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Ionizing radiation is a type of electromagnetic radiation which passes through tissues and organs. It displaces electrons from atoms and therefore makes them ionized. These ionized cells are damaged and either repair themselves or form what we know as cancer cells. These malignant cells proliferate uncontrollably and form tumours which could then infest other tissues. This event of the alteration of a cell is referred to as mutagenesis; when mutations occur. A high radiation dose can result in miscarriage, foetal growth restriction, mental retardation and an increased risk of childhood cancer.
The sources of ionizing radiation are gamma rays, x-rays, radioactive isotopes and radiation therapy machines. The foetal threshold from which mutagenesis is likely to occur from ionizing radiation exposure in America is 20000 mrad (mrad= the amount of radiation which is absorbed by the tissue). The radiation from diagnostic conventional x-rays ranges from 60-290 mrad and depends on the duration of the x-ray and therefore poses very little risk of causing foetal mutagenesis. CT and fluoroscopy of the abdomen expose the foetus to high doses of radiation over 3000 mrad and there is a higher risk of mutagenesis. Therapeutic radiation therapy destroys cancer cells and exposes the body to hundreds of thousands of mrad and a pregnancy would result in a miscarriage or an abortion depending on the stage of foetal development.
We are exposed to radiation each day. It stems from the soil, air and even cosmic rays. The word 'radiation' invokes anxiety and fear in many. Reasons behind this are the atomic bombings of Hiroshima and Nagasaki and the nuclear explosion in Chernobyl. The populations in these areas are still affected today. In Hiroshima and Nagasaki, the radiation had a more severe affect on the offspring from women who were exposed during the third and fourth month of gestation when the central nervous system is being formed, which is the most vulnerable foetal stage. These children were prone to brain damage and anatomical malformations. Yet foetuses exposed during other developmental stages did not exhibit such deformities. Currently, in Chernobyl only five percent of the children are healthy and all others suffer from some form of cancer and many have various deformities due to the ongoing consumption of irradiated water and local crops grown in irradiated soil.
There are some alternatives to clinical procedures involving ionizing radiation. Ultrasounds only enable a partial image of the foetus, even with the new 3D technology. MRIs give very detailed images and can aid the diagnosis of a patient almost as well as an x-ray, yet the processing of these images is more time-consuming and costly. The refusal of treatment due to religious reasons is an alternative and is probably more harmful than the x-ray itself.
This concludes that clinical diagnostic procedures are harmless, while high radiation levels from therapeutic medicine or nuclear events are harmful depending on the foetal developmental stage during which such exposure takes place.
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Forms of ionizing-radiation include gamma and x-rays which are emitted from radioactive-isotopes and therapeutic-machines. Conventional x-rays pose a low risk and expose the foetus to 60-290 mrad, far below the threshold radiation dose of 20000 mrad. However, fluoroscopy and abdominal-pelvic CTs can lead to mutagenesis due to increased radiation, hence are harmful. Continuous radiation exposure, e.g. from the soil, as in after the Chernobyl accident has lead to more mutagenesis. Following the atomic bombings of Hiroshima and Nagasaki this effect was more pronounced and in utero exposures resulted in severe malformations. The alternatives to clinical ionizing-radiation are MRIs and ultrasounds.
There is wide ranging consensus that frequent exposure to clinical x-rays increases the risk of cancer (Nave, 1993), but it is most controversial when applied to pregnant women and their foetuses. The following dissertation discusses the risks from the exposure of pregnant women and their foetus to both clinical and involuntary ionizing radiation. The hypothesis of this dissertation is that low radiation used in clinical diagnostic procedures are harmless, while high radiation levels from therapeutic medicine or nuclear events is harmful depending on the foetal developmental stage during which such exposure takes place.
Ionizing radiation is a kind of electromagnetic radiation which penetrates tissues and includes gamma and x-rays, produced by radioactive isotopes (radionuclides) and radiation therapy machines (Brent, 2009). Yet not all electromagnetic radiation is ionizing, only the high frequency portion of the spectrum is ionizing.
X-ray examinations include the conventional x-ray, which was first discovered by the German physicist, W. C. Roentgen in late 1895 (Solomon, 1982), tomography (CT) and fluoroscopy. The discovery of x-rays introduced a new era of diagnostic medicine. The reported dose of radiation which results in an increased incidence of birth defects is above 20000 mrad in America (Brent, 2009) yet it is 10000 mrad in Canada (Bentur, 2001). This shows that there are different policies in different countries and that different treatments are regulated and offered accordingly. A conventional x-ray usually involves low amounts of radiation in the range of 60-290 mrad. A CT involves approximately 3000 mrad, while a fluoroscopy exposes the patient to more than 3000 mrad (Brent, 2009) and would not be offered during pregnancy.
A high radiation dose can result in 'miscarriage, foetal growth restriction, congenital malformations, mental retardation and an increased risk of childhood cancer' (Valentin, 2000). It causes mutagenesis which can lead to all of the above mentioned effects (Solomon, 1982). It should be noted that every healthy woman, even without any family or personal history of developmental or reproductive problems 'begins her pregnancy with a three percent risk of birth defects and a 15 percent risk of miscarriage' (Brent, 2009)
The different stages of embryogenesis and organogenesis play a role in the sensitivity to radiation. According to Brent, in the first two weeks the embryo is not susceptible to the malforming effects of x-rays and doses higher than 5000 mrad are needed to cause a miscarriage. From the second to the eighth week of pregnancy the embryo is only affected by radiation levels above 20000 mrad. Radiation doses higher than 30000 mrad, which are rarely if ever reached using diagnostic methods, have a severe effect on the central nervous system of the developing foetus from weeks eight to fifteen and effect IQ in later life. This observation was the result of IQ tests taken by children who were exposed in utero to the radiation from the atomic bomb in Hiroshima and Nagasaki (Neel, 1991). From the twentieth week to birth the foetus is fully developed and any radiation exposure has the same effects on the foetus as it would on the mother (Brent, 2009).
Radiation exposures that pose a low risk:
The overall average annual dose in the U.K. is about 270 mrad, and the average annual dose from natural radiation is found to be 220 mrad (Hughes, 2005). Everyone is exposed to some form of radiation, whether it is from the soil, air or cosmic rays. Most 'embryos usually receive less than 100 mrad during the nine month gestation period in Canada' (Bentur, 2001).
In the general population, the term 'radiation' alarms people conjuring up images of malformed children from the atomic bombs dropped on Hiroshima and Nagasaki during World War II or of abandoned houses from the nuclear accident at Chernobyl. If the radiation, to which a pregnant woman is exposed to, is below a certain threshold, it is similar to the background radiation that we are exposed to everyday and is therefore relatively harmless. The prospect of an x-ray harming an unborn child often causes anxiety which in itself can be more harmful to the foetus than the x-rays themselves. Physicians specify the amount of radiation used for each patient, dependant on their radiological history, which takes the frequency, dosage of previous radiological examinations or treatments and the cumulative exposure to radiations into account. With this, the uterine and the foetal dose are calculated (Bentur, 2001).
Cumulative exposure is a constant concern as mentioned by the then congressman Al Gore. During the 1979 congressional hearing concerning medical and dental x-rays (Solomon, 1982) congressman Al Gore recalled taking his daughter to the emergency room because she had inhaled some pillow stuffing. There the doctors wanted to take an x-ray. Her father objected, arguing that the stuffing would not show on the x-ray, which would unnecessarily expose his daughter to unhealthy radiation, the doctor explained that as a routine an x-ray would be useful as a basis for future examinations. Al Gore decided against allowing the x-ray examination (Solomon, 1982). This anecdote illustrates the often controversial issue of deciding the necessity of clinical radiation exposure in the light of its harmful effects, which depends on the doctor's level of expertise and can even become an ethical issue.
The United States Centers for Disease Control and Prevention's Radiation Safety Committee recommends that a cumulative dose of 5000 mrad should not be exceeded throughout the entire gestation period. The United States National Council on Radiation Protection states that the 'risk of miscarriages, malignancies or major congenital malformations in embryos or foetuses exposed to doses of 5000 mrad or less is negligible compared with the spontaneous risk in non-exposed foetuses' (United States Department of Health, 2003).
Most of the radiodiagnostic examinations which are performed on pregnant women expose the foetus to less than 5000 mrad, 20000 mrad being the threshold from which mutagenesis can occur. As long as the radiation exposure is not directed at the embryo or ovaries, it poses no threat to the foetus. Even the amount of scatter radiation would be so small that it would not represent an increased risk of birth defects. This also includes other methods such as 'computerized tomography (CT or CAT) scans and fluoroscopy of the non-abdominal or pelvic areas' (Brent, 2009).
Examples of common procedures when x-rays are used during pregnancy:
'x-rays of the back (lumbar spine) for evaluating a lower back pain or a nerve route pain
intravenous pyelogram (IVP) to examine kidney function
upper GI series for evaluation of gastrointestinal symptoms
lower GI series (barium enema) to examine the structure and function of the large intestine
x-ray studies of bladder function
x-ray studies of the gallbladder and gallbladder function
x-ray studies of the structure and function of the uterus and tubes with the procedure known as a hysterosalpingogram (HSP)
x-ray studies of the pelvis and hips due to hip pain
standard abdominal x-rays' (Brent, 2009)
In all of the above mentioned procedures the x-ray beam can be directed away from the ovaries or the foetus, therefore causing no harm to the foetus; the x-ray beam itself can also be narrowed to target small areas and therefore decrease x-ray scattering.
'X-ray examinations of the abdomen are associated with 250 mrad, and an abdominal CT scan with 3000 mrad' (Archer, 1999). A dental x-ray is only 0.01 mrad; therefore 100000 dental x-rays would be required to accumulate only one rad. Brent showed that there was no significant increase in the incidence of major foetal malformations in pregnant woman who were exposed to radiation doses within these limits. This shows that the benefits of x-rays on pregnant women outweigh the almost non-existent consequences for the foetus. In this case the use of x-rays is justified to help diagnose the mother, because there are no detrimental effects on the foetus.
Harmful radiation exposures:
Although conventional x-rays are harmless, an abdominal/pelvic tomography and fluoroscopy can lead to mutagenesis in the foetus due to these technologies' inherent higher doses.
These diagnostic methods would not be used unless the patient is unaware of the pregnancy or to treat a malignant tumour in the abdominal region which poses a threat to the mother. The most common form of radiation therapy is the oral administration of radioactive iodine to treat hyperthyroidism or thyroid cancer (Brent, 2009). Radioactive seeds can also be placed in various tissues and organs to treat the tumour. Larger machines used in radiation therapy such as teletherapy units and linear accelerators are used to battle the most aggressive cancers. In these instances the patient should not be pregnant because of the exposure to high radiation doses, i.e. in the hundreds of thousands of mrad, which would cause mutagenesis or more likely miscarriage (Brent, 2009). Later in the gestation period, the foetus has a decreased sensitivity to the effects of radiation, but is still vulnerable to the cell-killing effects. The treating doctor would have to inform the mother of the possible risks of foetal malformations and genetic mutations, for her to then be able to make an informed decision. High doses of radiation can also impact the fertility of the woman, especially when the radiation is concentrated on her abdomen/ pelvic region. The benefits to the mother in cases where there is a very aggressive cancer outweigh the consequences to the foetus, and in most cases a miscarriage or an abortion would take place. This shows that too high doses can have grievous effects on the foetus, and that therefore certain forms of ionizing radiation can be very harmful to the foetus as well as to the mother.
Involuntary exposure to radiation:
Probably the most famous examples of a large population being unwillingly exposed to a high radiation dose are the incidents in Hiroshima, Nagasaki and Chernobyl.
The atomic bombings of Hiroshima and Nagasaki exposed the immediate population within 1000 metres to a dose of 400000 mrad (Nave, 1993) which is 20 times as much as the foetal threshold in America and is almost 1,500 times as much as we are exposed to annually.
Figure 1 below shows a much higher risk of leukaemia compared to other types of cancer, because radiation from the atomic bomb probably had a greater effect on the structure of the blood.
Figure 1: Risk of developing various types of cancer for the Hiroshima and Nagasaki bomb victims who received a dose of 400000 mrad at 1000 metres away from the bomb site (1 gray=100000 mrad) (Nave, 1993).
There is a fundamental difference between the atomic bombings and the Chernobyl event in regards to the type of gamma radiation exposure. While the A-bomb survivors were exposed to one high instantaneous radiation dose, those in Chernobyl were exposed to many more doses due to the radiation leaking into the environment and were therefore exposed externally and internally. Even though the accident in Chernobyl took place in 1986, there are still very high radiation levels present in the soil, water and consequently in local crops and animal products, which is still affecting the large population living in Chernobyl. Within this population only five percent of the children are healthy and all others have leukaemia or other forms of cancer, and many children are born with various different deformities (Mulvey, 2006). 'A New Safe Confinement structure will be built by the end of 2011, and then will be moved into place on rails. It is to be an 18,000 tonne metal arch 105 metres high, 200 metres long and spanning 257 metres, to cover' (Mulvey, 2006) the most radioactive sites of the former reactor.
There was a sharp increase in reproductive disorders in Ukraine and Belarus resulting from the accident in Chernobyl. The 'Ministry of Health in Ukraine recorded an increased number of miscarriages, premature births and stillbirths' (Otto, 2001) due to increased foetal mutagenesis rates. Simultaneously, the rate of deformities and developmental abnormalities in newborns exposed in utero to radiation was tripled (Otto, 2001) compared to normal rates.
In the case of the A-bomb survivors there were no consistent effects of parental exposure on birth weight, neonatal mortality, or increased malignancies in their children (Neel, 1991). But there was a significant difference seen between children who were exposed in utero from those conceived after the bombings (Neel, 1991), which is similar to the findings from Chernobyl. This led to the formation of the hypothesis that children conceived before the bomb hit, showed an increased rate of mutagenesis. While these high levels of radiation usually lead to spontaneous abortion, in Hiroshima and Nagasaki there were increased incidences of children with mental retardation which correlated with the degree to which the mother was exposed depending on her distance from the centre of the A-bomb explosion (Neel, 1991). The radiation appeared to have a more severe affect on the offspring from women who were exposed in 'the third and fourth month of gestation' (Neel, 1991) when the foetal central nervous system is being formed. These children were prone to brain damage and anatomical malformations (facial clefts and excess toes and fingers). Yet babies born to women exposed during other foetal developmental stages did not exhibit such deformities. 'Among the 800 in utero survivors included in the study's population, twenty-one severely mentally retarded individuals were identified' (Anhalt, 2000). IQ tests from these in utero survivors showed that the radiation exposure had a negative effect. These individuals had lower IQ scores and performed worse in school (Anhalt, 2000). 'This result is supported by the apparent decreased level of intelligence seen in large portions of the generation born during the first years after the war' (Neel, 1991).
The effects that the exposure to radiation had on the foetus were very severe, and can still be felt today. There is no question that such high radiation levels were harmful, yet they formed an important basis for comparing the worst cases to and for generating radiation threshold values which are used today as guidelines for the maximum exposure of foetuses to ionizing radiation.
Alternatives to the use of clinical ionizing radiation and technological advances:
Alternatives to the exposure of ionizing radiation through x-rays do not only have physical benefits but also psychological ones by decreasing the anxiety that the mother experiences. Yet it should be noted that conventional x-rays can in today's medicine not yet be completely substituted.
Magnetic resonance imaging (MRI) facilitates an accurate diagnosis without the exposure to ionizing radiation, yet it takes longer to process than x-rays and is costly. Prenatal MRIs can be taken which 'enhances foetal anatomic evaluations and facilitates perinatal management and family counselling' (Adzick, 1998). Ultrafast imaging sequence MRI enables foetal diagnosis without the risk of mutagenesis from radiation and can aid in the diagnosis of some life-threatening birth defects (Adzick, 1998).
Another alternative method is ultrasound, which involves no ionizing radiation and helps evaluate foetal abnormalities. This method provides a limited amount of information. Various afflictions could affect the accuracy of the sonogram. Anatomic features such as 'liver herniation, pulmonary hypoplasia in congenital diaphragmatic hernia (CDH) and airway patency in giant neck masses' (Adzick, 1998) can obscure the image and therefore affect the prognosis and selection for foetal therapy.
3D-sonography is the newest x-ray based technology and enables the parents to see the unborn child in three dimensions. This allows some diagnosis, yet is limited due to its lower depth of field.
A rarely taken alternative is the simple refusal to undergo examinations involving ionizing radiation, which can be due to religious reasons or the patient's principal concerns about radiation. The refusal of clinical diagnostic x-rays is negligent and can lead to an undiagnosed disease progressing to the point where it can not be cured and unexpected complications can occur which lead to an unnecessary risk to one's own or the foetus's life. While Al Gore's x-ray refusal was justified, in most cases it may be more harmful than beneficial.
Common diagnostic procedures involving x-rays that a pregnant woman would receive expose the foetus to so little radiation that there is almost no risk of mutagenesis, so that the foetus will remain healthy. As hypothesized and furthermore, when the woman is suffering from certain forms of cancer which are located in the abdomen/pelvis, then the chances of mutagenesis in the foetus dramatically increase when using x-ray radiation as a therapeutic method. This can then lead to malformations and more likely to a miscarriage. The levels of ionizing radiation exposure are specified carefully, especially in light of a patient's radiological history.
The radiation exposures in Hiroshima, Nagasaki and Chernobyl had the most severe effect on the foetuses during the third and fourth month of gestation, when the central nervous system is being formed. These incidents are the worst case scenarios, where a large population was affected and still is to this day.
The alternatives to x-rays are MRIs', which are probably the most comparable to x-rays and produce a detailed internal image which enables diagnosis. The only negative aspect is that the image takes longer to process than an x-ray and is costly. Another alternative is the ultrasound, which is of a very low resolution and can therefore only give a limited image, therefore allowing a limited diagnosis. The final alternative method is refusal of the examination which can lead to severe problems such as the metastasizing of tumours and possibly death due to the absence of a timely diagnosis.
X-rays are useful medical tools without which many people would live shorter with a lower quality of life. Notwithstanding this, levels and timing of ionizing radiation exposure have to take the foetal developmental stage into account.