Chapter 2 – Principles of Radiation Biology and Radiation Protection

Classification of Radiation Damage

Non-Stochastic (Deterministic):

The potential for adverse effects is related to total dose i.e. if a threshold is surpassed there is a significant risk of negative effects of the radiation. This does not mean that there is not damage at the cellular level it only means that no clinically evident damage is evident. This theory is most evident related to skin burns secondary to fluoroscopy but can also be noted related to temporary sterility in males and females and the delayed formation of cataracts in the eye.

Threshold levels for tissue damage have been elucidated. Observable thresholds for deterministic effects are widely variable depending upon the type of effect, the degree of severity used to characterize what is observable and the population group exposed. Some examples of deterministic effects and the typical (photon) thresholds at which they occur include:

– Irradiation of the gonads: Condition Radiation Dose
Temporary sterility (male) 0.15 Gy
Permanent sterility (male) 3.5 to 6 Gy
Permanent sterility (female) 2.5 to 6 Gy
– Irradiation of the eyes:
Cataracts 2 to 10 Gy
– Irradiation of red bone marrow:
Loss of bone marrow function 0.5 Gy
– Whole body irradiation
Death (LD50/60*) 3 to 5 Gy

{* The survival-dose relationship is often described using the LD50/60 (i.e., the dose at which half the exposed individuals would be expected to die within 60 days). The principal cause of death being loss of bone marrow function. (3)

Stochastic (Probabilistic):

There is no threshold that must be reached i.e. injury can occur with even the smallest exposure. This theory is applicable to genetic mutations and cancers suspected to be related to radiation exposure (leukemia, lymphoma). In reality the adverse effects seen associated with this theory are also dose related.

There have been no true, longitudinal, randomized, trials that have attempted to determine the potential adverse effects of radiation in a large population. Most data on large populations is related to variable exposure to radiation for a group involved in a military deployment of radiation or in an industrial accident. Suffice it to say, the complexity and longevity of such a research venture makes it very unlikely that it will ever be performed.

The benefits to patient health outcomes and management of diseases detected using ionizing radiation are substantial, widespread, and have considerably altering the well-being and treatment of patients over the 100 years radiation has been in use clinically. However, one should always consider the risk vs. benefit of imaging with ionizing radiation and balance this against the clinical situation of the patient in question.