Radiation

  • Ionizing radiation

  • Measuring radiation

  • Effective dose & types of examinations

  • Artificial radiation sources & natural background radiation

  • Harmful effects of radiation (children & pregnancy)

  • Radiation protection

 

This course will briefly address the (ionizing) radiation used in X-ray & CT examinations.
Most people know that (ionizing) radiation may be harmful to the human body. But why is this exactly? 
Important questions such as ‘how do we measure radiation load?’, ‘how dangerous is radiation?’ and ‘how high is the risk of developing cancer when undergoing a CT examination?’ will be answered in this course.

Ionizing radiation

Ionizing radiation is a collective term for multiple types of radiation (including X-rays, alpha radiation, beta radiation and gamma radiation). These various types have one property in common, which is that they can ‘ionize’ matter. Explanation: radiation contains so much energy that it can release a negatively charged electron from the outer shell of an atom. The atom then looses its neutral charge and gets a positive charge - an ion is formed. 
When ionizing radiation comes in contact with living tissue, it can cause biologic effects, leading to tissue damage.
Humans are exposed to ionizing radiation from natural sources and artificial sources (e.g. X-ray). This is explained further in the Artificial radiation sources & natural background radiation section.  

For your convenience, in the remainder of this course the word ‘radiation’ is understood to be ionizing radiation. 

Measuring radiation

There are several ways to describe the radiation load. In the literature, the terms CTDIvol, DLP and effective dose are used.

  • Volume CT dose index (CTDIvol) 
    Description of the mean local patient dose of a CT scan. This is expressed in mGy (milligray, 1 mGy = 1 joule/kg absorbed radiation energy). 
    Benefit: easy and quick comparison of the radiation dose between various CT scanners (Note: not all CT scanners are the same). The CTDIvol can be read directly on the CT scanner display.
    Drawback: no exact dose measurement for individual patients. 
     
  • Dose-length-product (DLP)
    Measurement of the total patient dose and accounting for scan length. So this is the radiation dose of the entire scan. The DLP is expressed in mGy/cm.
    The DLP can be calculated as follows:  DLP = CTDIvol x scan length (cm). 
     
  • Effective dose (E)
    Effective dose includes the radiation dose of the entire body and describes the risk of developing cancer. The effective dose is expressed in mSv (millisievert).
    The effective dose can be used to compare the relative risks of the various X-ray examinations. However, this measurement is unsuitable for determining individual patient risks. Gender, build, age and organ sensitivity to X-rays must be included in individual risk calculations (see also the Children section).

Not all tissues/organs are equally susceptible to ionizing radiation. The International Commission for Radiological Protection (ICRP) has developed various so-called tissue weighting factors. The tissue weighting factors can be used to estimate individual tissue/organ risks. Through the years and based on research, ICRP have adjusted the tissue weighting factors (see table 1). The figures are virtually unchanged. Remarkably though, the mammary gland is more susceptible to radiation-induced cancer than was thought originally in 1991 (tissue weighting factor from 0.05 to 0.12). Conversely, it has now been shown that the gonads are less susceptible (original tissue factor 0.25, now 0.08).

Table 1. Tissue weighting factors defined by ICRP in 1977, 1991 and 2006.
Source: ICRP (www.icrp.org)

Effective dose & types of examinations

Tables 2&3 roughly outline the effective dose in various X-ray/CT examinations. As it turns out, there is significant variation in effective doses for the same examination.  This can largely be accounted for by the difference in scanner type/manufacturers and scan parameter settings.

Important: the tables present rough indications of the radiation dose and should not be used for individual patients.

Table 2. Mean effective dose in mSv (millisievert) in various examination types. *using X-ray examination. 
Source: Mettler FA Jr, Huda W et al. Effective doses in radiology and diagnostic nuclear medicine: a catalog. Radiology (2008).

Table 3. Mean effective dose in mSv (millisievert) in various examination types.
Source: Mettler FA Jr, Huda W et al. Effective doses in radiology and diagnostic nuclear medicine: a catalog. Radiology (2008).

In summary:
The mean dose for conventional X-rays varies from 0.001 - 10 mSv and in CT tests from 2 – 20 mSv. For intervention procedures (radioscopy), the effective dose varies from 5 – 70 mSv and in nuclear medical procedures from 0.3 – 20 mSv. 
Despite the fact that the number of diagnostic tests has dramatically increased in recent years, the mean per capita dose has remained virtually constant as a result of digitization and dose optimization (total radiation burden:  2.47 in 2000, 2.37 in 2008 and 2.51 in 2012). New techniques enable continued reductions in radiation dose. 
There is currently much research into the recent development of iterative reconstructions in CT examinations (= image reconstruction method). Depending on the examination type, a dose reduction of 30% is described and in isolated cases even a 70% reduction.  This all sounds very good, but the drawback is that dose reduction is often associated with loss of image quality. The images contain more noise and evaluation becomes more difficult.  In short, a precondition for dose reduction techniques is that the image quality remains sufficient to draw reliable conclusions.
Many radiology departments have so-called low-dose protocols for children and screening.  Ask the radiology department in your hospital for more information on these specific protocols. 

 

Artificial radiation sources & natural background radiation

The total radiation burden can be subdivided into artificial radiation and natural background radiation.

In artificial radiation sources, you are deliberately exposed to radiation. Medical diagnostics are the major contributor. For other non-medical artificial sources, think of residual radioactivity in the atmosphere after nuclear bomb tests in the past and the nuclear reactor accident in Chernobyl (total contribution +- 1% of artificial sources). Worldwide, the mean burden of artificial radiation sources is 0.6 mSv/year.  Some countries are above this average value. For instance, in the United States, in view of the relatively high number of diagnostic tests, the mean artificial radiation burden is 3.0 mSv/year.

Everywhere in the world, you are exposed daily to natural background radiation; you can't escape it. The mean annual natural background radiation dose is about 2.0 mSv for Dutch citizens.  The worldwide mean value is 2.4 mSv/year. 
Radon is the primary radiation source and is released from the soil and stony building materials.  Also cosmic radiation contributes to the natural background radiation (it is increasing in level). For instance, during a return flight from London - New York you are exposed to about 1.0 mSv.
Natural background radiation levels vary strongly. The worldwide mean dose is 2.4 mSv/year. 
The highest natural background radiation values have been measured in:

  • Ramsar (Iran): 260 mSv/year.
  • Guarapari (Brazil): 70 mSv/year 
  • Karunagappaly (India): 15 mSv/year

In summary:
Worldwide our annual exposure to artificial radiation sources is 0.6 mSv on average and 2.4 mSv from natural background radiation. Total: 3.0 mSv/year.

The Netherlands is below average (2.4 mSv/year in 2008 and 2.51 mSv/year in 2012).  Figure 1 lists the various sources contributing to the total radiation burden in the Netherlands and the United States.

Figure 1. Proportional contribution from radiation sources in the Netherlands (2008) and the United States (2006). Total radiation burden in mSv (millisievert).
Source: RIVM – Radiation Burden in the Netherlands.

Harmful effects of radiation

Two harmful effects are distinguished:

  1. Deterministic effects; the likelihood and severity of the effects are dose-dependent. From a certain threshold dose, the body is no longer able to repair the radiation-induced cellular damage; think of red skin after prolonged radioscopy.  Under the threshold dose, no effect occurs; above the threshold dose the severity of the effect increases with the radiation dose.
    These are effects in the short term after high radiation dose
  2. Stochastic (risk-bound) effects; the likelihood of these effects occurring depends on the dose. The higher the exposure, the higher the risk that the effect will occur. There is no threshold dose.
    These are potential effects in the long term after exposure to a low dose. All radiation to which someone is exposed in his/her life is added up. This can be compared with the concept of number of pack-years in smokers.

The primary radiation risk is the development of cancer.
Much is known about the effect of acute high radiation doses (cancer, red skin, hair loss etc).  Think particularly of examinations related to the nuclear bomb explosions in Hiroshima & Nagasaki in Japan. 
Unfortunately, we still have much to learn about the long-term risks following exposure to low-dose radiation. In addition to environmental factors that have their impact, it takes several decades for cancer to develop (long incubation time). What it comes down to is that it is difficult to distinguish 'spontaneously’ developed tumors from tumors resulting from exposure to low-dose radiation.

IRCP estimate the risk of radiation-induced cancer in low dose and low dose rate at 5.5% per Sv
So this is 0.0055% per mSv (1 Sv = 1000 mSv).

For your reference, some data:

  • the mean dose for CT tests varies from 2 – 20 mSv.
  • general ‘basic risk’: +- 1 in 6 women will develop breast cancer at some point in their lives (= 16.7%). 
  • death risk due to smoking is 0.005% per 100 cigarettes.  One could say that the risk of 1 cigarette about equals 0.01 mSv. If someone smokes 2000 cigarettes a year (= about 2 packs/week), this will 'expose’ this person to 20 mSv annually.

In view of the uncertain long-term effects of radiation, rules have been formulated in the Netherlands:

  • per capita dose limit:  1 mSv per year (above the natural background radiation). 
  • dose limit at hospital site perimeter:  < 0.1 mSv annually.
  • dose limit for radiology worker: 20 mSv annually (calculation: 30 working years = 600 mSv = 0.6 Sv). 

Note: there is no official dose limit for radiation in medical diagnostics (radiology and nuclear medicine) and medical treatment (intervention radiology, radiotherapy and nuclear medicine). 

Primary risk groups include pilots (cosmic radiation), intervention radiologists/intervention cardiologists and professions in industrial applications such as isotope production.  Using personal dosimeters (fig. 2), information is obtained on professional exposure to ionizing radiation. This is documented in the National Dose Registration and Information System (NDRIS).

Children

Children are more sensitive to radiation than adults. This is because children have many replicating cells. Consequently, DNA errors and/or changes may occur relatively more frequently, which could lead to the development of cancer. Additionally, the child has more time to develop cancer (long incubation time!) compared to someone of 70 years old. 
A correct indication is essential to minimize the child's radiation burden.  In each X-ray/CT test, it must be considered whether the question cannot be answered using an imaging technique without ionizing radiation (such as ultrasound or MRI).

Pregnancy

Radiation risks in unborn children depend strongly on the phase in which the child is exposed to the radiation.

Pregnancy may be subdivided into three phases;

  • the preimplantation phase (ending about 10 days following conception) 
  • the organogenesis (= period of organ development, about 10 – 40 days after conception) 
  • the fetal period (= fetal development during the remainder of pregnancy)

Animal tests have shown that the organogenesis phase is particularly susceptible to radiation effects.  High-dose radiation in this phase may cause malformations; particularly in organs, skeletal system, eyes and central nervous system. In the most severe situation, the embryo may even die.
Susceptibility to malformations rapidly decreases in the fetal phase. It should be noted here that during the cerebral development phase (between the 8th and 15th week), exposure may lead to brain damage, resulting in lower IQ or mental retardation. 
No obvious effects have been observed in the preimplantation phase. Animal tests have shown that the embryo may be damaged and fail to implant.  The fetus is then rejected.  In practice, this may happen more or less unnoticed seeing an estimated one in three fertilizations end in incomplete implantation. 
The deterministic effects in unborn children in the above phases occur from a threshold value of 100 mSv (= exposure in one dose).  These high dosages are virtually non-existent in diagnostic tests.  The threshold value could be exceeded in a prolonged (acute) therapeutic intervention procedure, but this is very rare.

The rule is that an unborn child may be exposed to a maximum of 1 mSv during the entire pregnancy.  In some cases the 1 mSv is exceeded; think in particular of abdominal/pelvis examinations where the fetus is exposed to direct radiation (fetal radiation burden during an abdominal CT is 10 mSv). Examinations where the X-ray beam does not come near the uterus, as in a chest X-ray, can in principle be made without any danger (fetal radiation burden < 0.01 mSv).  Nevertheless, for each X-ray/CT examination, the question whether the test is truly necessary must be considered carefully. If in any way possible, the examination should be postponed until after pregnancy or changed into an examination that does not use radiation (e.g. ultrasound and MRI).

The stochastic (risk-bound) effects:
Much is still unclear about the relationship between intrauterine exposure to radiation and childhood cancer (leukemia in particular). 
According to IRCP, the risk of intrauterine tumor induction is 0.015% per mSv (= lifetime risk of cancer).  The occurrence of childhood cancer/leukemia is estimated at 0.0006% per mSv. These percentages are far below the general risk of developing (childhood) cancer.
Note that the general radiation risks for pregnant women are not different from those for non-pregnant women.
Also, no clear correlation has been found yet between congenital defects in offspring resulting from gonad exposure prior to conception.

Radiation protection

Due to the (potentially) negative long-term effects of radiation, the question whether the examination is indicated should be considered critically. Key questions include:

  • Is an X-ray/CT examination the only way to answer the question? Consider alternatives such as ultrasound and MRI.
  • Will the examination impact treatment? Will the outcome impact strategy?

When an examination is indicated, an effort is made to protect hospital staff as much as possible against the radiation; during the test, the staff will stand behind a lead screen/lead wall.
Radioscopy images are used for interventions and in some cases surgery. When during radioscopy people cannot leave the room, there are various protective options (fig. 2).

  1. Lead apron: the lead apron protects the mammae/lungs/colon/stomach/esophagus/liver/bladder/gonads.
  2. Thyroid shield: in addition to the lead apron, a thyroid shield may be used when a large number of (prolonged) procedures with high-intensity scattered radiation are used. 
  3. Lead glasses: the use of lead glasses is not standard, but they can be worn in view of the increased risk of radiation-induced cataract. Intervention radiologists and intervention cardiologists in particular are at increased risk of reaching their dosage in view of the large number of (prolonged) procedures. 

Other options include lead gloves, face shield and lead caps. These are used only very rarely. 

Figure 2. Protective options: lead glasses, thyroid shield and lead apron. The personal dosimeter measures professional exposure to ionizing radiation.

Sources

  • United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionizing radiation (2008) 
  • Bethesda. MD. Ionizing radiation exposure of the population of the United States. National Council on Radiation Protection and Measurements. 2009,  NCRP No. 160.
  • Padole A, Ali Khawaja RD et al. CT Radiation Dose and Iterative Reconstruction Techniques. AJR Am J Roentgenol. (2015)
  • Mettler FA Jr, Huda W et al. Effective doses in radiology and diagnostic nuclear medicine: a catalog. Radiology (2008)
  • Brenner DJ, Elliston CD et al. Estimated Risks of Radiation-Induced Fatal Cancer from Pediatric CT. AJR Am J Roentgenol. (2001)
  • M. Prokop; Spiral and Multislice Computed Tomography of the body (2003)
  • Ghiassi-nejadd M, Mortazavi SM et al. Very high background radiation areas of Ramsar Iran: preliminary biological studies. Health Phys. (2002)
  • RIVM – Dossier Stralingsbelasting in Nederland.
  • Rehani MM. Am Confused About the Cancer Risks Associated With CT: How Can We Summarize What Is Currently Known? AJR Am J Roentgenol. (2015)
  • Kleinman PL, Strauss KJ et al. Patient Size Measured on CT Images as a Function of Age at a Tertiary Care Children’s Hospital. AJR Am J Roentgenol. (2010)
  • Gezondheidsraad. Risico’s van blootstelling aan ioniserende straling. Den Haag: Gezondheidsraad, 2007 (publicatienr. 2007/03)
  • Aliyu AS, Ramli AT. The world's high background natural radiation areas (HBNRAs) revisited: A broad overview of the dosimetric, epidemiological and radiobiological issues. Radiation Measurements (2015)
  • Gordic S, Morsbach F et al. Ultralow-dose chest computed tomography for pulmonary nodule detection: first performance evaluation of single energy scanning with spectral shaping. Invest Radiol. (2014) 
  • Kleiverda G, van Tienhoven G. Bestraling tijdens de zwangerschap: denk nooit nooit. Ned Tijdschr Oncol (2007)
  • Berlin SC, Weinert DM et al. Successful Dose Reduction Using Reduced Tube Voltage With Hybrid Iterative Reconstruction in Pediatric Abdominal CT. AJR Am J Roentgenol. (2015)
  • Mahesh M. NCRP Report Number 160: its significance to medical imaging. J Am Coll Radiol. (2009) 
  • Bethesda, Md. Ionizing radiation exposure of the population of the United States. National Council on Radiation Protection and Measurements (2009)
  • Cira-Bielac O, Rehani MM et al. Risk for radiation-induced cataract for staff in interventional cardiology: is there reason for concern? Catheter Cardiovasc Interv. (2010)
  • Rajabi AB, Noohi F et al. Ionizing Radiation-Induced Cataract in Interventional Cardiology Staff. Res Cardiovasc Med. (2015)

Author

Annelies van der Plas, resident radiology LUMC.

23/08/2015 (translated 20/09/2016)