mammographic X-ray unit, containing at least the name of the manufacturer, model designation, operating X-ray tube voltages and X-ray tube current; and
• the location of the darkroom and film storage area.
  
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4.1.2 Shielding recommendations

The thickness of lead, concrete or gypsum wallboard required to reduce radiation levels to the recommended dose limits can be determined through calculations. The following recommendations apply to mammographic facility shielding.

1. Planned and existing structural materials should be fully considered when calculating a barrier requirement.
2. For installations under federal jurisdiction, the shielding required must reduce dose to individuals in uncontrolled areas to 1 mSv per year and in controlled areas to 20 mSv per year. Mammographic X-ray facilities that fall under provincial jurisdiction must meet the requirements set by the responsible agency in their respective provinces. These requirements can be obtained by contacting the appropriate agency listed in Appendix IV.
3. Acoustical type lead is not suitable for lead shielding.
4. For mammographic X-ray equipment, the image receptor support intercepts most, but not all of the primary X-ray beam. Therefore, there may be a need for primary barrier shielding for some specific areas of the room.
5. For secondary barriers shielding, the orientation factor is always equal to 1.
6. To determine the shielding necessary for a mammographic facility, the following information is required:
   • the distance between the nearest point of the area to be shielded;
   • the usual location and position of the X-ray unit;
   • the area designation (controlled or uncontrolled), (primary or secondary);
   • occupancy factors of adjacent rooms;
   • the maximum operational X-ray tube voltage;
   • the location of the darkroom and film storage area; and
   • anticipated maximum workload of the mammographic X-ray unit (in milliampere-minutes per week).

4.2 Equipment purchase

All new, used and refurbished medical X-ray equipment, and accessories, purchased or leased in Canada must conform to the requirements of the Radiation Emitting Devices Act and Regulations. Mammographic X-ray equipment must comply with Radiation Emitting Devices Regulations, Part XII: "Diagnostic X-ray Equipment", and it is the responsibility of the manufacturer or distributor to ensure that the equipment conforms with the requirements. The appropriate sections of this regulation are reproduced in Appendix VII of this Safety Code. In addition, all mammographic X-ray equipment must meet provincial requirements established for such equipment. The Canadian Standards Association and provincial electrical utility should be consulted for further information.

4.2.1 Performance specification writing

During the purchasing process, performance specifications for the mammographic X-ray equipment and accessories such as film cassettes and screens must be prepared with the full knowledge of the clinical needs, operational conditions and requirements, manufacturer's specifications, and regulatory requirements. The level of performance should be such that most manufacturers should be able to meet these performance requirements with readily available components and product lines.

Testing equipment not already available and required to perform Quality Control procedures must be purchased at the same time as the mammographic X-ray unit. Only testing equipment which is to be used by the facility needs to be purchased. This equipment is described is Section 8.1.

The performance specifications should include all relevant requirements stated in Section 5.3 and any further requirement as specified by the agency responsible for the facility. The performance specifications should include relevant electrical, mechanical and environmental conditions which may affect the performance of the equipment.

4.2.2 Acceptance testing

Acceptance testing must be performed before any clinical use of the equipment. Acceptance testing is a process to verify compliance with the performance specifications of the mammographic X-ray equipment as written in the purchase agreement. It must also verify that the equipment performance meets the manufacturer's specifications and complies with federal and provincial regulations. It is recommended that acceptance testing be performed by a medical physicist, or other individual knowledgeable in mammographic X-ray equipment testing and relevant regulations. For mammographic X-ray equipment, the federal regulation is the Radiation Emitting Devices Regulations, Part XII and is stated in Appendix VII. Acceptance testing of a mammographic system includes several major steps. They are:

1. the verification that delivered components or systems correspond to what was ordered;
2. the verification of the system mechanical integrity and stability, including safety mechanism, automatic patient release, power drives, interlocks;
3. the verification of electrical installation, including electrical safety, powerline fluctuation; and
4. the verification of mammographic X-ray performance.

X-ray performance tests performed during the acceptance testing should also reflect the requirements described in Section 5.3. In addition, the testing must evaluate the image quality produced by the X-ray system and determine typical surface and glandular dose. The results from the acceptance testing should be used to set baseline values and limits on operational performance of the mammographic X-ray equipment. These baseline values and limits are essential to the Quality Assurance program.

4.3 Radiation protection inspection

Radiation protection inspections must be established on a regular basis to assess that:

1. the mammographic X-ray equipment functions properly and according to applicable standards and legislative requirements;
2. the mammographic X-ray equipment is installed in a safe environment and is used in a way which provides maximum radiation safety for patients, technologists, and the public;
3. an adequate Quality Assurance program is properly implemented and maintained; and
4. the level of diagnostic image quality is optimized and maintained.

For installations under Federal jurisdiction the responsible agency is the Radiation Protection Bureau, Health Canada, Ottawa, Ontario, K1A 1C1. Mammography facilities that fall under provincial jurisdiction must meet the requirements of the responsible agency in their respective provinces. These requirements can be obtained by contacting the appropriate agency listed in Appendix IV.