6- biological effects of ionizing radiation

Simple explanation

Ionizing radiation is used safely and effectively in medicine, industry and science. However, when it interacts with biological systems, it can produce harmful effects.

To manage the risks of ionizing radiation, it is important to understand three things:

• Energy deposition: how energy is deposited at micro- and macroscopic scales.

• Cellular damage: what lesions radiation produces in cellular components.

• Consequences: how these lesions can lead to tissue damage, disease or cancer risk.

This chapter brings together current knowledge relevant to radiation protection practice.

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What can the interaction of ionizing radiation with biological systems produce?

Harmful effects.

What must be understood about energy deposition to manage radiation risk?

How energy is deposited at micro- and macroscopic scales.

What must be understood about cellular damage caused by ionizing radiation?

What lesions radiation produces in cellular components.

What consequences can radiation-induced lesions have?

Tissue damage, disease or cancer risk.

Interaction of radiation with the cell

The cell is the functional unit of living organisms. The critical target of ionizing radiation is usually DNA, although other macromolecules and organelles also contribute to the response.

Energy deposition can cause damage through two main routes:

• Direct action: The radiation track ionizes or excites DNA directly. This can produce single- and double-strand breaks, base loss or modification, DNA–protein crosslinks and other structural changes.

• Indirect action: Radiation ionizes water and generates reactive oxygen species (free radicals). These have a nanometric range and react with DNA and other biological targets.

At the DNA level, the most relevant lesions are double-strand breaks and spatially clustered, complex damage, which are particularly frequent when linear energy transfer (LET) is high.

The cell has multiple repair pathways. If repair is faulty or incomplete, mutations or cell death may occur

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Interaction of radiation with the cell

What is the functional unit of living organisms?

the cell

Interaction of radiation with the cell

What is usually the critical target of ionizing radiation in a cell?

DNA and other macromolecules and organelles

Interaction of radiation with the cell

What are the two main routes through which energy deposition causes damage?

Direct action and indirect action.

Interaction of radiation with the cell

What is direct action of ionizing radiation on DNA?

The radiation track ionizes or excites DNA directly.

Interaction of radiation with the cell

Which DNA lesions can result from direct action of ionizing radiation?

Single- and double-strand breaks, base loss or modification, DNA–protein crosslinks and other structural changes.

Interaction of radiation with the cell

What is indirect action of ionizing radiation?

Radiation ionizes water and generates reactive oxygen species (free radicals), which react with DNA and other biological targets.

Interaction of radiation with the cell

What does indirect action of ionizing radiation generate by ionizing water?

Reactive oxygen species (free radicals).

Interaction of radiation with the cell

What is the range of the reactive oxygen species

Nanometric range.

Interaction of radiation with the cell

Which two types of DNA damage are particularly relevant at the DNA level?

Double-strand breaks and spatially clustered, complex damage.

Interaction of radiation with the cell

When is spatially clustered, complex DNA damage particularly frequent?

When linear energy transfer (LET) is high.

Interaction of radiation with the cell

What can happen if DNA repair is faulty or incomplete?

Mutations or cell death may ensue

Interaction of radiation with the cell

• physical factors

What does radiation quality (LET) affect?

The complexity of damage caused by radiation and its reparability.

Interaction of radiation with the cell

• physical factors

an increase of radiation quality (LET) causes what

more complex and less repairable damage

Interaction of radiation with the cell

• biological factors

Which phases of the cell cycle are generally more radiosensitive?

G2 and M phases.

Interaction of radiation with the cell

• biological factors

Which phases of the cell cycle tend to be more radioresistant?

G1 and S phases.

Interaction of radiation with the cell

• biological factors

What biological factor affects cellular sensitivity to ionizing radiation?

The cell cycle.

Types of radiation-induced biological effects

How are radiation-induced biological effects classified?

Into deterministic effects (with threshold) and stochastic effects (no accepted threshold).

Types of radiation-induced biological effects

• DETERMINISTIC EFFECT (WITHIN THRESHOLD)

When do deterministic effects occur?

When a sufficient number of cells in a tissue or organ die or lose function.

Types of radiation-induced biological effects

• DETERMINISTIC EFFECT (WITHIN THRESHOLD)

Do deterministic effects have a dose threshold?

Yes, they exhibit a dose threshold.

Types of radiation-induced biological effects

• DETERMINISTIC EFFECT (WITHIN THRESHOLD)

How does the severity of deterministic effects change with dose?

Severity increases with dose.

Types of radiation-induced biological effects

• DETERMINISTIC EFFECT (WITHIN THRESHOLD)

Which dosimetric quantity describes deterministic effects?

Absorbed dose (D), measured in Gy.

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

What are stochastic effects?

Effects that occur when a cell survives with a fixed genetic lesion and, after a long latency, gives rise to cancer or hereditary effects.

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

Is there an accepted threshold for stochastic effects?

NO

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

How does dose affect the probability of stochastic effects?

The probability of occurrence increases with dose.

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

Which model is adopted by the ICRP for radiation protection in relation to stochastic effects?

The linear-no-threshold model.

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Hereditary effects

Have human genetic diseases been unequivocally attributed to parental radiation exposure to date?

no

(hereditary effect = myth)

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Hereditary effects

What is radiation in relation to mutations?

A universal mutagen.

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Hereditary effects

What do mammalian studies support regarding radiation exposure?

Its potential to induce germline mutations.

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Hereditary effects

How is hereditary risk in humans estimated?

Indirectly, from baseline frequencies of genetic disease, the doubling dose for mutations and recoverability factors.

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Dosimetric quantities

READ

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Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Dosimetric quantities

What is absorbed dose (D)?

Energy deposited per unit mass.

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Dosimetric quantities

What is the unit of absorbed dose (D)?

Gray (Gy).

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Dosimetric quantities

what does this formula relate to

Ht =WR . DT

equivalent dose

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Dosimetric quantities

What is the unit of equivalent dose (H_T)?

Sievert (Sv).

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Dosimetric quantities

what does equivalent dose mean

weighting radiation quality via radiation weighting factor WR

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Dosimetric quantities

what does that formula mean

effective dose

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Dosimetric quantities

What is the unit of effective dose (E)?

Sievert (Sv).

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Dosimetric quantities

What does the tissue weighting factor (wT) account for in effective dose?

Tissue and organ sensitivities.

Types of radiation-induced biological effects

• STOCHASTIC EFFECTS (NO ACCEPTED THRESHOLD)

• Dosimetric quantities

What is the purpose of effective dose (E)?

To estimate overall detriment.

Risk of stochastic effects and dose limits

Which publication recommends nominal risk coefficients per sievert to guide the setting of dose limits?

ICRP Publication 103.

Risk of stochastic effects and dose limits

What is the overall nominal risk coefficient for mixed populations?

Approximately 5.7%/Sv.

Risk of stochastic effects and dose limits

What is the nominal risk coefficient for cancer in mixed populations?

Approximately 5.5%/Sv.

Risk of stochastic effects and dose limits

What is the nominal risk coefficient for hereditary effects in mixed populations?

Approximately 0.2%/Sv.

Risk of stochastic effects and dose limits

What is the nominal risk coefficient for cancer in adults?

Approximately 4.1%/Sv.

Risk of stochastic effects and dose limits

What is the nominal risk coefficient for hereditary effects in adults?

Approximately 0.1%/Sv.

Risk of stochastic effects and dose limits

• dose limits (Spain)

What is the annual effective dose limit for exposed workers and students under 18 ?

≤ 20 mSv/year.

Risk of stochastic effects and dose limits

• dose limits (Spain)

What is the annual effective dose limit for the public?

≤ 1 mSv/year.

Risk of stochastic effects and dose limits

• dose limits (Spain)

What is the annual effective dose limit for students aged 16–18?

≤ 6 mSv/year.

Risk of stochastic effects and dose limits

• dose limits (Spain)

What is the fetal dose limit following declaration of pregnancy?

≤ 1 mSv from the time of declaration for the remainder of the pregnancy.

Risk of stochastic effects and dose limits

• dose limits (Spain)

Which tasks must a worker avoid following declaration of pregnancy?

Tasks with significant contamination risk.

Risk of stochastic effects and dose limits

• dose limits (Spain)

What work restriction applies during breast-feeding?

The worker must not be assigned to tasks with significant contamination risk.