I’m going to level with you. This is not a favourite section of mine. Let’s get the basics down, and move on to bigger and better things. If you remember anything, make sure its the difference between stochastic and deterministic effects. We’ve already gone over these concepts in the first lesson in this module. If you remember something else, let it be that immature and highly mitotic cells are the most sensitive to ionising radiation.

Ok, I’m just stalling writing this section now. Let’s get into it.

Pass your radiology physics exam first time

Ionising radiation can damage living tissue by depositing energy in cells, producing ionisations and free radicals that alter molecular structures.

The biological outcome depends on the type of radiation, absorbed dose, and the cellular radiosensitivity of the tissue.

Mechanisms of Radiation Interaction with Tissue

A. Direct Action

Occurs when radiation interacts directly with critical biomolecules (mainly DNA).
Causes ionisation, excitation, or molecular bond breakage.
More significant for densely ionising radiation (e.g. alpha particles, neutrons).

B. Indirect Action

More common in X-ray and gamma exposure.
Radiation interacts with water (≈70% of cell mass) → radiolysis: H₂O → H^∙ + OH^∙
These free radicals are highly reactive and cause:
- Single-strand and double-strand DNA breaks.
- Base deletions, cross-linking, or misrepair → mutations.
- Cell death if damage is irreparable.

A couple of things can happen when the DNA is damaged:

Type of Damage	Cellular Outcome
DNA correctly repaired	Cell survives normally
DNA misrepaired	Mutation, possible carcinogenesis
DNA unrepaired	Cell death (apoptosis or reproductive death)

The biological impact depends on:

Dose magnitude
Dose rate
Cell type (rapidly dividing and immature tissues more sensitive)
Presence of oxygen (oxygen enhances free radical damage).

Radiosensitivity of Tissues

Tissue sensitivity correlates with mitotic activity and maturity of cells described by the Law of Bergonie and Tribondeau (1906):

Cells are more radiosensitive if they are undifferentiated, rapidly dividing, and have a long mitotic future.

Highly Radiosensitive	Moderately Sensitive	Radioresistant
Bone marrow, gonads, GI epithelium	Skin, liver, lens of eye	Nerve, muscle, mature bone

This underlies why foetuses, children, and haematopoietic tissues are particularly vulnerable.

Dose–Response Relationships

Let’s briefly recap deterministic and stochastic effects.

A. Deterministic (Tissue Reaction) Effects

Have a threshold: below which no effect occurs.
Severity increases with dose (e.g. skin injury, cataract).
Represent cell death or loss of function in large cell populations.
Typically appear at doses >1 Gy.

B. Stochastic Effects

No threshold: risk increases linearly with dose.
Severity independent of dose.
Represent DNA mutation or misrepair.
Include carcinogenesis and heritable effects.
Estimated population risk ≈ 5% per Sv (fatal cancer).

Key Takeaways and Exam Tips:

Radiation damage occurs via direct ionisation or indirect free radical formation.
DNA is the critical target molecule; damage outcomes = repair, mutation, or cell death.
Deterministic effects → threshold, tissue reactions (cell death).
Stochastic effects → no threshold, probabilistic (cancer risk).
Oxygen, dose rate, and cell type modify radiosensitivity.
Diagnostic doses (mGy range) are far below deterministic thresholds but contribute cumulatively to stochastic risk.
Common exam question: “Describe the biological effects of ionising radiation and distinguish between deterministic and stochastic effects.”

Up Next

Next, we’ll move on to Patient Dose Reduction in X-ray Imaging, where we’ll outline concise, practical methods to minimise patient dose while maintaining diagnostic quality.