04 - Radiation Biology, Safety, and Protection

measurements of radiation

• exposure → capacity of x-rays to ionize air

  • Roentgen: 1R = 2.58 × 10^-4 C/kg

• absorbed dose → amount of radiation energy absorbed by patient

  • Gray: 1 Gy = 1 J/kg

• equivalent dose → adjusted amount of dose, taking into account the type of radiation

  • Sievert: 1 Sv = 1 Gy

  • equivalent dose = absorbed dose x radiation weighting factor

• effective dose → takes into account the specific types of tissues being irradiated

  • also measured in Sievert, but has a different meaning than equivalent dose

radiation weighting factors

• x-rays, gamma rays, beta particles → 1

• protons → 2

• alpha particles → 20

tissue weighting factors

relative measure of stochastic effects

• W_T = 0.12 → stomach, colon, lung, red bone marrow, breast, remainder tissues

• W_T = 0.08 → gonads

• W_T = 0.04 → urinary bladder, esophagus, liver, thyroid

• W_T = 0.01 → bone surface, skin, brain, salivary glands

direct effects of radiation

x-rays directly producing free radicals makes up 1/3 of x-ray effects

• RH + x-radiation → R’ + H⁺ + e^-

• molecule in body associated with hydrogen is irradiated, resulting in a free radical + H⁺ + e^-

• radiation interacts with body, where ionizing radiation can be dangerous

• free radical dissociation fates:

  • dissociation: R’ → X + Y’

  • cross-linking: R’ + S’ → RS

• radiolysis of water causes formation of hydroperoxyl free radicals, and can contribute to the formation of hydrogen peroxide in tissues which are the primary toxins produced in tissues by ionizing radiation

  • H’ + O₂ → HO₂’

  • HO₂’ + H’ → H₂O₂

  • HO₂’ + HO₂’ → O₂ + H₂O₂

indirect effects of radiation

free radicals cause damage to organic molecules by removing hydrogens, happening 2/3 of the time

• RH + OH’ → R’ + H₂O

• RH + H’ → R’ + H₂

• chain reaction of radicals interacting to form more radicals

DNA effects from radiation

DNA damage

• double or single-strand breaks

• cross-linking DNA strands

• formation of rings and dicentrics → lethal changes

  • dicentric: chromosome with two centromeres

• DNA base deletion or substitution

• hydrogen bond disruption

deterministic effects

• lethal DNA damage → lethal to cell

• occurs when radiation exceeds a threshold level

  • does not occur below threshold level

• severity of effect is proportional to dose

• decreased tissue and organ function

  • xerostomia, osteoradionecrosis, cataracts, fetal development effects

• acute radiation syndrome (at very high levels)

prodromal period

early stage of radiation sickness, experienced for minutes to hours after exposure

• anorexia, nausea, vomiting, diarrhea, weakness, fatigue

• the higher the dose, the more rapid and more severe the symptoms

latent period

time between radiation exposure and when the effects can be seen, occurring after exposure of >2Gy

• interim period between prodromal period and syndromic periods

• apparent well-being, where subject is temporarily asymptomatic 

• the higher the dose, the shorter the latent period

hematopoietic syndrome

acute radiation syndrome when bone marrow is damaged by dose of ionizing radiation

• occurs with exposure of 2-7Gy

• rapid fall in granulocytes, platelets, and erythrocytes

  • mature cells are resistant to radiation, but new cells are greatly affected and can cause mutations in newly formed cells

• death may or may not occur 10-30 days after irradiation

  • rare at 2Gy, much more common at 7Gy

gastrointestinal syndrome

acute radiation syndrome that occurs when body receives a very high dose of ionizing radiation

• occurs with exposure of 7-15Gy

• rapid loss of epithelial layer of intestinal mucosa → cannot get nutrients

• lethal, with death within two weeks

• fluid and electrolyte loss, infection, and nutritional impairment

  • cannot intake through IV because circulatory system is also not working

cardiovascular and central nervous system syndrome

deadliest acute radiation syndrome happening at extremely high doses of ionizing radiation

• occurs with exposure >50Gy

• very few reported cases

• total collapse of circulatory system

• death in 1 to 2 days

stochastic effects

• sub-lethal → doesn’t kill the cell but can cause mutations that can lead to cancers

• no minimum threshold for causation

• probability of occurrence increases as dose increases

• replication of mutated cells

  • leukemia, thyroid cancer, salivary  gland tumors, heritable disorders

potentially affected oral tissues from x-ray radiation

• mucous membranes → mucositis

• taste buds → reversible loss of taste

• salivary glands → xerostomia, resulting in difficulty with food intake

• teeth → caries secondary to xerostomia

• bone → damage to vasculature, osteoblasts, and osteoclasts that lead to osteoradionecrosis

• musculature → inflammation and fibrosis leading to trismus, difficulty opening mouth

• general → candidiasis or fungal infection due to suppression of immune system

radiosensitivity

governed by law of Bergonie and Tribondeau

• the most radiosensitive cells have a high mitotic rate, undergo many future mitoses, and are most primitive in differentiation

  • spermatogenic and erythroblastic stem cells, basal cells of oral mucous membrane

  • lymphoid organs, bone marrow, testes, intestines, mucous membranes

• cell cycle phases from most to least sensitive → mitosis, G2, G1, S

radiation effects on embryos and fetuses

• 1-3Gy radiation in the first few days after conception can cause embryo death

• congenital malformations can occur with 0.1Gy threshold

  • developing organs are most susceptible at 3-8 weeks

  • brain is most susceptible at 8-15 weeks

• suspected dose from FMX with leaded apron = 0.25mcGy

• natural background dose to embryo and fetus = 2250mcGy = 8mcGy/day

• radiation increases probability of leukemia and other types of cancers

• assumed that embryos and fetuses have the same risk as children, which is 3x that of adults

consideration for thyroid cancer

• children are up to 5x more susceptible

• females are 2-3x more susceptible than males

• when thyroid is in direct path of radiation and it won’t interfere with image, use a thyroid collar in children

linear energy transfer (LET)

pattern or rate of how energy is transferred from the radiation to the body as it penetrates further into the tissues

• x-rays are low LET, but particulate radiation like alpha particles have higher LET

• the greater the LET, the greater the effect on the patient since effects are more localized

modifying factors for radiation

• dose → the greater the dose, the greater the effects on the patient

• dose rate → the more rapidly the dose is administered, the greater the effects on the patient

• oxygen → the more oxygen in the environment, the greater the effect on the patient

• linear energy transfer (LET) → the greater the LET, the greater the effect on the patient

background radiation

radiation from the environment 

• at higher elevation,  the atmosphere is less thick and has less protection from cosmic radiation

• makes up about 50% of average annual effective dose of ionizing radiation

cancer risk

primary risk from dental radiography due to stochastic effects

• common disease that affects 40% off all people and accounts for 20% of all deaths

• ample evidence that links large radiation exposure to risk (>100mGy)

• uncertainty regarding risk from low-dose exposure

theories for radiation

• linear no-threshold theory

  • assume linear relationship from background dose and incidence to data points from high doses

• threshold theory

  • no change to danger until threshold is reached

• hormetic model

  • radiation can be beneficial at low radiations and is dangerous when it reaches a threshold

radiation protection guiding principles

1. justification → identify situations where benefit exceeds risk

2. optimization → use very reasonable mean to reduce exposure to patients, staff, and yourself to follow ALARA

3. dose limitation → legal limitations are placed on occupational and public exposures, but no limitation on patient exposure but justification ensure benefit outweighs risk

means for reducing x-ray exposure

• use selection criteria to assist in determining type and frequency of radiograph examinations

• use E/F-speed films or digital sensors

• use holders to support film or digital sensors intraorally

• make exposures with 60 to 70 kVp

• use long collimators

• use rectangular collimation for periapical and bitewing images

• use thyroid collars

• stand at least 6ft away from patient and away from x-ray machine when making exposure

• use rare-earth screens for panoramic and cephalometric film imaging or use digital systems

• reduce cone-beam CT beam field of view to region of interest

source to skin distance

use of long source-to-skin distances of 40cm, rather than short distances of 20cm

• decreases exposure by 10-25%

• distances between 20-40cm are appropriate but longer distances are optimal

rectangular collimation

rectangular collimator decreases radiation dose by up to fivefold as compared with circular one

• radiographic equipment should provide rectangular collimation for exposure of periapical and bitewing radiographs

filtration

low-energy photons are mostly absorbed by the patient, contributing to patient dose but not to the image

• preferentially removing photons decreases patient exposure with no loss of radiographic information

• with filtration of 3mm aluminum, surface exposure is reduced to about 20% of the exposure with no filtration

• federal government has instigated a minimum filtration of an equivalent half-value layer of 1.5mm aluminum

leaded aprons

lead aprons are now unnecessary according to NCRP, ADA, and AAOMR, but reducing exposure in main beam is more important

• very slight gonadal exposure from dental radiographs

• heritable effects are essentially insignificant

• most states still require them

• potential to trap internal exposure

• use thyroid collar if minimize radiation exposure

kilovoltage

optimal operating potential of dental x-ray units is between 60 to 70 kVp

• too low → increases patient dose due to increased amount of non-diagnostic lower-energy photons

• too high → increases patient dose and decreases contrast of the resulting image

milliampere-seconds

operator should set amperage and time settings for exposure of dental radiographs of optimal quality

• referred to as “mAs”

• radiograph is of diagnostic density, neither overexposed (too dark) nor underexposed (too light)

• correct density should show very faint soft tissue outlines

position-and-distance rule

operator should stand at least 6ft from the patient, at an angle of 90-135 degrees to the central ray of the x-ray beam

• use barrier protection when possible, with barriers containing leaded glass window to enable operator to view patient during exposure

talking with patient about radiation

1. speak clearly and confidently

2. allow them to express thoughts/concerns

3. acknowledge their concerns

4. explain the reason for the exam

5. describe how you try to reduce patient exposure

6. point out how small the exposure is in terms of natural background radiation