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70 terms

QUIZ 5 RADIATION PHYSICS

STUDY
PLAY
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
LET
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
1
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
3.0
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
extrapolated
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.