Fear of radiation-induced cancer is a commonly cited reason for not pursuing annual screening mammography. Every time a medical imaging examination is performed with radiation the potential risks of performing the exam (in this case, radiation-induced cancer) must be weighed against the potential benefits of the examination (diagnosing breast cancer at a stage where the chance of successful treatment is high).
Not every medical imaging technique produces radiation. X-rays are a form of radiant energy, which also includes light and radio waves. Unlike light rays, the x-rays penetrate the body allowing an image to be made of the internal structures. The imaging modalities that use ionizing radiation include x-rays, computed tomography (CT), fluoroscopy, mammography, and stereotactic-guided biopsy. MRI and ultrasound do not utilize x-rays to obtain images of the body.
Cancer risk due to radiation exposure is currently estimated with the Linear No-Threshold model. This model was established in the 1950’s using exposure outcomes related to very high doses of radiation from atomic bombs. The long-term radiation exposure effects were documented in a group of patients who survived the atomic bomb blasts at Hiroshima and Nagasaki. This information was then plotted on a graph with assumptions made concerning low dose radiation exposure risk based on the effects of very high radiation dose exposure on these patients. These assumptions are based on the belief that radiation exposure is always harmful at any dose above zero and that the risk does not exponentially increase with increased exposure.
Research is now showing that high radiation dose and low radiation dose exposure affect the body differently and the previous assumptions concerning low radiation dose risk based on high radiation exposure outcomes may not be accurate, as discussed in this article. Additionally, the data is based on the effects of a single large dose of radiation versus smaller doses spread over a period of time. Unfortunately, all decisions regarding low-dose medical imaging radiation exposure risk are assumptions that are not scientifically validated through a clinical trial.
Researching medical imaging radiation exposure on “Dr. Google” can be confusing at best. Radiation dose can be measured and reported using different units. A few of the radiation dosage units include millisievert (MSV), rad, rem, roentgen, and gray. The type of radiation measurement also varies. The effective or equivalent dose represents the radiation risk averaged over the entire body. The mean glandular dose is another commonly used radiation measurement, particularly in mammography. There are many articles correlating the amount of radiation exposure from natural background radiation, effects of elevation on radiation exposure, and different medical imaging exams. Unfortunately, when different units of measure are used, the apples-to-oranges comparison leaves the reader at a disadvantage to truly understanding the level of radiation exposure for each medical imaging examination.
Mammography radiation exposure is tightly regulated. The Mammography Quality Standards Act (MQSA) was enacted in 1994 by the United States Congress to regulate the quality and care in mammography. The U.S. Food and Drug Administration (FDA) is responsible for inspecting facilities to ensure compliance with MQSA. To confuse the issue, the standard radiation measurement for mammography is the mean glandular dose reported in mGy, NOT the effective dose measured in mSv. The MQSA dose limit for a screening mammogram is 3 mGy.
The approximate radiation dose exposures are as follows for a 4-view examination of both breasts (data obtained from this presentation). Doses given are the mean glandular dose in mGy (to be compared with the MQSA limit of 3 mGy for a screening mammogram), and the effective dose in mSv (to compare to other sources of radiation listed below):
- 2D mammogram: approximately 1.2 mGy/ 0.5 mSv
- 3D mammogram with reconstructed 2D images: approximately 1.45 mGy/0.5 mSv
- 2D and 3D mammogram: approximately 2.65 mSv/ 1 mSv
Even the 2D and 3D combination screening mammogram is well under the MQSA dose limit of 3mGy.
Although it is reassuring that the average radiation dose exposure for annual screening mammography is well under the national limit, how does that amount of radiation exposure compare to other forms of radiation exposure? We are all exposed to background radiation in our environment and in the food we eat. Additionally, people who live at higher altitudes experience a higher level of background radiation exposure. The effective dose provides the most straightforward way to compare different types of radiation exposure and is reported in mSv units.
Radiation Examples
Here are a few examples of effective radiation doses that may be helpful for comparison:
- Plane trip from LA to New York: 0.04 mSv
- Standard 2 view chest x-ray: 0.04 mSv
- CT abdomen and pelvis: 10 mSv
- Average natural background radiation: 3 mSv/yr
- People living in the plateaus of Colorado or New Mexico receive approximately 1 mSv more radiation exposure per year compared with those living at sea level.
Although there is a small amount of radiation exposure associated with screening mammography, the actual radiation exposure is equivalent to the background radiation difference between living in the mountains of Colorado versus the beaches of California (for a combined 2D and 3D screening mammogram).
Remember, there is no clinical trial data to truly know the lifetime risk of developing cancer induced by a medical imaging examination. The best estimate of risk assessment, albeit not scientifically proven, is based on the Linear No-Threshold model discussed above. Based on this model the estimated lifetime additional risk of cancer per exam from a screening mammogram is 1 in 100,000 to 1 in 10,000. In other words, the exam may cause 1 cancer per every 10,000 to 100,000 exams performed.
Breast cancer affects 1 in 8 women. This brings us back to the original point: the benefits of every medical imaging examination must be weighed against the risks. In my mind the risk of developing breast cancer far outweighs the risk of developing cancer from having a mammogram, especially given what we know about how risk is currently assessed for exposure to low dose radiation. It is a shame to see women come in with a palpable lump that is larger in size with metastases to the lymph nodes and they have not been having annual mammograms due to fear of inducing cancer with radiation exposure. This cancer would have had a much higher chance of cure if caught at a smaller size before the patient was able to feel it through annual screening mammography. By the time the cancer is that large the chance of cure is much smaller.
The choice is an individual one. The hope is that you can consider the data, understand that the radiation exposure risk models are not clinically proven and flawed at best, and truly consider the risks and benefits before deciding if annual screening mammography is the right choice for you.