Principles of Radiobiology Flashcards
what is radiation biology?
Branch of biology concerned with the effects of ionizing radiation on living systems
what are the 3 things encompassed by radiation biology?
- Sequence of events occurring after the absorption of energy from ionizing radiation
- Action of the living system to make up for the consequences of this energy assimilation
- Injury to the living system that may be produced
Tissue and radiation interactions?
- a probability
- damage may or may not result
- 90% of interactions are harmless
visible damage to tissue due to radiation?
- Indistinguishable from other causes (ie. Smoking, chemicals, etc.)
- Damage that we see may occur after a latent period of time (some time later)
- Latent period duration decreases as dose increases
what is ionizing radiation?
- Damages living systems by ionizing the atoms comprising the molecular structure of these systems
- Biologic damage begins with the ionization produced by various types of radiation such as: X-rays, Gamma rays and Alpha particles
- Ionized atoms will not bond properly in molecules.
charge, mass and energy of radiation
- vary among the different types of radiation
- these attributed determine the extent to which different radiation modalities transfer energy into biological tissue
what are the three ways ionizing radiation causes injury
- Linear energy transfer (LET)
- Relative biologic effectiveness (RBE)
- Oxygen enhancement ratio (OER)
what is a determinant?
a factor that affects the outcome
what is Linear energy transfer?
- the average energy deposited as ionizing radiation passes through a medium
- described in units of keV/micron
- is a very important factor in assessing potential tissue and organ damage from exposure to ionizing radiation
what happens to the probability of producing a significant biological response when LET increases?
probability also increases
what are the radiation categories according to LET?
- low-linear energy transfer radiation
- high-linear energy transfer radiation
which ionizing particles have the highest LET? the lowest?
alpha particles have the highest LET, followed by beta, and lastly X-ray has the lowest LET
EMR
- produce electrons
- low LET
particulate radiations
- can be high mass, and/or high charge
- higher LET
Low LET
- quite penetrating
- sparsely ionizing
- random interactions along the length of its track - spread out - few ionizations/distance travelled
- most damage to the cell through an indirect action
High LET
- less penetrating
- dense ionizations
- transfers a large amount of energy into a small area - many ionizations/distance travelled
- harder for the body to heal as there is a lot of damage in a small area
High LET radiation damage
- particles possess substantial mass and charge
- can produce dense ionizations along its path and is therefor more likely to interact significantly with biological tissue
What is Relative biologic effectiveness?
a measure of the damage that will occur in comparison with x-rays for the same radiation dose
– high LET radiation will have high RBE
what is radiation weighing factor?
- More practical for radiation protection dose levels in humans
- Used to calculate the equivalent dose (EqD) - Determines the ability of a dose of any kind of ionizing radiation to cause biological damage
what is the oxygen enhancement ratio?
the ratio of the radiation dose needed to cause the same biological damage when there is oxygen absent to when there is oxygen present
cell radiosensitivity
- Equal doses of ionizing radiation produce different degrees of damage in different kinds of human cells because of differences in cell radiosensitivity.
- The more mature and specialized in performing functions a cell is, the less sensitive it is to radiation.
what is the law of bergonie and tribondeau?
radiosensitivity of cells is directly proportional to their reproductive activity and inversely proportional to their degree of differentiation
the most radiosensitive cells are
- immature (early stages of its life cycle)
- unspecialized (undifferentiated) (not yet formed into a specific/certain tissue type)
- high reproductive activity (rate at which the cell is multiplying and dividing, more rapid more sensitive)
what are radioinsensitive cells?
least sensitive to radiation
- brain cells
- muscle cells
- nerve cells
radio sensitivity with age?
varies with age
- very young and very old are more sensitive to radiation
what does the suffix blast mean?
immature, still in development, not fully specialized, rapidly dividing/multiplying
what does the suffix cyte mean?
mature, fully developed, specialized, not rapidly dividing/multiplying
stem cell versus differentiated cells?
stem cells/precursor cells/undifferentiated have much higher sensitivity to radiation that differentiated/functional cells
LET and cell radiosenistivity?
- The amount of radiation energy transferred to biologic tissue plays a major role in determining the amount of biologic response
- As LET increases, the ability of the radiation to cause biologic effects also generally increases until it reaches a maximal value.
- LET can influence cell radiosensitivity.
OER and Cell radiosensitivty?
Oxygen enhancement effects
- Oxygen enhances the effects of ionizing radiation on biologic tissue by increasing tissue radiosensitivity.
- During diagnostic imaging procedures, fully oxygenated human tissues are exposed to x-radiation or gamma radiation.
- In radiotherapy, when radiation is used to treat certain types of cancerous tumors, high-pressure (hyperbaric) oxygen has sometimes been used in conjunction with it to increase tumor radiosensitivity - may increase the amount of oxygen in cancer cells, which may make them easier to kill with radiation therapy and chemotherapy
Target theory?
- master, or key, molecule that maintains normal cell function is believed to be present in every cell
- master, or key, molecule is necessary for the survival of the cell
- target theory may be used to explain cell death and nonfatal cell abnormalities caused by exposure to radiation
What are the 2 different classifications of ionizing radiation interaction on a cell?
direct action - (DNA)
indirect action - (H2O)
- essentially the majority of effects of irradiation in living cells result from indirect action, because the human body is composed of 80% water and less than 1% DNA
Radiolysis of water
- dissociation of water due to ionization (splitting apart and reforming)
- ionization of water molecules
- increases effective target size
- think of skeet shooting with a .22 versus shotgun
what are the ways water molecules can be ionized?
- free radicals
- undesirable chemical reactions and biological damage
- cell-damaging substances
- organic free radical formation
what does lysis mean?
breakdown
what is an ion pair?
HOH+ and e-
HOH+ on its own is unstable
HOH+ from unstable to stable
HOH+ + e- = H2O - Positively charged water molecule may recombine with the electron
Stable molecule (water)
No damage will occur
- often happens
what happens is HOH+ does not recombine with an electron?
break apart into smaller molecules
decomposes into:
- H+ hydrogen ion
- OH* (hydroxyl radical)
what is the hydroxyl radical?
- free radical
- molecule with an uneven number of electrons (unpaired valance electrons)
- very reactive
what happens if the free e- combines with another water molecule?
Negative water ion
H2O + e- = HOH-
what does HOH- decompose into?
OH- (hydroxyl ion)
H* (hydrogen radical)
what is the hydrogen radical?
- free radical
- molecule with an uneven number of electrons (unpaired valance electrons)
- very reactive
interaction of radiation with water?
formation of:
- Ion pair (H+ and OH-)
- Hydrogen ion and a hydroxyl ion
- Two free radicals (H* and OH*)
- Hydrogen radical and a hydroxyl radical
what happens if the H* and OH* bond?
H* and OH*
OH* could bond with another OH*
OH* + OH* = H2O2 (hydrogen peroxide)
Poisonous to the cell
HO2* hydroperoxyl radical
H* + O2 = HO2*
The two are believed to cause the most damage
what are free radicals?
Uncharged molecule
Reactive
Diffusible
