Radiation Safety

Question 1

How does ionisation cause damage to biological molecules?

Answer 1

-Direct damage: breaking molecular bonds directly via the energy released from ionisation events. Proteins are particularly sensible to direct damage from ionising radiation)
-Indirect damage: Ionising water molecules within the tissue to produce H+ ions and OH* free radicals, which then damage biological molecules.
*Both dependent on LET.

Question 2

What is 1 gray?

Answer 2

1 joule/kg

Question 3

What is exposure?

Answer 3

defined in air as the charge deposited in a unit mass when all the electrons have stopped. Measured in Coulombs and not very useful unit for practical radiation protection.

Question 4

What is KERMA?

Answer 4

(kinetic energy released per unit mass): kinetic energy of the charged particles released by the passage of ionising radiation through a unit mass of material. Measured in Greys.

Question 5

How do Kerma and absorbed doses compare at high and diagnostic energies?

Answer 5

-At Dx energies of up to 140 KeV: air kerma is used to define the output of x-ray sets (usually in mGy/mAs) at a distance of 1 metre.).
-At Dx energies air kerma and absorbed dose can be used interchangeably
-At high energies: the air kerma and absorbed dose will differ because there is more absorbed dose/energy deposited per unit mass (more spread within the material) and this will differ from the energy released to matter (kerma)

Question 6

What is absorbed dose? unit and ?density

Answer 6

= energy deposited/mass in Gy (or J/kg). Describes energy deposited by the charged particles liberated by the radiation as it passes through a unit mass of material (has unit Gy)
*Organ absorbed dose is independent of organ mass due to being divided by it.

Question 7

What is equivalent dose? unit

Answer 7

• Equivalent = absorbed dose x radiation weighting factor (in Sv).
  -Used to quantify the biological damage to an organ.
  -Doses to body parts are listed as equivalent doses.

Question 8

What is effective dose? Unit?How is it measured?

Answer 8

-Effective dose = equivalent dose x tissue weighting factor in Sv. Sum of the equivalent dose to each tissue multiplied by a tissue weighting factor (no units) dependent upon it radiosensitivity.
-It represents the uniform whole-body dose which produces the same radiation risk as the non-uniform absorbed dose and is also measured in Sv.  primarily concerned with stochastic risk b/c incorporates the tissue weighting factor for all irradiated organs (whole body).
-Effective dose can only be computed – cannot be measured directly.

Question 9

What is the effective dose of a : barium meal, enema

Answer 9

-Barium meal: 1.5-3mSv
-Barium enema: 3-6mSv

Question 10

What is the effective dose of a : radionuclide bone scan

Answer 10

5mSv

Question 11

What is the effective dose of a : CT head, AP, thorax

Answer 11

-CT head: 2mSv
-CT AP 10-20mSv
-CT thorax 8mSv.

Question 12

What is the effective dose of a CXR (equivalent to how many hours on a plane?)

Answer 12

-Chest x-ray: 0.0015 mSv
-5h on a plane

Question 13

What is the annual effective dose limit for a member of the public?

Answer 13

1mSv

Question 14

What is a radiation weighting factor? Units?

Answer 14

-Estimate of the effectiveness per unit dose of the given radiation relative to a low-LET.
-Dimensionless constant that accounts for the relative biological effectiveness of ionising radiation (or how good something is at causing damage to biological tissue). Used to calculate equivalent dose.
-Equivalent dose = absorbed dose x radiation weighting factor.

Question 15

What is the radiation weighting factor for alpha, neutrons and xrays/beta radiation?

Answer 15

• alpha = 20.
  -Neutrons = 5-20
  -X-ray/beta = 1.

Question 16

What is the Dose are product?

Answer 16

-used in xray = dose x area.
-It is calculated as the product of dose and beam area (Gy.cm2), and is measured using an ionisation chamber placed between the x-ray tube/collimator set up and the patient (which in theory is independent of its position in the beam). The dose area product can be divided by the area of exposure (in cm2) to give the incident accumulated exposure (air kerma) at that site. The dose area product is linked to a patients stochastic risk.

Question 17

What is the dose length product? unit?

Answer 17

-CT: effective dose doesn’t depend on area of patient exposed (as in xray using DAP) but on the volume of the patient imaged, which depends on the scan lengths – dose length product in mGycm.
-This is derived from the CT dose index (CTDI)measured in tissue-equivalent phantom or using Monte Carle based computer simulations from CT data to give a dose in Gy.

Question 18

What is the linear energy transfer?

Answer 18

-Energy deposited in tissue per unit path length and primary mechanism for ionising effects of x-rays at diagnostic range  occurs when a charged particle moves through tissue.
-Radiation with high let will cause more damage.

Question 19

What is the LET for alpha/neutrons and for gamma/beta/x-rays?

Answer 19

• high in alpha, neutrons (10-100keV/mm)
  -low in gamma, beta, xray (0.2-10keV/mm).

Question 20

What is radiosensitivity? which tissues are considered more radiosensitive?

Answer 20

-rate of division of cell types determines their sensitivity to specific dose of radiation.
-High radiosensitive: bone marrow, GI tract, breast tissue, gonads. (1.20)
-Low: bone, muscle, nervous system, BVs. (0.02)

Question 21

What is UK background radiation? Cornwall?

Answer 21

-Background radiation: Mostly radon from ground, potassium etc. Average dose is 2.7mSv/year
-closer to 6-7 in Cornwall (Radon decay –> primarily alpha particles.)

Question 22

What non medical source contributed the most to internal radiation

Answer 22

-Sources of radiation: cosmic, terrestrial, radon, food (K-40 in bananas contributes to 60% of our internal radiation)