Radiation Exposure: Safe Limits and Health Risks

Radiation Issues

Hello Doctor:
1.
What is the acceptable standard range of radiation exposure for an individual in one year?
2.
I underwent a nuclear medicine bone scan, mammography, and low-dose computed tomography scan this year; has my radiation exposure exceeded the standard?
3.
Will these radiation doses be expelled from the body, or will they accumulate over time?
4.
Reports indicate that individuals frequently exposed to radiation are at a higher risk of developing cancer; what constitutes frequent exposure?

Ms. Chen, 40~49 year old female. Ask Date: 2012/09/16

Dr. Yang Jinghua reply Radiology

1.
Currently, there are no regulatory dose limits for medical radiation exposure received by patients; the dosage is primarily determined by clinical physicians based on medical necessity.
Regulatory authorities both domestically and internationally have only established reference dose levels (DRLs) for certain high-dose examinations.
These dose levels do not limit the number of medical exposures a patient can receive, nor do they set a maximum allowable dose of medical radiation.
Instead, they aim to establish reference standards for radiation doses associated with various medical devices and examinations, which can further regulate the radiation doses of these examinations and gradually reduce the radiation exposure for patients.
2.
A nuclear medicine bone scan typically involves a radiation dose of about 4.4 mSv per procedure, mammography about 0.7 mSv per procedure, and low-dose computed tomography (CT) scans less than 7 mSv per procedure, totaling approximately 12.1 mSv or less.
However, due to the body's automatic repair mechanisms, the doses received at different body sites and times cannot simply be summed mathematically as described above.
Generally, it is only when the whole body receives a dose exceeding 250 mSv that acute changes in white blood cell and lymphocyte counts may occur.
3.
The radioactive isotope used in nuclear medicine bone scans, 99m-Tc, is excreted through urine, and its radiation level decreases by half every six hours.
Therefore, after one day, it will have reduced to at least one-sixteenth of its original amount.
When accounting for the amount excreted in urine, the residual radioactive isotope in the body after one day is very minimal.
Additionally, the radiation from mammography and CT scans does not accumulate in the body.
4.
According to recommendations from the International Commission on Radiological Protection (ICRP), any exposure to radiation carries a risk of cancer (this is known as the probabilistic effect hypothesis of radiation).
The likelihood of this risk is directly proportional to the radiation dose received; however, there is no concrete evidence supporting this hypothesis at low doses (hence it is termed a hypothesis).
In 1993, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) defined doses below 200 millisieverts as low dose.
Furthermore, a single chest CT scan may increase the lifetime risk of cancer by approximately 0.01% to 0.1%, which is nearly negligible compared to the natural background cancer incidence rate of about 25% in Taiwan.
Therefore, various medical radiation examinations are justified as they assist clinical physicians in diagnosing diseases or treating life-threatening conditions.
These examinations have been carefully evaluated for safety and substantial benefits by experts and scholars internationally, ensuring that their safety and efficacy far outweigh the potential risks associated with radiation exposure for patients.

Reply Date: 2012/09/16

More Info

Understanding radiation exposure and its associated health risks is crucial for making informed decisions about medical imaging and treatments. Let's address your questions one by one.

1. Acceptable Annual Radiation Exposure Limits: According to the United States Environmental Protection Agency (EPA), the average person is exposed to about 6.2 millisieverts (mSv) of natural background radiation each year. This includes cosmic rays, radon gas, and terrestrial sources. For occupational exposure, the limit is typically set at 50 mSv per year for radiation workers, but the goal is to keep exposures as low as reasonably achievable (ALARA principle). The International Commission on Radiological Protection (ICRP) recommends that the cumulative dose over a lifetime should not exceed 1,000 mSv (or 1 Sv).

2. Radiation from Medical Procedures: The radiation doses from the procedures you mentioned can vary. A nuclear medicine bone scan typically exposes a patient to about 5-10 mSv, a mammogram (breast X-ray) is around 0.4 mSv, and a low-dose CT scan can range from 1 to 3 mSv. If we sum these values, your total exposure from these procedures could be approximately 6.4 to 13.4 mSv, depending on the specific techniques and equipment used. This is within the range of natural background radiation and does not exceed the annual limit for the general population.

3. Radiation Excretion and Accumulation: Most radiation exposure from medical imaging does not result in any residual radiation in the body. For example, the radioactive tracers used in nuclear medicine are designed to be eliminated from the body through urine or feces within a few hours to days. Therefore, these exposures do not accumulate in the body over time. The body does not retain the radiation; rather, it is the biological effects of radiation that can pose risks, not the radiation itself lingering in the body.

4. Frequent Radiation Exposure and Cancer Risk: The term "frequent exposure" can vary based on context. Generally, individuals who undergo multiple imaging studies involving radiation (like CT scans or X-rays) within a short period may be considered to have frequent exposure. The risk of developing cancer from radiation exposure is cumulative; however, the increase in risk from medical imaging is relatively low compared to other lifestyle factors such as smoking, poor diet, and lack of exercise. The risk of cancer from radiation exposure is often expressed as a small percentage increase in lifetime risk. For example, a dose of 10 mSv might increase the lifetime risk of cancer by about 0.01% to 0.05%, depending on various factors including age and gender.

In conclusion, while it is important to be aware of radiation exposure, the benefits of necessary medical imaging often outweigh the risks. It is crucial to have open discussions with your healthcare provider about the necessity of each imaging study and to consider alternative methods when appropriate. If you have concerns about your radiation exposure, discussing them with your physician can help clarify the risks and benefits specific to your situation. Remember, maintaining a healthy lifestyle and managing other risk factors for cancer can significantly impact your overall health.

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