The motivation for gonad shielding is to reduce the hereditary specific risks of radiation, and it is still common practice at many institutions. The reasons that shielding will soon be phased out include: hereditary risk in humans has not been demonstrated even at high doses, the radiation doses have reduced greatly from the 1950s (orders of magnitude), stray radiation is significantly improved on modern systems, gonad shielding can block important anatomical information, and gonad shielding can negatively influence automatic exposure control systems. These are some of the reasons that the AAPM issued a consensus statement suggesting to discontinue gonad shielding.
Motivations for Gonad Shielding
The motivation for gonad shielding is to reduce the radiation dose to the ovaries and the testis. In this post we will review the motivations for gonad shielding, the reduced need for shielding with modern x-ray equipment, and the potential negative consequences of using shielding.
There is a general fear of radiation in the public, and one of the measures passed into public policy is the requirement to use gonad shielding for x-ray imaging procedures. First, we will discuss the motivations for gonad specific shielding.
There are a couple of unique potential radiation harms for radiation received by the gonads:
Hereditary Effects (Increased risk of mutations in offspring)
Reduced fertility and sterility can be caused by radiation, but those doses need to be much higher than a patient receives during diagnostic examination. This requires radiation doses about multiple Gray for sterility and a significant fraction of a Gray for reduced fertility. Diagnostic exams are at a radiation dose much lower than fertility effects.
Therefore, since fertility effects are not the primary concern it must be the potential hereditary effects that motivate gonad specific shielding. As discussed above in the hereditary section there are theoretical hereditary risks that have been demonstrated in animals, but as mentioned in BIER V these have not been directly observed.
From BIER V, “Due to a lack of direct evidence of any increase in human heritable effects resulting from radiation exposure, the estimates of genetic risks in humans are based primarily on experimental data with laboratory animals.”
This is the context for gonad shielding in clinical practice.
Gonad Shielding was introduced into U.S Code in 1976, more than 40 years ago, following given the potential for hereditary effects.
Motivation for Shielding (Stray Radiation)
Early x-ray equipment produced significant stray radiation from off focal radiation or x-ray scatter internal to the x-ray tube. This existing stray radiation was a major motivation for introducing gonad shielding.
The purpose of this shielding is blocking x-rays outside of field of view. This reduces secondary radiation dose as shown in the top row of the figure. The lighter shade of yellow in this figure indicates the secondary radiation outside of the field of view. In x-ray systems from more than forty years ago this could be a significant source of radiation.
In modern x-ray systems, there is very low radiation outside of the primary field of view, so gonad shields, placed to block x-rays, no longer have significant benefits (lower row of the figure).
Since the secondary incident radiation is very low on modern x-ray systems, the gonad shield have any other way to reduce radiation dose to structures outside the field of view?
The answer is NO, Because, the radiation dose outside the field of view comes from scatter of x-rays inside of patient’s body. As you can see in this figure the gonad shield does not reduce the impact of x-ray scatter.
These are the reasons, why having a gonad shield outside of the field of view is not very effective at reducing radiation dose on modern x-ray systems.
Motivation for Shielding (Within Field of View)
Shielding can also be used to block primary radiation. So, rather than trying to block stray radiation outside of the beam, you can put the shield actually directly within the primary beam path.
Placing the shield in the way of primary beam path is effective to reduce radiation dose. However, since the shield is so heavily attenuating for the x-rays there can be some un-intended consequences (discussed below).
First, we will review how dramatically x-ray doses have been reduced since the middle of last century. These dramatic reductions in x-ray dose reduce the impact of shielding as well.
If we look in the late 1950s, the reported dose in the literature to the testis or to the ovaries was in the order of 2.5 or 1.2 milligray (mGy), that dose in 2012 was down to 0.06 mGy or 0.01 mGy. So, radiation dose reductions have been achieved of 98% and 99% compared with historic doses. This means that radiation doses are very low compared with the dose during the years preceding the gonad shielding regulations.
Another way of saying this is that having an x-ray exam on a modern system without gonad shielding would likely result in less radiation to the gonads than if gonad shielding was used in the 1950s.
Issues with gonad shields
Gonad shields placed directly in the path of primary radiation, also cause secondary problems related to measurement of signal in the detector.
In modern x-ray systems, the mA used for the x-ray exposure is often determined by automatic exposure control (AEC). AEC operates using either the signal on the detector or an ion chamber at the detector in order to set the tube current such that acceptable image quality is obtained for a wide range of patients (i.e. patients with varying body habitus).
So the question is what will happen when a gonad shield is placed within the field of view? The AEC control system will detect that the signal reaching the detector is lower than expected and request higher mA. This leads to an overall increase of the radiation dose for the procedure. This is contrary to the purpose of using gonad shields.
The second problem is reduced visualization of key structures in the radiographs. If you want to have clear visualization of all the structures, then you need to have repeatable and ideal positioning of the gonad shields.
In practice the gonad positioning is not always optimal and structures of interest can be blocked. In the literature the detailed studies often find that ideal positioning is not always achieved. “For girls, gonad shields were placed incorrectly in 91% of the radiographs; for boys, in 66%” (Link to the Article).
Additionally, in many cases when re-scanning is warranted due to poor shield positioning and is not performed the radiologist will not have access to the information about the important structures when conducting the diagnostic read.
Therefore, there is a clinically significant risk that gonad shields can reduce the diagnostic information of the exam.
Rad Take-home Point:
- The goals of gonad shielding are to reduce the potential hereditary impact of x-rays.
- Gonad shielding was put into the Code of Regulations over 40 years ago.
- Modern x-ray equipment has significant reduction in both the stray radiation and primary radiation (98-99% dose reduction compared with 1950s).
- Gonad shielding can impact AEC leading to a higher radiation dose, and in clinical practice gonad shields can reduce the ability visualize all the anatomy of interest.
Summary of Five Reasons Gonad Shielding will become extinct
- Hereditary risks in human have not been demonstrated even at high doses.
- Radiation doses on modern x-ray scans are orders of magnitude better than in the 1950s.
- Stray radiation outside the field of view is significantly improved on modern systems.
- Gonad shielding within the field of view can block important anatomical information.
- Automatic exposure control (AEC) can reduce dose and provide more uniform quality but gonad shielding can lead to problems with AEC leading to higher dose.
What can you do that is best for the patients?
As a Radiologic technologist the best thing that you can do is to be informed of the known risks of radiation and the misconceptions that many of the patients will have about radiation. You can help the patients to understand that the best acquisition will not include gonad shielding.