4: radiation biology and protection

Radiobilology

the study of the effects of ionizing radiation on living system

initial interaction between ionizing radiation and matter occurs...

at the level of electron within the first 10^-13 seconds after exposure

the effect of a living system interacting with ionizing radiation

alteration in the cell and organisms

Radiation injury

injury caused by x rays being absorbed into patient's issues and energy being transferred

two possible effects of energy transfer

- ionization
- free radical formation

DNA

- located in the nucleus of the cell
- serves as a command or a control molecule
- contains hereditary information
- a radiosensitive target molecule

When is a cell most radiosensitive in the cell cycle?

Mitosis and G2

When is a cell less sensitive in cell cycle to radiation

G1 Phase

When is a cell least sensitive in the cell cycle to radiation

DNA Synthesis period

Radiation affects living system through...

direct effects
indirect effects

direct effects

if any form of radiation is absorbed in biological material, there is a possibility that it will interact directly with the critical targets in the cells
- the atoms of the target itself may ionize to initiate the chain of events that leads to biologic change

what causes approximately one third of biologic effects of x ray exposure?

direct effects

Indirect effects

The radiation may interact with other atoms or molecules in the cell (H2O) to produce free radicals which have the ability to damage the critical structures in the cell

free radical

an atom or a group of atoms that has one unpaired electron
(highly unstable and reactive)

what happens when radiation interacts with water?

ionization of a water molecule

how do free radicals achieve stability?

1) recombine without causing any changes in the biological molecule
2) combine with other free radicals to produce chemical changes
3) combine to produce toxins such as H2O2

Deterministic Effects

Killing of large numbers of cells

Stochastic effects

sub-lethal damage to cell genome

deterministic effects characteristics

- lethal DNA damage
- cell death
- reduced tissue and organ function

stochastic effects characteristics

- sub-lethal DNA damage
- gene mutation
- replication of muted cells

Examples of deterministic effects

- xerostomia
- osteoradionecrosis
- cataracts
- decreased fetal development

examples of stochastic effects

- leukemia
- thyroid cancer
- salivary gland tumors
- heritable disorder

Deterministic effects are caused by

killing of cells

stochastic effects are caused by

sublethal damage to DNA

it there a threshold dose for deterministic effects?

yes
sufficient cell killing is required to produce a clinical response

it there a threshold dose for stochastic effects?

no
a single photon can cause changes in DNA

severity of clinical effects and dose - deterministic effects

severity is proportional to dose

severity of clinical effects and dose - stochastic effects

severity is independent of the dose.
- all or none response

probability of having effect and dose - deterministic effects

probability of effect is independent of dose
- all individuals show effect when dose is above threshold

probability of having effect and dose - stochastic effects

frequency of effect is proportional to the dose
- greater dose = greater chance of having the effect

Organs with high radiosensitivity

lymphoid organs
bone marrow
testes
intestines
mucous membranes

Organs with intermediate radiosensitivity

fine vasculature
growing cartilage
growing bone
salivary glands
lungs
kidney
liver

Organs with low radiosensitivity

neurons
muscles

common consequence of stochastic effects

radiation induced cancer

sources of radiation

- background radiation
- medical exposure
- consumer products

Average Background Millisievert

3.1

Average medical Millisievert

3.0

Total Millisievert

6.2 mSv

dose limits - occupational

50 mSv/year

dose limits - public

1 mSv /year

dose limits - pregnant workers

0.5 mSv/month

what is the average dose in dental x rays for occupationally exposed individual?

approx 0.2mSv - 1% of allowable dose

Round collimation on average has how much more mSv compared to rectangular collimation

round collimation has approximately 5 times more mSv

patient exposure

- reported as effective dose, which is the measure of radiation dose received by various organs

is there a limit on exposure that a patient can receive for diagnostic imaging?

no

guiding principles in radiation protection

- justification
- optimization
- dose limitation

Justification

identifying a situation where the benefit to patient from diagnostics exposure likely exceeds the risk of harm

Optimization

dentists should use every reasonable means to reduce unnecessary radiation exposure to their patient, staff, and themselves

ALARA

principle of radiation protection
As Low As Reasonably Achievable

Dose Limitation

- exposure should be limited to occupational and public
- principle is applied to the dentist and their staff
- dentist is responsible for the design and conduct of radiation protection program

Patient Selection Criteria

clinical examination must precede diagnostic imaging

FDA Guidelines for Dental Radiographs

"radiographic screening for the purpose of detecting disease before clinical examination should not be performed"

"radiographs should be taken only when there is an expectation by dentists that the diagnostic yield will affect patient care"

Decision to take radiographs

general health and age
medical history
dental history
clinical findings

aluminum filtration

a method to reduce dose

what does removing low energy photons do?

decrease beam intensity
increase mean energy

Mandated minimum total filtration for <50 kV

0.5 mm

Mandated minimum total filtration for 50 - 70 kV

1.5 mm

Mandated minimum total filtration for > 70 kV

2.5 mm

collimation

metallic barrier with an aperature in the middle to restrict size of x ray beam

Dental x ray beam is collimated to the...

circle of 7cm diameter at the patient's face

Rectangular PID

60-70% less skin area
80-85% less head volume
45-95% reduction in dose

Recommended Beam Collimation

maximum 2.75" diameter
Rectangular preferred

low kVp generates...

low energy photons

long PID (focal length)

27% less head volume
reduced effective dose
sharper image

Safe position and distance from x ray beam

stand at least 6 feet away
90 - 135 degrees to the primary beam

Types of personnel dosimeters

Film Badge
OSL dosimeter
TLD