DiagnosticImaging.com.

Breast Shields Better at Reducing Exposure Than Posteriorly-Centered Partial CT

By DiagnosticImaging Staff | April 28, 2011

The use of breast shields is the technique of choice to protect the breasts of women from radiation exposure while undergoing chest CT examinations, according to a new study.

Rafel Tappouni, MD, and colleagues at Penn State Hershey Medical Center compared the radiation dose to the front and back of a breast phantom using a breast shield and using posteriorly-centered partial CT.

“We found that posteriorly centered partial CT does decrease skin entrance radiation dose to the breast by 16 percent, but increases overall radiation dose to the chest by 8 percent,” said Tappouni. “The bismuth breast shields, on the other hand, reduced skin entrance dose to the breast by 38 percent without an increase in overall radiation dose,” he said.

Tappouni notes that they now use breast shields at his facility for all female patients up to age 90 who undergo chest CT examinations.

The efficacy of breast shields has a long precedent, but this report specifically compares their use to posteriorly-centered partial CT. Also, the issue has become more important since the International Commission of Radiation Protection (ICRP) increased the tissue weighting factor for the breast from 0.05 to 0.1.

To put the risk into perspective, the delivery of 1 rad to a 35 year-old woman can increase her lifetime risk of breast cancer by 13.6 percent; each CT exam delivers at least twice that amount, Tappouni said.

The study is being presented during the American Roentgen Ray Society Annual Meeting on May 4 in Chicago.

by john miller | May 14, 2011 10:39 AM EDT

I have used for fixed mA low dose cardiac CT (Impact software 0.8mSv to 4mSv) and the image noise is only in the breast tisse and not the ribs or heart. So diagnostically OK for me. We did some TLD work with local very helpful physicists who demonstrated some dose saving but are still sceptical.

I have drivewn dose down as far as I can with low kVp, reduced mA and zero padding with prospective software. We trick the smart mA by applying shields after the scanograms.

This will not be solved without close dialogue between radiologists and physicists. If a benefit then why not all chest abdo scans on females?

John Miller

Inverness UK

by jia wang | May 02, 2011 8:46 PM EDT

Note - the post below (listed under Jia Wang) was not from Jia. I used a link that he sent to me to open the article.

Cynthia

by jia wang | May 03, 2011 12:44 AM GMT
Having performed these experiments myself in carefully controlled conditions, I disagree with the conclusions summarized in this article. What is not controlled for in this work is image noise. To achieve equal image noise, the bismuth shielding requires more dose than the partial scan approach because it attenuated useful photons exiting the patient.

Bismuth shielding goes against the fundamental tenets of radiation protection in medicine. It attenuates x-rays AFTER they have interacted with the patient (that is, on the way out of the patient). It also causes streaks, increases image noise, and changes CT numbers. Radiologists state in multiple studies that the resultant image quality is acceptable to them. If that level of image quality is acceptable, then I can set up a technique to save as much or more dose that will have the same image noise but no streaks or bad CT numbers.

Please consult with a qualified CT physicist to discuss dose reduction approaches. These are complex medical devices and laying a piece of attenuating vinyl on top of the patient is not the best way to reduce dose.

The dose and risk values quoted in this story are also not correct. A chest CT exam results in about 5-7 mSv of effective dose in a typical adult. A 10 mSv effective dose raises the risk of a fatal cancer from the background rate of 22% to about 22.05% - i.e. an absolute increase in risk of less than 0.1%. These values are for all cancers. I'm not sure where the value of 13.6% is coming from ...

Cynthia McCollough, PhD
Professor of Radiological Physics

by jia wang | May 02, 2011 8:44 PM EDT

Having performed these experiments myself in carefully controlled conditions, I disagree with the conclusions summarized in this article. What is not controlled for in this work is image noise. To achieve equal image noise, the bismuth shielding requires more dose than the partial scan approach because it attenuated useful photons exiting the patient.

Bismuth shielding goes against the fundamental tenets of radiation protection in medicine. It attenuates x-rays AFTER they have interacted with the patient (that is, on the way out of the patient). It also causes streaks, increases image noise, and changes CT numbers. Radiologists state in multiple studies that the resultant image quality is acceptable to them. If that level of image quality is acceptable, then I can set up a technique to save as much or more dose that will have the same image noise but no streaks or bad CT numbers.

Please consult with a qualified CT physicist to discuss dose reduction approaches. These are complex medical devices and laying a piece of attenuating vinyl on top of the patient is not the best way to reduce dose.

The dose and risk values quoted in this story are also not correct. A chest CT exam results in about 5-7 mSv of effective dose in a typical adult. A 10 mSv effective dose raises the risk of a fatal cancer from the background rate of 22% to about 22.05% - i.e. an absolute increase in risk of less than 0.1%. These values are for all cancers. I'm not sure where the value of 13.6% is coming from ...

Cynthia McCollough, PhD
Professor of Radiological Physics

by Mitch Goodsitt | May 02, 2011 5:41 PM EDT

Most physicist believe radiologitsts have been sold a bag of goods. These bismuth breast shields absorb the CT signal for PA projections and do not perform as well as simply reducing the mAs by 30%. Even better resutlts can be obtained with organ specific tube current modulation (e.g., reduce mAs for AP projections of breast regions (where x-rays are incident on breast) and increase mAs for PA projections (where attenuation of intervening tissues decrease dose to breast),
See:
1) Eur Radiol (2006) 16: 2334-2340 "An increase in image noise could be
observed in the ranges where bismuth shielding was applied. The observed
reduction of organ dose and total energy imparted could be achieved
more efficiently by a reduction of tube current. The application of in-plane
selective shielding is therefore discouraged."

2) Eur Radiol (2008) 18: 1674-1682 "Bismuth shielding may compromise image
quality, increase noise level and introduce streak artifacts. Partial and
TCM (tube current modulaiton) examinations reduced dose to the breast without influencing image
quality."

3) Geleijns J., Wang J., McCollough C., "The Use of Breast Shielding for Dose Reduction in Pediatric CT: Arguments Against the Proposition,"Pediatric Radiology, August 2010.
"In summary, we strongly recommend against the use of
selective bismuth shielding, for all patients, and especially
for children. A far superior alternative for minimizing dose
in pediatric chest CT is reduction of the tube current to
achieve the required image quality at the lowest possible
dose [7, 8], and the use of angular and z-axis tube current
modulation, which has been shown to reduce dose to the
region of the breast by approximately 50% without altering
the accuracy of CT numbers or introducing streak and beam
hardening artifacts [10]."

It's time for radiologists to start consulting with their Medical Physicists.
Sincerely,
Mitch Goodsitt

by Hans Ringertz | May 02, 2011 1:50 PM EDT

It is clearly incorrect that a CT including the breast should deliver 1000 mSv. Should be corrected by whoever published it.
Hans Ringertz

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