Can the risk of breast cancer from CT radiation be reduced?

 

Breast radiation dose to females during CT imaging is a significant concern because of the potential for development of cancer. The percentage of patients developing cancer as a result of radiation exposure is dependent upon the age of the patient and the radiation dose to the breasts (Einstein AJ et al., JAMA 2007; 298(3): 317-323). Industry responses include partial volume scanning, improved collimation to reduce overscanning, iterative reconstruction techniques, bismuth shields, and breast displacement. These responses can be divided into hardware and software equipment developed by CT manufacturers and equipment actively protecting women being scanned by either shielding or moving their breast tissue.  There are advantages and disadvantages to all of these approaches.

 

Equipment Solutions

 

Partial volume scanning is available on some new units and includes software and hardware that markedly reduce tube output in the anterior 120◦rotation and increase tube output in the dorsal 240◦rotation. It is an expensive response not available on older scanners, but it has been shown to result in breast dose decreases of 30% to 40% and dose decreases in the anterior lungs (as achieved by Siemens Medical). Image quality is not reduced, and the use of this technique is easily accommodated in the workflow of a department. A downside is the increase in dose to the posterior lungs to compensate for the decrease in dose to the anterior of the chest and breasts, which is concerning in view of the susceptibility of lung tissue in women and the potential for development of lung cancer (BEIR VII). Another concern is the location of breast tissue in patients with large breasts that naturally fall to the side while in the supine position, out of the 120◦anterior arc and into the dorsal arc, with the attendant increase in dose. This effect is not appreciated in phantom studies in which the breasts are small cones attached to the anterior chest wall.  A solution to this problem would be to include breast size in departmental protocols for CT imaging; however, doing so would increase the complexity of the protocols.

 

Improved collimation, such as achieved through an Adaptive Dose Shield from Siemens Medical, is useful in reducing the breast dose when scanning the abdomen but would have less impact on coronary arterial and thoracic CT. It is both a hardware and software solution not available on all new scanners, and it is not available as an update for older scanners. Overall dose savings to the body in general range from 15% to 25%, depending on the length of the scan – the shorter the scan, the greater are the dose savings.

 

Iterative reconstruction techniques produce maximal overall dose reductions of 40% to 60% (as achieved by GE and Siemens) while maintaining image quality. These techniques require software and, in most cases, hardware upgrades. The downsides for this solution are cost and limited availability for each scanner.

 

Solutions That Shield or Move the Breasts

 

Bismuth shields are used to shield the breast tissue when in the scan plane. They are available in different thicknesses and sizes. They are most effective when placed after the scout image for GE scanners since the technique for GE is determined by the scout image and does not change during the actual scan. The effect is to “fool” the machine by adding the shielding after the scout image. Much of the research on bismuth shields has been done on GE units. Potentially, software could change the automatic exposure control to compensate for the use of bismuth shields. Breast dose reductions in published studies range from 25% to 50% depending on the thickness of the shield, with some variation between studies. Dose reductions realized will be additive to those reduction techniques of CT manufacturers, with minimal degradation of image quality. Downsides include the relatively small dose reduction of 50% in the best scenarios, need for compensation with all CT manufacturers depending on the type of exposure control, the potential for dose increase in the dorsal lungs, the need to actively place the shields when scanning patients, and difficulties in patients with particularly large breasts. 

 

Finally, breast displacement is a new idea for breast dose reduction as seen with the Chrysalis Breast Displacement system. In a presentation at RSNA 2009, median dose reductions of 88% and 94% at the nipples and outer quadrants, respectively, were quoted for coronary arterial CTA by moving the breasts upward out of the scan plane. Dose reduction in the inferior quadrants of the breasts was 32%. In patients with large breasts, displacement has the additional advantage of decreasing the amount of tissue in the scan plane. This approach is the only means of breast dose reduction in which image quality is improved in patients with large breasts. Dose reductions achieved will be additive to techniques used by CT manufacturers and would make partial volume scanning useful for patients with large breasts by moving them both upward, anteriorly, and into the low dose anterior 120◦ arc and should be synergistic with this technique. Additional dose reduction can be realized by using optional external lead shielding placed above the scan plane using scout images for accurate placement. Indications include coronary arterial CTA and abdominal CT for all women and thoracic CT, particularly if performed according to a pulmonary embolism protocol, for women with large breasts. Patient satisfaction and comfort are high, and patients respond favorably to the additional care taken to move the breasts out of the image plane. A downside includes the need to displace and secure the breasts, requiring two personnel for optimal results. Self-displacement is an alternative but is not as effective. Additionally, use in thoracic CT is contraindicated in those patients with small-to-medium breasts since the tissue would be displaced upward to the level of the shoulder girdle, thereby increasing breast dose. 

 

Conclusion

 

Radiation to breast tissue during CT imaging can increase the risk of developing cancer. Fortunately, many techniques to reduce radiation exposure exist and are undergoing refinement, including partial volume scanning, improved collimation to reduce overscanning, iterative reconstruction techniques, bismuth shields, and breast displacement.