Both breast-specific gamma imaging (BSGI) and positron emission mammography (PEM) have been recently approved for use in the United States. They are marketed as diagnostic adjuncts to mammography and breast ultrasound. But, they can also be considered for breast cancer screening, particularly in women at higher risk for breast cancer.

However, the risk of radiation from breast cancer imaging methods should be communicated to patients as part of the process of obtaining informed consent.

Although that risk is theoretical, however, recent findings suggest that this new breast cancer imaging methods expose radiation risks much higher than mammography.

Published in August 24, 2010 in Radiology, the study authors, Edward Hendrick, PhD, from the Department of Radiology at the University of Colorado-Denver in Aurora, said, “BSGI and PEM should not be used for screening because of the higher risk of cancer induction, compared with existing modalities such as mammography, breast ultrasound, and breast magnetic resonance imaging (MRI).”

Dr. Hendrick explained, currently, these new methods are being used for diagnostic evaluation after mammographic or physical detection of a suspicious finding.
He added, they are also used to evaluate the extent of disease when a woman has a biopsy-proven breast cancer. In such cases, the use of these new tests would have a different benefit-to-risk ratio, since the additional information obtained with the new imaging methods can “better guide treatment”.

He notes, these new methods are not currently being used for breast cancer screening, but, he points out that there “has been talk” about using them (particularly BSGI) for screening women who are at high risk or who have dense breasts.

He asserts, “But there are no scientific clinical studies to support that use. And much of that talk was before the radiation risks of these nuclear medicine studies were fully understood.”

In fact, he adds, the radiation risks might be even higher in the population suggested as targets for these new methods, because women who are genetically at high risk for breast cancer and women with dense breasts, who are more likely to be younger, are more radiosensitive than normal-risk and older women.

Dr. Hendrick, who acknowledges that there is “continuing controversy” over the best method to screen for breast cancer in women with different risks and ages, suggests that screening should be carried out with mammography for women who are 4o years and older.
According to Dr. Hendrick, women who are at high risk for breast cancer and younger than 40 years should undergo screening with breast MRI or ultrasound, whereas those at high risk and older than 40 years should undergo screening with some combination of mammography and breast MRI, probably annually, but staggered so that the tests are 6 months apart.

He said, “We know from randomized clinical trials that mammography has a mortality benefit. And from studies in high-risk women, we know that breast MRI is better than mammography in detecting cancers and evaluating the extent of disease; in high-risk dense-breasted women, we know that screening with ultrasound adds to mammography in terms of cancer detection.”

In contrast, he pointed out: “we know very little about BSGI and PEM, except in relatively small studies (a few hundred subjects) of highly cancer-enriched study groups (e.g., those already identified as having lesions suspicious for breast cancer).”

Dr. Hendrick explained that the new imaging methods use radiation in a completely different way. Mammography uses external-beam low-energy x-rays (avg. energy, ~20 keV ), whereas the new methods use internally administered high-energy gamma rays (avg. energy, 140 keV for BSGI and 511 keV for PEM).

In addition, the new imaging methods use injected radioisotopes – BSGI uses technetium 99m (99mTc) sestamibi, which is also used in cardiac stress tests, and PEM uses fluorine 18fluorodeoxyglucose (FDG), the same as whole-body positron emission tomography scans for detecting metastatic cancer.

Dr. Hendrick points out that the distribution of these radionuclides in the bloodstream, their uptake into tissues, and their clearance in the liver (for 99mTc used in BSGI) and kidney (for FDG used in PEM) result in radiation exposure of organs in the chest, abdomen, and pelvis. The highest risk is to the colon in BSGI and the bladder in PEM.

He notes, The organs receiving the highest doses and therefore at greatest risk for cancer induction from radionuclide administration are the colon, lungs, and bladder. Cancers occurring in these organs are less curable than breast cancer, so the risk ratios for BSGI and PEM, compared with mammography, are greater for cancer mortality than for cancer incidence.

However, Dr. Hendrick explains, all of these radiation risks are theoretical, because there is no direct evidence that exposures at the level of mammography, BSGI, PEM, or computed tomography exams can cause an increase in cancer incidence or cancer deaths.

He said, “We do know that much higher levels of radiation exposure, such as that received by survivors of the atomic bomb explosions at Hiroshima and Nagasaki, cause higher levels of cancer incidence and cancer deaths.”

Overall, he said, “The theoretical part is that we extrapolate these high-level exposures to low-level exposures using a linear no-threshold model, which is generally viewed to be the most prudent approach for radiation protection (and is best supported by the existing data).”