Chest CT plays a critical role in the diagnosis and management of most patients with complex thoracic and cardiopulmonary disease, and has become the gold standard for imaging pulmonary emboli, said Mark S. Parker, MD, associate professor of thoracic imaging at the Medical College of Virginia Hospitals, Virginia Commonwealth University Medical Center. However, the potential carcinogenic effects of ionizing radiation on radiosensitive tissues such as the female breast are often not deemed relevant to acute patient care. The latency period for potential cancer induction is estimated to be 10-30 years in the dose ranges used in CT pulmonary angiography. Thus, the younger the patient is at the time of exposure, the more radiosensitive the fibroglandular tissue, and the more time the patient has to develop potential nonfatal and fatal cancers, Dr. Parker said.
"We recently studied 1,325 patients who had undergone CT pulmonary angiograms; 60% of the patients were women--some of them as young as 15 years old," said Dr. Parker. The mean age of the female patients was 52.5 years. "We found that the calculated dose to the breast tissue of an average-sized woman during a CT pulmonary angiography examination was at least 2.0 rads per breast. The radiation dose to the breast from these examinations is equivalent to10-25, two view mammograms, or 100-400 chest radiographs. The estimated radiation dose is two to three times greater with techniques that employ overlapped scanned regions of interest (i.e., nonenhanced and enhanced scans), and 30-50% greater with the newer multidetector CT (MDCT) scanners, Dr. Parker added. One millisievert (mSv) of radiation exposure may be associated with five additional cancers in 100,000 exposed patients, he said.
There are a number of ways to reduce the radiation exposure, Dr. Parker said. "First, we need to reduce the number of female patients scanned by better selecting those patients who truly warrant the CT pulmonary angiogram. Arterial blood gases, D-dimer assays, Doppler studies and chest radiography are tools that can be appropriately used to triage patients with suspected acute pulmonary thromboembolic disease, especially in out-patient and emergency department settings." Radiologists often manipulate scan parameters (e.g., reduce tube current, reduce tube voltage, increase pitch) to reduce the radiation exposure. However, each manipulation has a trade-off with respect to image quality and potential loss of diagnostic information. "Radiologists and clinicians both must come to an agreement as to whether the detection of distal segmental or subsegmental emboli truly impact patient management and are worth the radiation dose to the patient," Dr. Parker said. Thin-layered bismuth breast shields may reduce breast radiation exposure by 57% without greatly affecting diagnostic interpretation and should be routinely used in reproductive age and perimenopausal women. Alternative, lower-dose examinations utilizing ionizing radiation (e.g., ventilation-perfusion lung scan in the setting of a normal chest radiograph) and nonionizing radiation exams (e.g., MR angiography) may be considered in stable female patients, he said.
CT is an excellent tool for diagnosing pulmonary embolism, a disease that if undetected, has a high morbidity and mortality rate, said Dr. Parker. "We just need to carefully weigh the benefits of the examination with the potential long-term risks before determining if it is the right examination for each patient," he added.
The study will be presented on May 4 at the American Roentgen Ray Society Annual Meeting in Miami Beach, FL
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