Part 1 – Radiation exposure
Should I get that scan?
Getting a scan sounds simple and straightforward but it is actually a more complex decision than most people would be aware. Using my own experience working in sports medicine, I order many scans. I am requested to order more than are required from my perspective while patients refuse others I consider necessary. There are a several important issues to consider when having a scan but the two main ones are:-
- Radiation exposure
- Utility of the test.
In this blog I will discuss radiation exposure and in the following I will discuss test utility.
What is radiation?
Radiation is the emission or transmission of energy in the form of waves or particles through space or through a medium. Light and sound are forms of radiation but are considered non-ionising radiation. This means that although light and sound can travel through structures in our body they are not strong enough to break down any of the chemical bonds in the molecules that make up our cells. Ionizing radiation however is radiation which is strong enough to change the subatomic structure of a molecule and leave it “ionised”. Ionising radiation has been shown to cause some changes in cells and increases the risk of cancer. Sunlight is a good example of this difference. Normal light poses no risk to the human body but UV radiation from the sun is a type of radiation which all North Queenslanders will understand is associated with sunburn, melanoma and other skin cancers.
What dose of ionising radiation do common scans emit?
When determining the safety of having a scan it is important to understand the radiation dose emitted by the scan. This level needs to be considered in context of understanding the radiation dose we are all exposed to daily due to our environment (background radiation dose) and the level of radiation dose considered to be harmful.
We are exposed to radiation from natural sources all the time. Estimates show that an average person receives an effective dose of about 3mSv per year (Sv or Sievert is the unit for absorbed dose) from naturally occurring radioactive materials and even cosmic radiation from outer space. These natural “background” doses vary throughout the world but 3mSv is considered the average background dose. This means that if you are exposed to a 3mSv dose of radiation at one time, it is equivalent to a year’s worth of background radiation. It is hard to put a number on how much radiation is safe or what dose will cause health problems. People have different genetics and different susceptibilities to different cancers so estimates need to be made. Below is a table which outlines ionising radiation dose from various imaging modalities and the associated increased risk of cancer with them.
The risk of cancer from all of the scans is reasonably low. Even the highest risk listed is 1/500. From a patients perspective an increased risk of cancer of 1/1000 may not be significant. As a doctor though, if I order 1000 of those tests in my career I will unfortunately have created a situation where a cancer could be predicted to occur in one of those patients. The risks are low enough not to cause undue stress if the scan is required but are high enough to exercise caution to avoid an unnecessary scan. I consider the x-ray, ultrasound, bone density scan and MRIs to be very safe. Ultrasounds and MRIs do emit radiation but it is non-ionising so there is no increased risk from the scan itself (the result of the scan can sometimes cause harm but we will discuss that in part 2). An x-ray and bone density scan (for the vast majority of people except for a newborn or pregnant woman) is the same amount of radiation as having been alive for an extra 3 hours. The radiation dose is extremely low and the increased risk of cancer is essentially zero. More caution is required however with a CT scan. Remember that the risk is low but not zero. So if the scan is required then there is no need to hesitate in having it done. It would be prudent however to ask your doctor if having the scan will change the management of the injury and if a better scan exists. If having a scan will not alter the management plan then you need to seriously consider a decision to progress with it. This is especially true for back and neck pain. Exercise / physiotherapy and medical therapy is normally not significantly altered by the results of a CT scan of these areas. If the symptoms have been present for several weeks and / or are severe enough to be considering an injection in to the spine or surgery, then a scan will potentially change the management and therefore should be considered.