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What is the fundamental principle of radiobiology

relates a dose of radiation to what happens in body

what is the law of Bergonie and Tribondeau

radiosensitivity of tissue varies with maturation and metabolism

when are cells the most radiosensitivie

in utero

which are the most radiosensitive cells

stem cells, young tissue, increased metabolic rate, increased proliferation and growth rate

which are the most radioresistant

mature cells, older tissue, decreased metabolic rate

what are the physical factors affecting radiosensitivity

LET, RBE, protraction and fractionation

what is LET

linear energy transfer

describe LET

rate at which energy is transferred to soft tissues per unit length (track length)

what is track length on LET

how far photon goes into tissue

what is measurement of LET

kiloelectronvolt / micrometer

what is diagnostic xray LET

3 kiloelectron volt / micrometer

what is radiation therapy LET usually

250 kiloelectron volt / micrometer

what is LET useful in describing

quality of radiation

how do you describe quality of radiation


what is the quality of radiation

size + charge

what radiation is low LET

EMR which is X-ray and gamma rays

what type of radiation quality is low LET

high energy but small particles

how does low LET cause tissue damage

indirect action, tissue brakes photon with absorption and attenuation


photon hits water then free radicals hit DNA

what type of DNA damage does low LET cause

causes single strand break of one base pair

what type of radiation is high LET

alpha particles, radiation that has substantial mass and charge

what type of track does high LET have

doesn't travel far due to mass, like a bowling ball

what type of track does high LET have

has dense ionization, energy is exhausted in a short length of track

what type of DNA damage does high LET cause

double strand break

what is RBE

relative biologic effectiveness

describe RBE

the capability of radiation with varying LET to produce a biologic response

how does LET affect RBE

as LET increases, RBE increases

what is RBE of diagnostic x-rays


how do you calculate RBE

dose of standard radiation to produce given effect Gamma divided by the dose or standard radiation to produce given effect Alpha

how do you calculate RBE

dose of standard radiation to produce effect of gamma divided by alpha

How is radiation therapy delivered

protraction and fractionation

why use protraction and fractionation in radiation therapy

to allow for intracellular repair, tissue recover and higher total doses

how is protracted treatment delivered

lower dose delivered continuously over long perior or time

how is fractionated treatment delivered

dose delivered in equal portions at regular intervals

how do biologic factors affect radiosensitivity

alters tissue response to radiation

what are the biologic factors affecting radiosensitivity

oxygen effect, age, recovery, chemical agent, hormesis

what is oxygen effect

tissues high in oxygen are more sensitive to radiation

what is oxygenation enhancement ratio OER

ratio of dose required to produce a particular biologic response in a hypoxic environment divided

what is oxygenation enhancement ratio OER

dose required to cause some response in oxygenated environment

what is OER of diagnostic x-rays


what is the biologic factor of age

humans are more radiosensitive in utero, decreasing with age until elderly

what is the recovery biologic factor

cells can recover from sublethal radiation damage

what is the recovery biologic factor

but if dose kills cell before next division, interphase death occurs

what is the recovery biologic factor

cells cannot recover from interphase death

what is recovery

intracellular repair + repopulation = recovery

how do chemical factors affect radiosensitivity

some chemicals can modify the radiation response

how do chemical factors affect radiosensitivity

radiosensitizers increase effects of raidation, called halogenated pyramidines

how do chemical factors affect radiosensitivity

radioprotectors decrease effects of radiation, call cysteine, but these are fatal

what is hormesis

possible beneficial effect of a small amount of radiation

how does hormesis work

stimulates hormonal and immune responses and extends life span

what is radiation dose-response relationship

relationship between radiation dose levels and the response observed

how are radiation dose-response relationships demonstrated

graphically through a line or curve

what types of graphs are there

linear, non linear, threshold, non threshold

what is linear radiation dose response relationship

response is directly proportional to dose in straight line

what is non threshold radiation dose relationship

regardless of size of dose it is expected to produce a response

what are dose limits based on

linear non threshold relationships

what type radiation dose relationship is radiation induced genetic damage and late effects

linear non threshold

how are low dose results obtained from high dose results


varied doses produce varied responses and are called

non linear

what is threshold radiation dose relationship

dose is larger than 0, the level below which there is no response

what can have a sigmoid curve and a sigmoid curve can only occur in

non linear threshold response

when would you see sigmoid curve

radiation therapy to show high dose cellular response

what is Gray

amount of radiation required to deposit one joule of energy in 1 kg of any kind of matter

what is dose to patient measured in

Rad or Gray, 1 Gray = 100 rads

what is Gyf

absorbed dose to patient

what is radiation dose to tech measured in

Rem or Sievert. 1 sievert = 100 rem

describe Sievert

the dose to tech of a given type of radiation in Gray that has same effect on human as 1 Gray of X-ray or gamma radiation

How is radiation in air meaused

Roentgen, or coulombs per kg or air kerma, or gray in air, 1 gray in air = 100 roentgens

what is the relationship between OER and LET

The relationship between the oxygen enhancement ration (O.E.R.) and the linear energy transfer (L.E.T.) is that the OER is LET-dependent. The O.E.R. is highest for low-L.E.T. radiation and decreases in value as the L.E.T. increases

Radiation Weighting Factor

Radiation Weighting Factor
a) The radiation weighting (Wt) factor is the factor by which the absorbed dose (rad or gray) must be multiplied to obtain a quantity that expresses, on a common scale for all ionizing radiation, the biological damage (rem or Sievert) to the exposed tissue. It is used because some types of radiation, such as alpha particles, are more biologically damaging to live tissue than other types of radiation when the absorbed dose from both is equal. This replaces the term quality factor in the latest system of recommendations for radiation protection. For example, the gonads have a Wt of 0.20; whereas the skin has a Wt of 0.01 as the gonads are more sensitive to radiation than the skin.

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