Radiation Biology, Safety and Protection

Biological effects of radiation exposure can be classified as either

1. Stochastic

2. Deterministic

What is the significance of different biological effects in dental imaging?

The radiation dose; there are varieties of radiation effects in the body

What are deterministic radiation effects?

A radiation dose threshold is required for the radiation damage. After the threshold is met, the severity of the damage is proportional to the radiation dose

What are stochastic radiation effects?

There is no radiation threshold and the probability of the effect is directly proportional to the radiation dose. Even one single x-ray can induce a cancerous effect. The chance of this effect increases with higher radiation dose

Tissue reaction characteristics

• Lethal DNA damage

• Occurs when radiation exceeds a threshold level

• Does not occur below threshold level

• Above dose threshold, the severity of the effect is proportional to dose

• Cell death

• Decreased tissue and organ function

Stochastic characteristics

• Sub lethal

• No minimum threshold for causation

• Probability, not severity of occurrence increases as dose increases

• Gene mutation

• Replication of mutated cells

What are some examples of tissue reactions effects

• Xerostomia

• Osteoradionecrosis

• Cataracts

• Fetal development

What are some examples of stochastic effects

• Leukemia

• Thyroid cancer

• Salivary gland tumors

What is radio-sensitivity and how does it translate to patient imaging?

• Sensitivity of cells to radiation varies between cells and tissue

• Cells that are more rapidly dividing (immature/non-specialized) are more sensitive

• The radiation effect varies depending on the area that is imaged

What are some areas that are radiosensitive (H&N)?

• Thyroid (most sensitive)

• Salivary glands (2nd most sensitive)

• Bone marrow

• Esophagus

• Skin

• Bone surface

• Brain

How are age and radiosensitivity correlated?

• Children are up to 5x more prone to carcinogenic effects of radiation

• This is d/t higher cell and tissue sensitivity to radiation & longer expected life span

Radiosensitivity to various organs

High radiosensitive organs

• Lymphoid organs

• Bone marrow

• Testes

• Intestines

• Mucus membranes

Intermediate radiosensitive organs

• Fine vasculature

• Growing cartilage

• Growing bone

• Salivary glands

• Lungs

• Kidney

• Liver

Low radiosensitive organs

• Neurons

• Muscle

Results of radiotherapy- deterministic effects after radiotherapy

• Taste loss

• Mucositis

• Hyposalivation

• Radiation caries

• Trismus

• Susceptibility to osteoradionecrosis

What is the model currently used in the Radiation Protection Guideline?

Linear non-threshold model (explains stochastic effect of ionizing radiation)

What is the linear non-threshold hypothesis?

At doses less than 100 mSv there is a linear relationship between dose and risk AND there is no threshold dose below where there is no additional rise

How does the linear non-threshold model explain stochastic effect

The probability of occurrence due to radiation effect increases as dose increase

What does this mean?

The gray-blue dot: there is a certain natural prevalence of cancer and a certain natural background radiation exposure

Green dots: doses of radiation greater than 100mSv results in a dose-dependant increase in the cancer rate

How would you compare radiation dose of different types of radiographs?

They are reported in Sieverts (Sv). The effective dose is measured with consideration of the type of tissue and degree of radiation sensitivity of the tissue that was exposed during a certain radiograph

Average annual effective dose of ionizing radiation

Natural

1. Total - 3.1

2. Radon- 2.3

Medical

1. Total -6.2

2. CT- 1.5

3. Dental- 0.007

Compare the different effective dose between a full mouth PSP film and CCD sensor

PSP is 35Sv, CCD sensor is 17. It is a 2:1 ratio

Compare the different effective dose between round collimation and rectangular

Round is 5: Rectangular is 1

Compare the effective dose of pan/FMX

There is less especially compared to round collimation FMX

Compare the effective dose of CBCT and FMX

They both are less compared to round collimation FMX

The largest field of view CBCT view is still higher than

Pano + Ceph

CCD FMX is similar to

One chest x- ray

Congenital malformations can occur with

0.1 Gy Threshold

Estimate dose from a FMX

0.17 mSv (approx. 0.1-0.2mGy)

Natural background dose to embryo and fetus

2250 microGy = 8 microGy/day

Compare a dental FMX to natural background dose and pregnancy

FMX is 20 days of natural background dose, and will not cause congenital malformation

Can you take dental x-rays on pregnant people?

Yes, it is allowed any time during pregnancy if there is a special indication. But in principle, defer optional imaging to the end of pregnancy like screening bitewings

What is the primary risk from dental radiography

Cancer

What is the primary concern from dental radiographs according to the linear non-threshold theory?

The stochastic effect of ionizing radiation and the primary risk is cancer

Cancer is a common disease. What are some stats regarding it?

It affects 40% of all people and

Accounts for 20% of all deaths

What is the risk from exposure in your childhood vs adulthood

The risk from exposure during childhood is 2-3x as great as the risk during adulthood

What is the evidence around cancer and radiation?

There is tons that links large radiation exposure to cancer risk (100 mGy), but the data is more uncertain regarding cancer risk from low-dose exposure

Define justification in radiation protection principles

Identify situations where benefit exceeds risk

Define optimization in radiation protection principles

Use every reasonable means to reduce exposure to patients, staff, and yourself. Follow ALARA

Define dose limitation in radiation protection principles

Legal limitations are placed on occupational and public exposures. No limitations on patient exposure but justification ensures benefit outweighs risk

What is the ALARA principle?

The goal is to remain below the dose limits and keep doses to patients and workers As Low As Reasonably Achievable. It is required by regulations

What does the ALARA Principle imply?

Any radiation dose that can be reduced without major difficulty, great expense, or inconvenience should be reduced or eliminated

What are the 3 principles of reduction of radiation exposure?

1. Limit time

2. Increase distance

3. Use sheilding

Time - ALARA Principle

1. Decreasing time spent in unshielded area while the x-ray beam is on

   1. No one else should be in the room during exposure

2. Avoid retakes

3. Avoid present intervals

Distance - ALARA principle

1. Apply the inverse square law - radiation dose is inversely proportional to the square of the distance from the source

2. Doubling your distance (x) from a radiation source decreases your dose by a factor of 4

3. Stand at least 6 feet away from the x-ray tube

Source to skin distance

Use of long source-to-skin distances of 40cm rather than short distances of 20cm to decrease exposure.

Distances between 20-40cm are appropriate but the longer distances are optimal

Use of the long cone results in

A reduction in exposed tissue volume because the x-ray beam is less divergent, and results in a smaller apparent focal spot size increasing the resolution of the radiograph

Position-and-distance rule operator dose

Operator should stand 6 ft from pt at an angle of 90-135 degrees to the central ray of the x-ray beam

Shielding - ALARA principle

x-rays travel great distances and penetrate through low-density materials like wood and plastic

Dense materials like lead, steel, and concrete are the most effective

What does shielding look like in ALARA?

Proper shielding will greatly reduce/eliminate the dose received (lead and concrete attenuate x-rays best

• Lead-lined doors/walls/windows, portable barriers

• Lead aprons, gloves, thyroid collars, glasses and gonadal sheilds

  • Hang aprons- do not fold

  • Do a visual examination of leaded PPE

What did the ADA change in 2024?

Use of abdominal aprons or thyroid collars on pts when conducting dental x-rays is no long recommended

The changes apply to all pts regardless of age or health status

Leaded aprons

• Reducing exposure in main beam is more important

• The only radiation exposure to anatomy below the neck is through

  • Scatter radiation

  • This passes through the body internally

• Gonad dose from FMX does not exceed 5 mGy

• Heritable effects are insignificant

What about patient thyroid shielding during diagnostic x-rays like intraoral, panoramic, cephalometric and CBCT imaging?

They should no longer be used in routine practice for pediatric or adult patients

How do thyroid collars and abdominal shielding work

They introduce artifacts by blocking the primary beam, potentially resulting in additional radiographs being taken and do not protect against internal scatter radiation

Why can’t you use a thyroid collar with panos?

Because of the beam’s upward projection from behind the patient during production of the pano- it obscures the anterior mandible

Why can’t you use a thyroid collar with CBCT?

It creates a streaking artifact and obscure anatomy

What is a collimator?

A metallic barrier with an aperture in the middle used to restrict the size of the x-ray beam and volume of tissue irritated

What kind of collimators are used in dentistry?

• Round

• Rectangular

Dental x-ray beams are usually collimated to a circle (2.75in) in diameter at the patients face

What is rectangular collimation?

• Device that limits the x-ray field to dimensions large enough to cover the region of interest

• Prevents exposing the patient to unnecessary primary radiation

• Reduces the amount of scatter radiation to image, patient and operator

• Decreases radiation dose by 5x as compared to a circular one

• Radiation dose to the thyroid was less using rectangular collimation alone vs. using both a round cone and thyroid collar shield

What is the dead man switch in radiation controls?

• x-rays will only be generated when the operator applies continuous pressure to the exposure switch

• When released, the exposure will stop immediately

• You have to release the exposure switch after the timer setting or rotation is completed and there is an audible warning signal

What are exposure indicators?

All radiation-producing devices must be equipped with visual and audible warning signals, activated when the device is emitting x-rays

-Warning lights are located on the device control panel

-Beep when x-rays are activated

What is the optimal operating potential of dental x-ray units?

60-70 kVp - kilovoltage

If everything is set correctly, what should the operator see radiographically?

Faint soft tissue outlines and they should set the amperage and time settings for exposure of dental radiographs of optimal quality

What is a dosimeter?

Recommended for workers who may receive more than 1mSv and for pregnant workers. Optically Stimulated Luminescence Dosimeter is a strip of Al2O3 radiosensitive crystal and is sensitive to 10µSv

SUMMARY - means for reducing x-ray exposure

• Use digital sensors

• Use holders to support films

• Use rectangular collimation for PA and BWX

• Replace short aiming tube with long x-ray tube

• Expose with 60-70kSv

• Use thyroid collars for intraoral radiographs and ceph

• Reduce CBCT beam FOV to region of interest

How should you talk to your patient about radiation?