Biological Responses to Radiation

Radiation Effects

Acute and Chronic Effects

Acute effects occur shortly after radiation exposure. Examples include inflammation and erythema.

Chronic effects occur after a delay period. Examples include alopecia and fibrosis.

Data from ICRP 103 and ICRP 60

Exposed PopulationExcess Relative Risk of Cancer per Sv
Entire population5.5% - 6.0%
Adults only4.1% - 4.8%

 Stochastic (Probabilistic) Effects

Stochastic effects are those in which the probability of effect increases with dose and the effect is binary. Because of their probabilistic nature, there is no dose threshold for stochastic effects.

Examples of stochastic effects

  • Cancer induction
  • Cell death

Non-Stochastic Effects

Non-stochastic effects are those in which the magnitude of effect increases with increased dose. Non-stochastic effects have a minimum dose, a threshold dose, below which the effect is not observed.

Examples of non-stochastic effects

  • Erythema (skin reddening)
  • Cataract induction

Total Body Exposure Responses

Total body exposure responses are non-stochastic effects. The magnitude effect is dependent upon absorbed dose.

Exposure Response Phases

1. Prodromal Phase: Minutes to days after exposure

Prodromal phase is marked by the emergence of initial symptoms. This often includes nausea, vomiting, diarrhea and general low-level symptoms.

2. Latent Phase: Days to a week after exposure

During the latent phase, symptoms which began in the prodromal phase are temporarily reduced or eliminated.

3. Manifest illness: Days to weeks after exposure

 In this phase, the primary symptoms, outlined in the table at right, occur.

4. Death or recovery

SyndromeDose RangeSigns/SymptomsSurvival
Hematopoietic Syndrome1-6Gy
  • Drop in blood cell count.
  • Infections due to low blood cells
  • Poor wound healing
  • Moderate fever
  • Possible headache
  • Short-term cognitive impairment
  • Alopecia (after 3 Gy)
  • 50-95%
  • Recovery in 6-8 weeks
Gastrointestinal Syndrome6-30Gy
  • Symptoms associated with Hematopoietic Syndrome
  • Nausea
  • Vomiting
  • Loss of appetite
  • Abdominal pain
  • Death is common without exotic treatments such as bone marrow transplant
  • 0-50%
  • >99% fatality above 8Gy
  • Cause of death is generally infection
  • Survival time of 2 days to 2 weeks
Neurovascular Syndrome>30Gy
  • Symptoms associated with Gastrointestinal Syndrome
  • Dizziness
  • Headache
  • Seizures
  • Tremor
  • Ataxia
  • Loss of consciousness
  • 0% survival
  • Survival time 1-2 days

Organ Responses

Parallel and Serial Organs

Organs vary in their sensitivity not only to the amount of radiation dose they receive, but also to the distribution of that dose. Organs may be divided into sub-units. Each sub-unit may be functional or non-functional following radiation damage. Sub-units may be as small as cells or as large as lobes of the liver.

Serial Organs are organs in which disabling any sub-unit causes the entire organ to fail.

Parallel Organs are organs in which many or all of the sub-units must be disabled to cause organ failure.

Serial OrgansParallel Organs
Spinal CordLungs
Brain StemLiver
Optic StructuresKidneys

Organ Specific Exposure Responses

OrganToxicityDose Threshold
Skin (Hair)Epilation (hair loss)3Gy
SkinErythema (skin reddening)6Gy
SkinDry Desquamation (sores)12Gy
SkinWet Desquamation (oozing sores)25Gy
SkinRadionecrosis (loss of skin)50Gy
Whole bodyDeath3-5Gy

Radiation Response Curves

Therapeutic Ratio

Therapeutic ratio is an indication of how "successful" radiotherapy is likely to be when incorporating both probability of cure and probability of adverse complication. Much of conventional fractionated radiotherapy centers on attempting to maximize therapeutic ratio.

There are two mathematical formulations of therapeutic ratio; the equal dose equation and the equal effect equation. In both cases, a value greater than 1 is desired.

The equal dose equation is defined as the ratio of damage to the tumor cells to damage to normal cells for the same dose. In research, this is often taken as the dose causing a 5% complication rate over 5 years (TD5/5).


The equal effect equation is defined as the ratio of the normal tissue dose to tumor dose required to reach the same endpoint (fraction of cell death).


Key Point: Radiotherapy fractionation attempts to maximize the therapeutic ratio, thereby offering the best trade-off between probability of cure and probability of adverse reaction.


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