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.