Linear Accelerator Shielding Guidelines

Listed below are some of the normal criteria that the Radiation Safety Section generally applies in evaluating the design and shielding performance of medical accelerator enclosures:

• The maximum integrated weekly radiation levels in occupiable areas outside the room, taking the workload, use factor, and occupancy factor into account, do not exceed the following:

0.1 mGy per week in air kerma average (5 mGy per year) for controlled areas

0.02 mGy per week in air kerma average (1 mGy per year) for persons in uncontrolled areas.

• The dose in any unrestricted area does not exceed 0.02 mGy in any one hour of operation with the occupancy factor equal to one.

The use of these limits for controlled and uncontrolled areas is in agreement with principles for maintaining radiation doses to personnel as low as reasonably achievable (ALARA).

• Occupancy factors for various areas should be no less than those suggested by the NCRP and should not normally be less than 1/40 for any area, including a roof.

Care should be taken when utilizing low occupancy factors such as 1/40 for uncontrolled occupiable areas.  In addition to meeting the design criteria of 1 mGy per year, additional calculations should be performed to ensure the dose into any unrestricted area does not exceed 0.02 mGy in any one hour.

• If a retractable beam stop is provided, room shielding is designed with the assumption that the beam stop either will not be in place or will be interlocked to ensure that it is in the beam when necessary to meet the dose and instantaneous dose rate levels referenced above.

• Beam stop design and room shielding adequately attenuate the x-ray beam and head leakage radiation as well as small angle scatter which does not strike the beam stop.

• Photon and neutron shielding methods for the door and any other penetrations meet the dose and dose rate levels referenced above.

• All shielding considerations are for worst-case scenarios.

• A "search and evict" time-delay switch is recommended to be located within the accelerator enclosure near the patient treatment area. In order to operate an accelerator, the last person leaving the room would need to manually reset this switch and then promptly close the door. If the door is not closed within a reasonable time delay allowed by the switch, or if someone subsequently enters the room and the door is closed behind him, accelerator operation would be prevented. This safety feature or an equivalent alternative would reduce the chance of staff or other persons being accidentally exposed to radiation while in the room with the door closed, especially if they are out of the view of the patient TV camera. For your information, such accidents have been reported in several Michigan medical accelerator facilities which did not have "search and evict" switches.

• Emergency beam cut-off switches are installed in appropriate locations in the accelerator room.

Plans submitted for linear accelerator facilities should include confirmation that the enclosure will meet the above design and safety criteria and concepts for maintaining radiation levels as low as reasonably achievable.

June 1, 2005