Ionizing Radiation

Slide 1 · Ionizing Radiation

DNA damage, carcinogenesis and tissue response

Pathologic mechanisms • Epidemiologic evidence • Clinical interpretation

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Ionizing radiation: Cellular injury to cancer

Radiation

Photon or energetic particle

DNA injury

Breaks, base lesions and cross-links

Cell fate

Repair, death or persistent damage

Late outcome

A clone may acquire transforming events

Carcinogenesis depends on survival and propagation of altered genetic information—not simply the amount of immediate tissue destruction.

Slide 3 · Learning outcomes: Molecular pathology

Recognize the ionizing modalities and the types of DNA lesions they produce.

Explain direct ionization, radiolysis of water and the modifying effect of oxygen.

Distinguish accurate DNA repair from mutagenic repair and persistent genomic injury.

Slide 4 · Learning outcomes: Clinicopathologic interpretation

Contrast early cell killing with the delayed emergence of radiation-associated neoplasia.

Compare radiation-associated leukemias, thyroid cancers and other solid tumors.

Interpret risk using exposure setting, age, latency, target tissue and clinical context.

Slide 5 · Ionizing versus nonionizing radiation

Ionizing radiation

Sufficient energy to remove tightly bound electrons.

Initiates cascades of ionization and chemical injury.

Includes x-rays, gamma rays and energetic particles.

Nonionizing radiation

Insufficient energy to directly remove tightly bound electrons.

Can cause molecular excitation or vibration.

Includes ultraviolet and infrared light comparison.

UV is a carcinogen, but through different characteristic DNA photolesions.

Slide 6 · Major forms of ionizing radiation

Electromagnetic

X-rays and gamma rays

Charged particles

Alpha and beta radiation; Protons

Uncharged particles

Energetic neutrons

All listed modalities can be carcinogenic; The spatial pattern of energy deposition is not identical.

Slide 7 · X-rays and gamma rays versus alpha particles

X-rays / gamma rays

Penetrating electromagnetic radiation.

Energy dissipated over a longer tissue path.

Less localized damage at equal deposited energy.

Alpha particles

Relatively heavy, charged particles.

Intense ionization along a restricted track.

Particularly consequential for locally deposited radionuclide products.

The biologic effect also depends on absorbed dose and exposure geometry.

Slide 8 · Exposure contexts in the Robbins discussion

Medical

Diagnostic imaging; Therapeutic irradiation

Occupational

Radiology pioneers; Miners

Environmental

Radon exposure and nuclear releases

Historical

Atomic bomb survivor cohorts

Evidence for carcinogenesis spans distinct exposure patterns, ages, doses and follow-up periods.

Slide 9 · Two convergent routes to DNA damage

Direct effect

Ionization alters DNA or adjacent molecules

Indirect effect

Water radiolysis generates reactive intermediates

Shared outcome

Potential DNA lesions and chromosomal damage

Indirect free-radical damage as the predominant pathway for DNA injury.

Slide 10 · Radiolysis of water and DNA injury

Ionizing track

Energy transferred to intracellular water

Reactive species

Free radicals arise from radiolysis

DNA attack

Bases and DNA backbone are damaged

Consequences

Repair, death or misrepair

This mechanism explains why the effects are not limited to the atoms directly struck by a photon or particle.

Slide 11 · Reactive intermediates: Why oxygen matters

The free radicals generated by ionized water damage DNA and other cellular constituents.

Higher oxygenation enhances radiation effects; Tumor hypoxia can decrease radiosensitivity.

Oxygen modifies the severity of injury, but oxygen tension alone does not determine tumor response.

Slide 12 · Irradiated tumor: Oxygenated and hypoxic zones

Better-oxygenated tumor region

Greater radiosensitivity associated with oxygen-dependent chemical injury.

Radiation damage is more likely to translate into lethal lesions.

Illustrates the oxygen effect.

Poorly oxygenated tumor region

Hypoxia reduces sensitivity to radiation-induced cell injury.

Hypoxic regions may persist despite irradiation.

The tumor microenvironment therefore modifies treatment response.

This is a conceptual comparison, not an imaging or histology finding.

Slide 13 · DNA damage has several molecular forms

Ionizing radiation

Base injury

Chemical alteration of nucleotides

Strand breaks

Single- or double-strand breaks

Cross-linking

DNA–DNA and DNA–protein lesions

Multiple lesion classes can coexist within a single irradiated cell.

Slide 14 · Single-strand breaks: Injured DNA backbone

Ionizing energy may break one DNA strand while the complementary strand remains intact.

Repair systems can use the complementary strand to restore genetic continuity.

Single-strand lesions nevertheless contribute to the overall burden of radiation injury.

Slide 15 · Double-strand breaks: A central lethal lesion

Both strands of the DNA duplex are interrupted.

Double-strand breaks are especially consequential because accurate restoration is difficult.

Incorrect rejoining can leave deletions, inversions or chromosomal translocations.

Slide 16 · Misrejoining can create chromosomal rearrangement

Two DNA breaks

Broken ends require rejoining

Aberrant repair

Ends joined in an incorrect configuration

Stable alteration

Inversion or translocation may persist

The illustration depicts a class of rearrangements, not a specific cancer-defining translocation.

Slide 17 · DNA–protein cross-links: A different obstacle

Ionizing radiation can cross-link DNA with itself or with proteins.

Such damage threatens DNA replication and normal genomic function.

The potential carcinogenic outcome depends on how the surviving cell handles the lesion.

Slide 18 · Checkpoint control limits propagation of damage

DNA damage

Lesions detected during cellular stress

Cell-cycle restraint

Replication or division is delayed

Repair opportunity

DNA repair processes act

Outcome

Recovery, arrest or elimination

Failed checkpoint restraint after double-strand breaks to persistence of chromosomal lesions.

Slide 19 · Two routes for repairing double-strand breaks

Homologous recombination

Uses a homologous sequence to guide reconstitution.

One of the two repair mechanisms named by Robbins.

Can provide more faithful sequence restoration.

Nonhomologous end joining (NHEJ)

Directly rejoins broken DNA ends.

Described as the commoner route in mammalian cells.

Can generate small deletions or duplications and rearrangements.

These are alternatives for repairing double-strand breaks; Their accuracy is context dependent.

Slide 20 · How NHEJ may generate a mutation

Broken DNA ends

A double-strand break is present

End processing

Sequence can be lost or duplicated

Rejoining

Continuity returns; Sequence may change

Clonal survival

The altered cell may proliferate

Repair of DNA continuity is not necessarily repair of the original nucleotide sequence.

Slide 21 · NHEJ and large-scale genome disruption

Deletions and duplications among possible NHEJ-related changes.

Translocations and inversions can occur when breaks are repaired aberrantly.

These abnormalities constitute plausible substrates for later oncogenic evolution.

Slide 22 · DNA damage is not synonymous with cancer

Irradiated cell

Effective repair

The original genome is restored

Cell death or arrest

A damaged clone does not expand

Mutagenic survival

Altered genome can persist

Carcinogenesis requires survival, continued proliferation and further transforming events.

Slide 23 · Why a cell that survives matters most

Massive radiation injury may kill affected cells rather than produce cancer from them.

A surviving cell with stable, oncogenic genetic damage has malignant potential.

Late disease reflects persistence and subsequent expansion of a mutated lineage.

Slide 24 · A mechanistic model of radiation carcinogenesis

Initial exposure

Ionization injures genomic DNA

Persistent lesion

Damage escapes faithful repair

Genomic instability

Altered or rearranged DNA remains

Clonal evolution

Additional events allow malignant transformation

The sequence is a teaching model; No single mutation is presented as sufficient for every cancer.

Slide 25 · Genomic instability after ionizing injury

Radiation can generate DNA breaks and structural chromosome abnormalities.

Imperfectly repaired lesions may be inherited by daughter cells.

Subsequent transforming events can make a radiation-associated tumor clinically apparent years later.

Slide 26 · Acute injury and delayed neoplastic effects

Early radiation injury

Dominated by cell damage, impaired proliferation and death.

Frequently affects rapidly dividing tissues.

May lead to cytopenias and mucosal injury after high exposures.

Delayed carcinogenesis

Requires survival of genetically injured cells.

Mutations and clonal expansion accumulate over time.

Leukemias and solid tumors may emerge years to decades later.

High acute radiosensitivity does not automatically imply maximal cancer incidence.

Slide 27 · Latency separates exposure and cancer manifestation

Exposure

Radiation dose delivered

Initial effects

DNA lesions and cell fate

Clonal evolution

Further transforming events

Clinical tumor

Cancer becomes detectable

Latency is variable; The schematic is not drawn to a numerical time scale.

Slide 28 · Determinants of biological effect

Type of ionizing radiation changes the density and distribution of damage.

Absorbed dose and delivery rate influence cellular repair opportunity.

Size of irradiated field, tissue proliferation and oxygenation modify outcomes.

Slide 29 · Single exposure versus fractionated exposure

Concentrated exposure

More injury delivered without substantial time for inter-fraction recovery.

Can overwhelm the ability of tissues to repair DNA.

Consequences vary with dose and irradiated volume.

Fractionated exposure

Separated exposures permit partial repair between fractions.

Cumulative damage remains when repair is incomplete.

This differential recovery is used in radiotherapy.

Fractionation changes biological effect; It does not imply that cumulative risk vanishes.

Slide 30 · Why fractionation can protect normal tissues

Radiation fraction

Both tumor and normal tissues are damaged

Recovery interval

Normal cells can repair some injury

Next fraction

Residual damage may accumulate in tumor

The principle as comparatively faster recovery of many normal cells than tumor cells.

Slide 31 · Small-field and large-field exposure

Small, shielded field

Local irradiation limits the volume exposed.

Higher local doses may be tolerated.

Aims to spare uninvolved tissues.

Broad or total-body field

Large exposed volume recruits multiple sensitive organs.

Even lower doses may produce severe systemic effects.

Marrow and intestinal injury are major risks.

Field geometry determines the clinical expression of radiation injury.

Slide 32 · Proliferation affects early tissue radiosensitivity

DNA-directed cellular injury

Highly proliferative

Marrow, gut mucosa and gonads

Intermediate context

Endothelial cells may sustain injury

Nondividing mature cells

Often more resistant to immediate cell killing

Radiation exposure damages all these compartments, but clinical consequences differ.

Slide 33 · Bone marrow: The cost of rapidly dividing cells

Hematopoietic precursors are sensitive to DNA damage and radiation-induced arrest or death.

Severe exposure may cause dose-dependent marrow hypoplasia or aplasia.

Peripheral blood effects develop according to the turnover of each cell population.

Slide 34 · The hematologic sequence after marrow irradiation

Early

Lymphopenia may appear rapidly

Days

Neutropenia develops

Later

Thrombocytopenia; Then anemia

Recovery

Depends on surviving precursors and dose

These are qualitative temporal phases; Actual timing and severity vary with exposure.

Slide 35 · Lymphoid tissue is especially radiation-sensitive

Radiation can directly kill circulating and tissue lymphocytes.

Severe exposures may shrink lymph nodes and spleen.

After sublethal injury, surviving precursors can repopulate the lymphoid compartment.

Slide 36 · Gastrointestinal mucosa: Early tissue loss

Crypt epithelial turnover depends on continuous stem and progenitor proliferation.

Radiation suppresses renewal and may cause mucosal injury and ulceration.

The severe acute intestinal response must be distinguished from relative resistance to induced gastrointestinal carcinomas described in the neoplasia discussion.

Slide 37 · Tissue killing versus tissue transformation

Sensitive to early cell killing

Lymphoid and bone marrow compartments.

Rapidly renewing gastrointestinal epithelium.

Germinal cells and supporting reproductive tissue.

At risk for later cancers

Myeloid hematopoietic lineages.

Thyroid tissue, particularly after young-age exposure.

Breast, lung and salivary tissues in the chapter’s hierarchy.

These lists answer different questions and should never be conflated.

Slide 38 · Radiation-induced cancer: Comparative tissue vulnerability

Category

Examples

Interpretation

Prominent

Myeloid leukemia

Important early epidemiologic signal

High in young

Thyroid carcinoma

Strong age dependence

Intermediate

Breast, lung, salivary cancers

Documented solid-tumor associations

Relatively less

Skin and gastrointestinal epithelium

Lower induction tendency in this comparison

This is the textbook’s qualitative hierarchy; Not a universal numerical ranking.

Slide 39 · Why leukemias are an early signal

Atomic bomb follow-up found a marked increase in certain forms of leukemia.

An average latent period of approximately seven years for the early leukemia excess.

Hematopoietic malignancies therefore supplied an important epidemiologic signal before many solid tumors became apparent.

Slide 40 · Atomic bomb survivor follow-up: Two temporal patterns

1945

Exposure event

Later years

Leukemia incidence rises

Longer interval

Solid-tumor excess becomes evident

Long follow-up

Multiple cancers are identified

The timeline demonstrates ordering only; The distance between markers is not proportional to incidence or years.

Slide 41 · Solid tumors require prolonged observation

Longer latent intervals followed the earlier radiation-associated leukemia excess.

Breast, colon, thyroid and lung carcinomas are examples cited among later tumors.

Observation lasting decades is needed to capture the full burden of delayed neoplasia.

Slide 42 · Early radiology workers: An occupational warning

Pioneers using x-rays had substantial unprotected exposure compared with current practice.

Skin cancers among early x-ray workers.

The historical pattern helps establish that medically useful ionizing radiation can also be carcinogenic.

Slide 43 · Uranium miners: Radiation and lung carcinoma

Miners of radioactive elements were observed to have higher rates of lung cancer.

Historical cohorts showed a tenfold excess in historical European and Rocky Mountain mining cohorts.

Inhaled radon progeny and alpha emission to the lung exposure pathway.

Slide 44 · Radon exposure: From source to lung injury

Uranium decay

Radon gas forms naturally

Inhalation

Radon decay products enter airways

Alpha emission

Polonium products deposit energy locally

Long-term outcome

Lung cancer risk can rise

The mechanism concerns inhaled radioactivity; It is not the same exposure geometry as external x-rays.

Slide 45 · Radon daughters and high local ionization

Robbins specifically names polonium-214 and polonium-218 as relevant radon decay products.

Their alpha emission delivers dense local energy in tissues where radioactive particles deposit.

Chronic inhalation is a plausible route to accumulating lung-cell DNA damage.

Slide 46 · Residential radon risk: Interpreting context

Concern about homes with very high radon concentrations.

The text also raises the possibility that lower household radon contributes to risk, especially with smoking.

Exposure history and concurrent carcinogenic insults matter when interpreting lung cancer causation.

Slide 47 · Atomic bomb survivor studies as causal evidence

Hiroshima and Nagasaki cohorts were observed after a sudden radiation exposure.

The subsequent excess of leukemia and multiple solid malignancies supports radiation carcinogenesis.

Cancer incidence and latency required many years of survivor follow-up.

Slide 48 · Chernobyl: A thyroid-cancer association

Thyroid cancers in people exposed during the Chernobyl nuclear accident.

This evidence complements the age-dependent thyroid susceptibility identified in the neoplasia section.

Organ-specific exposure and age must be considered before generalizing population risk.

Slide 49 · Marshall Islands: Consequences of fallout

Exposed Marshall Islands populations as a radiation-risk example.

Reported effects included elevated thyroid cancers and leukemias, among other consequences.

Nuclear fallout illustrates a complex environmental exposure rather than a single uniform tissue dose.

Slide 50 · Second malignancies after therapeutic irradiation

Cancer therapy

An intended radiation field is delivered

Surviving normal cells

Some DNA lesions persist after repair

Long latency

Genomic changes accumulate in clones

New malignancy

Second cancer can eventually emerge

The risk is distinct from local recurrence of the original malignancy.

Slide 51 · Second cancers after treatment for Hodgkin lymphoma

Radiation therapy for Hodgkin lymphoma as an example of later secondary neoplasia.

Myeloid malignancies, including acute myeloid leukemia and myelodysplastic syndrome, are listed.

Solid tumors may also occur after sufficient latency; Prior treatment history is a critical interpretive variable.

Slide 52 · Adolescent chest irradiation and breast cancer

Breast cancer risk after chest radiotherapy in adolescent females.

Susceptibility is age-dependent, with young exposure of particular concern.

The radiation field and treatment history should be included in cancer risk assessment.

Slide 53 · Diagnostic imaging: Illustrative textbook dose examples

Investigation

Textbook dose

Why this is contextual

PA chest radiograph

0.01 mSv

Illustrative low-dose reference

Lateral chest radiograph

0.15 mSv

Different projection and exposure

Chest CT

10 mSv

Illustrative higher-dose modality

Values are those printed in the radiation-injury discussion, not contemporary protocol-specific dose guarantees.

Slide 54 · Pediatric CT: Relative and absolute risk

Studies linking repeated pediatric CT to a measurable excess risk of leukemia and brain tumors.

It reports roughly a threefold relative risk under certain scan-count comparisons.

The absolute excess was small; Interpretation must distinguish relative from absolute risk.

Slide 55 · Relative risk and absolute excess are not interchangeable

Relative increase

A ratio compares the rate in exposed and reference groups.

A threefold association may sound large without baseline context.

Does not identify an individual child who will develop cancer.

Absolute excess

Counts extra outcomes against a baseline population rate.

A small baseline may yield a small number of additional events.

Needed to place diagnostic benefit and exposure risk in context.

That pediatric imaging risk is small but measurable.

Slide 56 · Balancing imaging benefit against carcinogenic risk

Ionizing imaging is diagnostically valuable and often clinically necessary.

Repeated or avoidable examinations add to cumulative exposure considerations.

The textbook’s practical emphasis is to minimize unnecessary radiation while preserving appropriate diagnostic care.

Slide 57 · Three different ways of describing radiation

Measurement

Unit

Meaning

Radioactivity

Curie (Ci)

Disintegrations from a source

Absorbed dose

Gray (Gy)

Physical energy delivered to tissue mass

Biologic dose

Sievert (Sv)

Radiation-weighted biological impact

Do not confuse emitted activity, absorbed physical dose and biologically weighted dose.

Slide 58 · Why physical dose alone is insufficient

Equal absorbed doses delivered by different radiation types may cause different biological damage.

Radiation type, tissue distribution and exposure conditions modify the resulting injury.

The sievert is intended to capture weighting relevant to biologic effect rather than emitted source activity.

Slide 59 · Low-dose uncertainty versus established exposure effects

Strongly established at higher doses

Major accidents and historical exposed cohorts demonstrate cancer excess.

The textbook considers exposures above 100 mSv clearly consequential.

Risk can include leukemia and organ-specific solid cancers.

Harder to quantify at lower doses

Small cancer increments are difficult to estimate in limited populations.

The textbook stresses the very large samples required for low-dose comparisons.

A weak individual-level signal does not demonstrate absence of risk.

Quantitative risk attribution should not be overstated where statistical power is limited.

Slide 60 · Why radiation risk studies need large cohorts

Cancers also occur spontaneously in people without known ionizing exposure.

At low doses, any radiation-attributable excess may be small relative to background cancer occurrence.

Distinguishing signal from random variation therefore requires substantial follow-up and population size.

Slide 61 · Clinical assessment of radiation-associated cancer

Exposure history

Modality, site, timing and age

Potential target

Organ in an exposed field or pathway

Biologic plausibility

Cancer type and latent interval

Interpretation

Association, not automatic proof of causality

No routine morphology alone is described as pathognomonic for radiation etiology.

Slide 62 · Radiation-induced cellular atypia

Surviving irradiated cells may have enlarged or pleomorphic nuclei.

Multinucleated giant cells, chromatin abnormalities and atypical mitoses may be seen.

Robbins warns that therapy-related atypia can mimic residual malignancy.

Slide 63 · Post-treatment radiation atypia versus persistent tumor

Radiation-injured tissue

Can show enlarged, irregular or multinucleate cells.

Damage may coexist with vascular sclerosis and fibrosis.

History, timing and architecture are essential to interpretation.

Persistent neoplasm

Requires an integrated assessment of malignant morphology and context.

Not established by nuclear atypia alone after treatment.

Clinical and tissue correlation helps avoid overcalling therapy effect.

No generated specimen is shown; These are conceptual diagnostic distinctions.

Slide 64 · Late vascular injury produces ischemic fibrosis

Endothelial injury

Cells swell or may be lost

Vascular remodeling

Intimal thickening and hyalinization

Luminal compromise

Narrowed or occluded vessels

Chronic change

Ischemic atrophy and tissue fibrosis

Vascular damage contributes to delayed organ dysfunction after radiotherapy.

Slide 65 · Radiation fibrosis: Pathogenesis

Death of parenchymal cells can lead to replacement by scar tissue.

Vascular injury causes ischemia and impaired recovery.

Inflammatory mediators and fibroblast activation amplify collagen deposition.

Slide 66 · Radiation fibrosis: Typical tissue settings

Lung fibrosis is a recognized consequence of radiation treatment.

Salivary glands may undergo scarring after irradiation for head and neck malignancies.

Colorectal and pelvic tissue fibrosis can follow treatment for nearby cancers.

Slide 67 · Three outcome domains after radiation exposure

Ionizing radiation injury

Acute tissue response

DNA damage, cell death, cytopenias

Chronic tissue response

Vascular sclerosis and fibrosis

Oncogenic response

Mutagenic survival and delayed cancer

Exposure conditions, tissue type and time govern which effects predominate.

Slide 68 · Clinical reasoning: Child with repeated CT examinations

HYPOTHETICAL TEACHING SCENARIO

A child undergoes several clinically indicated CT examinations over a period of years. The family asks whether the examinations will cause leukemia.

How should relative and absolute risk be distinguished?

Why does patient age affect risk interpretation?

What is the appropriate message about necessary imaging?

Slide 69 · Case analysis: Pediatric CT exposure

Repeated CT can be associated with a small increase in later cancer risk; Risk is not deterministic.

The textbook’s reported relative-risk estimates should not be interpreted as a prediction for one child.

Indicated imaging benefits must be considered while minimizing avoidable exposure.

Slide 70 · Clinical reasoning: New lung cancer in a miner

HYPOTHETICAL TEACHING SCENARIO

A retired uranium miner develops lung carcinoma many years after working underground. The history also includes tobacco exposure.

Which inhaled radioactive agent and decay products are relevant?

How does alpha radiation produce local DNA damage?

Why should causation consider concurrent tobacco exposure?

Slide 71 · Case analysis: Mining, radon and smoking

Inhaled radon decay products can expose lung epithelium to local alpha radiation.

Historical mining cohorts demonstrate increased lung cancer incidence.

Coexisting tobacco exposure complicates individual attribution but does not negate radiation-associated risk.

Slide 72 · Clinical reasoning: Atypical cells after radiotherapy

HYPOTHETICAL TEACHING SCENARIO

A post-treatment biopsy contains enlarged pleomorphic cells and interstitial fibrosis in a previously irradiated field.

Can radiation effects imitate malignant cytology?

Which histologic and clinical features help contextualize atypia?

Why is nuclear pleomorphism alone insufficient to prove recurrence?

Slide 73 · Case analysis: Radiation effect versus recurrence

Irradiated tissues may contain persistent pleomorphic, multinucleated or abnormal mitotic cells.

Fibrosis and chronic vascular injury provide supporting evidence of a treatment effect.

Residual cancer cannot be excluded by these findings alone; Architecture and clinical context remain decisive.

Slide 74 · Contrast with ultraviolet carcinogenesis

Ultraviolet radiation

Characteristic lesion: Pyrimidine dimers.

Associated primarily with sun-exposed skin tumors.

Nucleotide-excision repair defects heighten susceptibility.

Ionizing radiation

Characteristic lesions: Strand breaks and cross-links.

Associated with hematologic and many organ-based malignancies.

Double-strand break repair integrity is central.

Both are carcinogenic radiation exposures, but DNA damage signatures differ.

Slide 75 · Synthesis: From exposure to pathologic consequence

Energy deposition

Ionization directly or via free radicals

DNA fate

Repair, death or erroneous rejoining

Clinical expression

Acute injury, late fibrosis or cancer

Mechanism and outcome must always be interpreted with organ, dose, age and latency.

Slide 76 · Five pathology principles to retain

Ionizing radiations are carcinogens because they damage DNA.

Double-strand breaks and aberrant repair can create durable genomic changes.

Acute cell death and late tumor incidence are biologically distinct endpoints.

Age, organ, dose distribution, oxygen and delivery schedule influence effect.

Late cancers require long follow-up and contextual risk interpretation.

Slide 77 · Robbins sources and lecture boundaries

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition (2026).

Neoplasia section: ionizing radiation, DNA injury and long-term carcinogenesis.

Radiation-injury section: cellular effects, fibrosis, clinical exposures and delayed tumors.

All illustrations are original editable schematics; no textbook clinical figures are reproduced.