Radiation induced damage

Question 1

What is stochastic damage? What is the threshold? 2 examples

Answer 1

-Events arising by chance -no threshold.
-Probability increases with dose but severity does not, (either do or do not happen) – occur many years after exposure (if at all).

Examples:
-Induction of cancer in a subject exposed to radiation
-Production of genetic abnormalities in the offspring of subject/subsequent generations.

Question 2

What is deterministic damage?

Answer 2

-Tissue effects which occur rapidly at high doses and above a certain threshold (tissue type specific) of radiation.
-Severity of deterministic effect is related to dose.

Question 3

Give the deterministic thresholds for:
-Cataracts
-Whole body fatal dose

Answer 3

-Cataracts: 2-6Gy cumulative over person’s lifetime
-Whole body fatal dose: 2-6Gy

Question 4

Dose threshold for foetal abnormalities. When are the risky period for a foetus?

Answer 4

> 100mGy
Foetal abnormalities - max at 3rd to 8th week.
Mental retardation at 8-15 weeks.
Death at implantation phase. Growth retardation 8-25 weeks.

Question 5

Give the deterministic doses and timings for: BM failure, GI/lung failure, CNS failure

Answer 5

● >2Sv death in weeks from bone marrow failure
● >5Sv death in days from GIT/lung failure
● >20Sv death in hours from CNS failure

Question 6

What is the risk of inducing a fatal cancer risk in adults?

Answer 6

5% per Sv (1 in 20,000 mSv for adults)
A CT AP can be 10-20mSV

Question 7

What is the Linear no threshold model?

Answer 7

-Recommended by the International Commission on Radiological Protection (ICRP)
-Linear no threshold model (LNT): implies no safe dose of radiation (contradicts evidence of a threshold dose form atomic bomb data) but is overall accepted model.

Question 8

What are the 3 types of dosimetry badges

Answer 8

Film badges
Thermoluminescent
Electotronic dosimeters

Question 9

Film badges MOA

Answer 9

These use a silver-halide film (similar to that used in plain film radiography). They are an old technology and have been largely replaced with TLDs.

Question 10

Film badges: advantages

Answer 10

Cheap
Can distinguish between different energies of photons
Can measure doses from different types of radiation
Provide a permanent record
Accurate for exposures > 100 millirem

Question 11

Film badges: disadvantages

Answer 11

Film fogging over time
Prolonged exposures can adversely affect the film
Not accurate to exposures < 20 millirem
Must be developed and read by a processor, which is time consuming
Must be changed every 1 month due to fogging over time

Question 12

TLDs: MOA and advantages

Answer 12

-Most common
-TLD is heated and visible light is released from the crystal in proportion to absorbed radiation –> measured to calculate radiation exposure.
-Casing acts as filter to correct for deep and superficial absorption through skin.
-Advantages: can be very small and can be reused

Question 13

TLD disadvantages

Answer 13

Cannot distinguish between different types of radiation
More expensive than film badges
Once read out, record is lost i.e. can’t provide permanent record

Question 14

Electronic dosimeter: MOA

Answer 14

Most commonly used electronic dosimeter uses silicone diode detector. They can provide a direct electronic readout and live/real time readouts and don’t need the processing that is required for the other types of dosimetry badges. Require yearly battery replacement and checking.

Question 15

Electronic dosimeter: advantages and disadvantages

Answer 15

Advantages
-Very sensitive. Nearly 100x more sensitive than a TLD and can measure to nearest 1 µSv
-Good for measuring pregnancy doses

Disadvantages
-High initial cost