Chest Imaging and Radiation Exposure Reduction

Media coverage of excessive radiation doses from CT scans has led to increased public awareness of the health risks of ionizing radiation from CT scans. The number of CT scans performed annually in the United States increased from 3 million in 1980 to more than 70 million in 2007 (Amis et al. J Am Coll Radiol 2007;4[5]:272). The National Cancer Institute projects 29,000 excess cancers, with an estimated 50% case fatality rate, from the 72 million CT scans that Americans received in 2007 alone (Berrington de González et al. Arch Intern Med 2009;169[22]:2071).

While there is some disagreement over the extent of the cancer risk associated with exposure to radiation from medical imaging, there is broad agreement that steps can and should be taken to reduce unnecessary radiation exposure.

On February 9, 2010, the US Food and Drug Administration (FDA) announced an initiative to reduce unnecessary radiation exposure from three types of medical imaging procedures: computed tomography (CT), nuclear medicine studies, and fluoroscopy. The FDA is advocating the adoption of the following two principles of radiation protection:
1. Appropriate justification of the radiation procedure.
2. Optimization of the radiation dose used during each procedure.

Dr. Jeffrey Shuren, JD, director of the FDA Center for Devices and Radiological Health, elaborated by saying, “The goal of FDA’s initiative is to support the benefits associated with medical imaging while minimizing the risks …to help patients get the right imaging exam, at the right time, with the right radiation dose” (www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm200085.htm). In support of this goal, the FDA is launching a cooperative, “Initiative to Reduce Unnecessary Radiation Exposure from Medical Imaging.”

Radiation Doses
Background radiation is estimated at 3 millisievert (mSv) per year at sea level. Of this, 2 mSv is estimated to come from radon exposure in American homes. Annual background exposure may range up to 4.5 mSv at high altitude. A transatlantic airplane flight increases radiation exposure by 0.03 mSv.

Radiation dose is often equated to a number of chest radiographs for comparison. A single chest radiograph is equivalent to 10 days of background radiation. A single conventional chest CT scan may provide 7 to 10 mSv of radiation exposure. Dose reduction strategies can reduce this exposure by at least two-thirds. The radiation exposure from a PET-CT scan is approximately 25 mSv. This higher level is more difficult to reduce further since the CT scan for attenuation correction is performed with a low radiation dose.

Incorporation of greater awareness of radiation dose in the selection of medical imaging is particularly important when imaging chest disease. These radiation doses are important as radiosensitive breast tissue is unavoidably included in chest CT scan examinations. Radiation-induced lung cancer and lymphoma are also of specific concern as a result of chest CT scans.

One chest CT scan leads to another, whether to verify the benign nature of an indeterminate pulmonary nodule or to assess treatment for cancer, suggesting more careful consideration of scan timing, as well as type of scan and CT scan dose. As a result, the three-prongedFDA Initiative to Reduce Unnecessary Radiation Exposure from Medical Imaging will (1) promote the safe use of medical imaging devices; (2) support informed clinical decision-making; and (3) increase patients’ awareness of their own exposure.

The FDA intends to issue targeted requirements for manufacturers to incorporate important safeguards into the design of their machines to develop safer technologies and provide appropriate training to support safe use by practitioners. Input for establishing these requirements is being addressed with a public meeting, March 30-31, 2010.

Examples could include the following: a requirement that these devices display, record, and report equipment settings and radiation dose; an alert for users when the dose exceeds a diagnostic reference level (the optimal dose for most patients); training for users; and a requirement that devices be able to capture and transmit radiation dose information to a patient’s electronic medical record and to national dose registries.

In addition, the FDA and the Centers for Medicare & Medicaid Services are collaborating to incorporate key quality assurance practices into the mandatory accreditation and conditions of participation survey processes for imaging facilities and hospitals. These quality assurance practices will improve the quality of oversight and promote the safe use of advanced imaging technologies in those facilities.

The FDA recommends that health-care professional organizations continue to develop, in collaboration with the agency, diagnostic radiation reference levels for medical imaging procedures and increase efforts to develop one or more national registries for radiation doses. A dose registry would pool data from many imaging facilities nationwide, capturing dose information from a variety of imaging studies. This registry will help define diagnostic reference levels where they do not yet exist, validate levels that do exist, and provide benchmarks for health-care facilities to use in individual imaging studies.

To empower patients and increase awareness, the FDA is collaborating with other organizations to develop and disseminate a patient medical imaging history card. This tool, which will be available on the FDA Web site, will allow patients to track their medical imaging history and share it with their physicians, especially when it may not be included in their medical records.

As a result of the recent publication of higher than expected CT scan dosages at noted institutions, CT scan volume has diminished in many institutions, with a coincident rise in requests for an MRI examination, often by patient request rather than by clinician. Unfortunately, these requests are not always appropriate for patient care. MRI is not a useful modality for following indeterminate pulmonary nodules. MRI is equivalent to CT scanning for evaluation of mediastinal lymphadenopathy and quite valuable for evaluation of disease spanning compartments, such as malignant pleural mesothelioma that may invade the chest wall, mediastinum, and abdomen.

MRI may be more useful for assessment of lungs in the future; but, at this time, the more common decision is between diagnostic chest CT scan and PET-CT scan. The information that is gained differs significantly, as does the radiation dose.

The chest CT scan provides superior anatomic information and the most precise detection and measurement for small pulmonary nodules. Chest CT scans include adrenal glands that are frequent sites of lung cancer metastasis. The brain is another important site of lung cancer metastases, although it is not adequately imaged by PET-CT scanning, even when brain images are obtained.

There are tumors that respond to chemotherapy by becoming biologically inactive without decreasing significantly in size. Evaluation of therapy for such a tumor may require PET-CT scanning. PET-CT dose differentiation is also less when disease in multiple organs would require CT scans of chest, abdomen, and pelvis. Minimizing the number of scans, including adoption of guidelines, such as the Fleischner Guidelines, for incidentally detected pulmonary nodules can decrease cumulative CT dose significantly (MacMahon et al. Radiology 2005;237[2]:395).

Chest CT dose can be reduced significantly without loss of diagnostic accuracy or adverse effect upon nodule measurement. Lungs offer maximal contrast between air and soft tissue, allowing marked reduction of radiation dose. Tube currents have been reduced up to 90% for lung cancer screening CT studies. The voltage also is now being reduced for CT pulmonary angiography.

While radiologists apply the principle of ALARA (as low as reasonably achievable), the radiation dose for a particular patient’s chest CT scan can often be reduced by specifically requesting a low dose technique. This approach can be used in novel ways, such as for CT scans evaluating resolution or progression of a pneumothorax or pleural effusion.

Two additional considerations will reduce cumulative dose to a patient from chest CT scanning. Treat an acute process adequately before obtaining a CT scan for evaluation of nonacute findings. Pleural effusions and inflammatory opacities may limit visualization of small pulmonary nodules. An acute process that precludes adequate breath-hold may also result in a nondiagnostic CT scan that will ultimately require repetition.

As a general guideline, incidental nodule follow-up should be performed on an outpatient basis when the patient is well rather than while hospitalized for an acute process.

Acute and dynamically changing processes are often studied too frequently by CT scanning. Understanding, utilizing, and treating the information gleaned from one CT scan for an acute process may allow confident radiographic follow-up for up to a week or more.

Dr. Francine L. Jacobson, MPH
Division of Thoracic Imaging
Brigham and Women’s Hospital
Boston, MA

Editor’s Insight
As pulmonary and critical care physicians, thoracic imaging is crucial in our care of patients. The concern that radiation dose may widely vary among CT scanners is of particular concern, especially as patients may require serial examinations to assess stability or resolution of lesions.

This article is a logical progression from the report by Kerri Wachter in the November 2009 CHEST Physician. In the current article, Dr. Jacobson describes the new initiatives and possible solutions to the problem of excess radiation exposure in patients. As a thoracic radiologist, she offers practical advice for choice of procedures.

We look forward to the development of new guidelines for standardization of radiation dose in order to limit excess radiation exposure in our patients.

Dr. Marilyn G. Foreman, FCCP
Editor, Pulmonary Perspectives

Dr. Loren J. Harris, FCCP
Deputy Editor, Pulmonary Perspectives