Radiobiology

Ionising radiation causes damage to cells and must be dealt with carefully. The lowest possible dose should be delivered at all times, and special cases, e.g. pregnancy and infancy, should be evaluated for examination necessity.

Stochastic Effects

Stochastic effects are those where the probably of the effect increases with dose. These effects do not have a threshold dose. The primary biological effect is cell modification, e.g. radiation-induced cancer. The principle health risk is at low doses.

Non-Stochastic Effects

Non-stochastic effects are those whose severity increases with dose. The effects have a threshold value below which no effects are seen. The primary biological effect is cell death, e.g. skin erythema, cataracts and fibrosis. The principle health risk is at high doses. 

Damage to Cells

When a photon interacts with biological tissues, it produces energetic electrons through Compton scattering. Through a process of thermal heating, ionisation and excitation, secondary electrons are then produced, known as delta electrons. These delta electrons then cause damage to critical structures, i.e. DNA, directly or indirectly. In direct damage, the electrons themselves damage the DNA however in indirect damage, the electrons produce free radicals which then travel to the critical structures to cause damage. Damage via free radicals is the most common way cells are damaged by radiation, as the delta electron would be more likely to interact with the water in the cell rather than the tiny genetic fragments deep in the cell.

DNA Damages

There are a range of effects that may result from radiation damage. These include base damages and strand breaks. The base damages include:
  • Base damage
  • Base deletion
  • Base substitution
  • Hydrogen bond disruption
The strand breaks include:
  • Single strand break
  • Double strand break
  • Complex strand break
For a dose of 1Gy, low LET radiation, approximately 1000 base damages result, 100 single strand breaks and 40 double strand breaks.

Apoptosis is defined as programmed cell death. Many radiation-induced damages are capable of repair by the cell, particularly given that the rate of mutation is low and there is plenty of time for the cell to repair. Only in catastrophic events does the cell undergo apoptosis, where the damaged genetic material is completely eliminated.

Chromosomal Aberrations

There are four possible chromosomal aberrations as a result of radiation damage:
  • One break in one chromosome
  • Two breaks in one chromosome (ring formation)
  • One break in two chromosomes (translocation)
  • One break in two chromosomes (dicentrics)
In ring formation, there are two breaks in one chromosome (either 'leg'), and the broken ends join to form a ring. The fragments combine themselves to form a single linear chromosome. The rings and smaller chromosomes then replicate and anaphasic separation is either equal (one ring and one small chromosome to each pole) or the rings break and there is uneven separation.

In dicentric formation, there are two breaks, one in each chromosome. The two chromosomes then combine at the site of break, and the fragments form themselves as a linear chromosome. Replication then occurs and even anaphasic separation occurs.


LET

LET stands for linear energy transfer and describes the energy deposited per unit length, largely determining biological consequences. High LET radiations are heavy ions, low energy neutrons and alpha particles. Low LET radiations are gamma rays, x-rays and electrons. High LET radiations produce more double strand breaks and complex damages in DNA, leading to cell death.

Skin Effects

Skin reactions to radiation are the most common effects seen. Radiation induced skin conditions are termed cutaneous radiation syndrome. High doses may cause chronic ulceration and necrosis. Early transient erythema arises minutes to hours after a dose of 2Gy. Main transient erythema arises 2 weeks after a high exposure or repeated exposures of lower dose. Late transient erythema arises 8-52 weeks after exposure.

Temporary hair loss occurs 3 weeks after an exposure of 3-6Gy.

Gonadal Effects

The male gonads are of higher sensitivity to radiation than female gonads. The primary effects of radiation on the male reproductive system are reduced sperm count, temporary sterility and permanent sterility. Reduced sperm count occurs at 150mGy, temporary sterility occurs at different doses (~150mGy) but if at 2Gy it will take up to 3 years to improve, and permanent sterility occurs at 6Gy. Before the onset of sterility, there may be a window of temporary fertility.

Although less sensitive in general, the intermediate follicles of the female reproductive system are most sensitive to radiation damage. The only effect is permanent sterility however. Prior to puberty, a dose of 10Gy is required to sterilise a female. Over the age of 40, a dose of 2Gy is required for sterilisation.

Optical Effects

The lens of the eye contains a population of radiosensitive cells and as there is no drainage system or removal of damaged cells, cataracts may develop from the accumulation of damaged cells. After an acute exposure of 2Gy, cataracts may develop, and also after a chronic exposure of 5Gy. The cataracts as a result of radiation damage, unlike senile cataracts, accumulate on the posterior aspect of the eye. The occupational dose limit to the lens is 150mSv per year.

Radiosensitivity and the Cell Cycle

Cells are most radiosensitive in the G2 and mitotic phases, as there is less time to repair the cell prior to division. Abnormalities are only expressed in the mitosis phase.

Law of Bergoine and Tribondeau

This law states that 'cells are radiosensitive if they are undifferentiated and have a high mitotic rate'.

Most Radiosensitive Cells

Below, from highest to lowest radio sensitivity, are ranked cells of the body:
  1. Spermatogonia
  2. Lymphocytes
  3. Hematopoietic stem cells
  4. Small intestine crypt cells
  5. Hair follicles
  6. Colon, stomach
  7. Skin, kidney
  8. CNS
  9. Muscle
  10. Bone
Changes to Imaging Parameters to Reduce Patient Exposure
  • Grid: use for large patients where scatter radiation will be high but do not use for thin or petite patients as this will increase patient dose (Bucky factor)
  • Image intensifier distance from patient: the II should be placed as close to the patient as possible to reduce patient dose and optimise image quality
  • Patient size: large patients require higher kVp for adequate penetration for image quality
  • kVp and mAs: increasing kVp and decreasing mAs (15% rule) will reduce patient dose but maintain image quality. Increasing kVp will decrease the skin dose and increase penetration through the patient, thereby decreasing dose. Thus, a kVp and mAs are selected which achieve best image quality with the lowest possible dose.
  • Collimation: decreasing collimation reduces area of patient irradiated, and also decreases scatter which is bad for image quality
  • Organ shielding: use gonad shielding when applicable, especially for paediatric patients, with a thickness of at least 0.5mm of lead. The shielding should not interfere with the examination but should be placed on their surface, facing the primary beam. NB: Gonad shielding is not very effective in females because of the significant internal scattering which contributes to absorbed dose by the ovaries
  • Distance: approximately 0.1% of patient radiation exposure reaches 1m from the patient and personnel should stand at least 2m away from the x-ray tube.
  • Beam on time: keep to a minimum
  • Beam filtration: beam filtration hardens the beam by removing 'soft' or low energy photons, which do not contribute to imaging but are only absorbed but the patient. 2.5mm of aluminium, the standard filtration used, may be increased to 3.5mm for abdomen exposures, for example. Although introducing thicker filtration induces tube loading, it significantly reduces patient dose. For exposures of 70kVp or above, at least 2.5mm of aluminium is required.
  • Source to surface distance: increasing SSD decreases beam divergence, decreasing the volume of tissue irradiated, increasing SSD reduces entrance dose due to the inverse square law and increasing SSD decreases patient dose as a result of tube leakage. For mobile radiography suites, greater than 30cm SSD is used and for fixed radiography suites, greater than 45cm SSD is used.
  • Automatic exposure control: automatic exposure control is almost always used, as it achieves optimal image quality at the lowest possible dose. Use of AEC prevents exposure creep.
10 Commandments
  • Patient size
  • Tube current
  • Tube kilovoltage
  • Source to surface distance
  • Keep II close to patient
  • Image magnification
  • Grid
  • X-ray beam collimation
In Utero Radiation Effects

The radiation effects seen in the pre-implantation, organogenesis and foetal growth stages do vary.

In the pre-implantation stage (0-9 days), we observe an 'all or nothing response'. There are only a few cells at this stage, they are undifferentiated and have a high repair rate. Thus, if only a few cells are damaged, these may either repair or cause a failure to implant on the uterine wall, terminating the pregnancy. The incidence of congenital abnormalities as a result of radiation in the pre-implantation stage are low.

In the organogenesis stage (2-8 weeks), the organs of the child begin to form. A dose of 150mGy is required to cause malformation of organs. In this stage, there is increased risk of microcephaly, CNS abnormalities and growth retardation. Approximately 5 pelvic scans and 100 conventional x-rays would be required to reach a dose of 150mGy.

In the foetal growth stage (8 weeks onwards), only weeks 8-15 is the foetus considered radiosensitive. Past this point, the foetus is not considered radiosensitive. A dose of 1Gy (1000mGy) is required to increase the probability of mental or growth retardation. 

In general, doses to pregnant women are limited to 50Gy and foetal effects are only seen at 150mGy. Therapeutic abortion is not advisable based on radiation exposure alone.
  • 6% of foetuses will be mentally retarded
  • 15% of foetuses will have microcephaly
  • 3% will develop cancer
  • 15% will have a lower IQ
NB: Infants have greater life expectancies and have more radiosensitive tissues, and should receive the lowest possible dose.

Dosimetry

Dosimetry allows us to calculate absorbed dose to a tissue or organ at a certain depth.

Common Foetal Doses
  • Abdomen: 1.4mGy
  • Chest: <0.01mGy
  • Pelvis: 1.1mGy
  • Barium enema - fluoroscopy: 6.8mGy
  • Barium meal - fluoroscopy: 1.1mGy
  • Head CT: <0.005mGy
  • Chest CT: 0.06mGy
  • Abdomen CT: 8.0mGy
  • Pelvis CT: 25mGy

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