Due to a change in the federal performance standard for mammography collimation, the Section has revised its mammography room shielding recommendations (effective August 1, 2000). The new federal collimation standard requires that machines manufactured after September 30, 1999, provide a primary protective barrier that intercepts the cross section of the useful beam along every direction except at the chest-wall edge. The x-ray tube shall not permit exposure unless the appropriate barrier is in place to intercept the useful beam.

Following are the revised shielding recommendations for mammography rooms that enclose machines manufactured after September 30, 1999:

• Two layers of 5/8-inch drywall are normally sufficient for these mammography rooms.
  
• Doors should be equivalent to 1 millimeter of steel. Except as noted below, wood doors without additional shielding should not normally be used in mammography rooms unless protected by the operator's barrier or other acceptable vertical barrier.

Special note regarding doors: To date, testing by the Radiation Safety Section has not confirmed the general adequacy of any wooden doors to substitute for steel or lead-lined doors in mammography rooms.  Based on Section 2.3.3.2 of NCRP Report # 147, issued November 19, 2004, "wooden doors exhibit limited attenuation efficiency, and not all wooden doors are constructed with equal integrity."  Some "drop-in-core" models or "lumber core" doors provide little shielding because of large gaps and voids, according to the NCRP.  In cases where a wooden door might be considered for a particular mammography room, it is important to ensure the uniform shielding integrity of the door and for a facility to demonstrate the adequacy of the door.  In this regard, NCRP notes that one should specify American Woodwork Institute type PC5 (solid wooden core), 43 mm thick doors or C-45 (mineral core), or equivalent, for mammography shielding applications in a room where a wooden door might be judged acceptable.  As shown under the Mammography Shielding Assumptions link below, a wooden door would need to be about 7 inches thick to provide the same radiation attenuation as two layers of 5/8-inch drywall.  Despite the lack of shielding equivalence of a solid wooden door compared to standard drywall construction, certain solid-wood-core or mineral-core wood doors may, according to the NCRP in the above section and in Section 5.5, be considered for mammography shielding applications in some limited situations, based on room design layout, workload, and occupancy factors.  Use of a wooden door is also dependent upon specific approval by the Radiation Safety Section.  Furthermore, room shielding design should not normally specify reduced shielding for a door or other area just because that area is behind the patient.  Although radiation behind the patient may be reduced from self-shielding by the body of the patient, it may not be practical to ensure a fixed and on-going permanent arrangement of a mammography machine within a room.

• Standard concrete construction found in the ceiling and floors of most hospitals and office buildings provides sufficient shielding for mammography rooms. However, if located in a room with only wood construction between floors, additional shielding may be required.
  
• Pursuant to Rule 655, the operator's barrier shall provide radiation protection equivalent to not less than 0.5 millimeter of lead when the maximum potential is limited to 35 kilovolts or less, and 0.8 millimeter of lead when the maximum potential is greater than 35 kilovolts.

For mammography rooms enclosing machines manufactured on or prior to September 30, 1999, the following shielding guidelines still apply:

• Typical wall construction (two layers of drywall) is not sufficient for these mammography machines. Three (3) layers of 5/8-inch drywall (or four layers of 1/2-inch drywall) are acceptable. However, due to the construction complications that this extra drywall creates, most facilities find it is better to use 1/32-inch leaded drywall to provide the needed attenuation. An additional advantage to using 1/32-inch leaded drywall is that radiation doses beyond the wall are reduced not just to permissible levels but to essentially undetectable levels.
  
• Mammography room doors should be provided with shielding equivalent to 1/32-inch thick lead or at least 1 millimeter of steel, even if exposed only to secondary radiation. Wood doors without additional shielding should not normally be used in mammography rooms unless protected by the operator's barrier or other acceptable permanent-design type of vertical barrier.  Solid core wood doors are not considered sufficient for rooms containing older mammography machines.
  
• Mammography room floors should also be equivalent to 1/32-inch lead shielding. In most cases, standard concrete construction found in most hospitals and office buildings is sufficient. Ceilings in these rooms are not considered primary barriers. However, if located in a room with only wood construction between floors, additional shielding may be required.
  
• Pursuant to Rule 655, the operator's barrier shall provide radiation protection equivalent to not less than 0.5 millimeter of lead when the maximum potential is limited to 35 kilovolts or less, and 0.8 millimeter of lead when the maximum potential is greater than 35 kilovolts.

Assumptions used for mammography shielding design recommendations

February 14, 2005