MOD 3 - Radiation Biology

Radiation Interactions

  • low energy radiations do not have sufficient energy to penetrate bio tissues and ionize atoms

    • UV light

    • microwaves

  • however, ionizing radiations do penetrate tissues and interact with the orbiting electrons of atoms, hence they are high energy radiations

    • x-rays

    • gamma rays

* higher energy → more biologic damage *

  • cell damage is stochastic

Radiobiology

= study of the effects of ionizing radiation on the human body and how the body compensates for the absorbed radiation

  • known negative effects

  • effects are dependent on dose and dose rate (how much dose you are getting in a certain time)

  • effect expressions:

    • deterministic (high dose and short rate)

    • stochastic (random effect, from lower doses over a long period of time/long rate)

  • source dependent

Low LET Interactions

low LET ionizing radiation = characterized as streams of photons without mass or charge

  • x-rays

  • gamma rays

Low LET photons interact random with tissue → causing sparse ionization which are primarily indirect actions that produce free radicals (but very sometimes direct action)

low LET indirect action

= do not effect critical cell structures directly but the free radicals produced can impact DNA and cause cell death

  1. low LET photons interact with (usually) water molecules which leads to →

  2. Radiolysis - caused from ionizing radiation dissociating water molecules

  3. this dissociation results in (no cell damage, yes cell damage)

    • the water molecule reforming = no cell damage

    • formation of a negative water ion or positive water molecule

    • water molecule dissolute into hydroxyl radical (OH*) or hydroxyl ion (OH-)

    • formation of cellular toxic hydrogen peroxide (H2O2) or hydroperoxyl radical (HO2*)

  • hydroxyl radical causes 2/3 of all radiation induced damage

low LET direct action

= ionizing radiation interacts with essential cellular structures

  • DNA

  • RNA

  • proteins

  • enzymes

  • mostly cellular damage is done by high LET radiation and the chance of DNA interaction occurring is VERY low

  • low LET radiation is typically capable of only single strand DNA breaks (point lesions) which are very repairable while double strand breaks are not

High LET Interactions

= includes radiation that has both mass and charge

  • very destructive to biologic matter and has a high probability of interacting with DNA.

  • High LET Interactions → multistrand breaks and double-strand breaks → chromosomal breakage (chromosome aberration) → cellular disfunction or death

    • double strand DNA breakage

      • most common with high let radiation

      • may be repaired but if not → separation of DNA → cell death

    • single strand DNA breakage

      • can occur in both low let and high let

      • repair enzymes repair this level of DNA damage

high let radiations:

  • alpha particles

  • heavy ions

Impact of Radiation Interaction with DNA

  • DNA damage is random (stochastic)

  • types of DNA damage caused by radiation

    • single strand break = typically repaired

    • base damage = cause mutation

    • double strand break = cell death

    • cross linking = cause lesions (area of abnormal or damaged tissue)

    • rung breakage = cause molecular lesions

Target Theory

= indicates that the cell will only die after exposure to ionizing radiation if the DNA is inactivated in the process (highly caused with high LET radiation exposure)

targets: vital biologic macromolecules that cause biologic effects when damaged by ionizing radiation

  • DNA

  • RNA

  • proteins

  • enzymes

targets being damaged are stochastic and generally require more than a single hit to cause cell structure damage

Five trends of Radiation Interaction with Biological Materials

  1. radiation and cell interaction is random, damage may not occur even if interaction did occur

  1. initial deposit of energy into cell is very rapid (10-17s)

  2. the area radiation interacts with is random

  3. radiation damage on cells, tissue, and organs are indistinguishable from other types of trauma.

  4. latent period for biological changes to manifest, length of time is dependent on initial dose (min-years)