Dose Response, Fractionation, LET and RBE Flashcards
LET
energy transferred per unit length of track (keV/𝝁m). Can be track or E average
fol sparsely ionizing radiation
track and E averages are similar
Correlates better with biological responses
E average
The higher the E
the lower the LET for a given particle
Relative Biological Effectiveness - RBE
of some test radiation (r) is the ratio Dx/Dr, where Dx and Dr
are the doses of 250 kV x-rays and the test radiation, respectively,
required to produce equal biological effects
The RBE generally increases
as the dose decreases
RBE is greater for many low dose fractions than for a single dose
true
RBE lower
for curves with little or no shoulder (X vs neutron)
Peak RBE reached at E
100keV/𝝁m. Same for many cells
The probability of causing DSBs of x-rays
is low, low RBE
LET > 100keV
waste of E, overkill- inefficient as deposits more E than needed for DSB
Factors that determine RBE
LET, D, Number of fractions, D rate, Biosystem
OER decreases
as LET increases
radiation weighting factor WR
the dimensionless multiplier used to place biological effects from exposure to different types of radiation on a common scale.
Equivalent Dose
Absorbed Dose x WR [Sv]
Tissue Weighting Factor WT
the relative contribution of each tissue or organ to the total detriment resulting from uniform irradiation of the whole body
Effective Dose
𝚺 Absorbed Dose x WR x WT
Prolonging overall time within the normal radiotherapy range
has a little sparing effect on late reactions, but a large sparing effect on early reactions
The dose-response relationship for late-responding tissues is
more curved than for early-responding tissues
𝛂/𝛃 ratio for early effects
larger than for late effects as 𝛂 dominates. at low doses.
Late-responding tissues are
more sensitive to changes in fractionation
patterns than early-responding tissues
biological effect formula
E=𝜶d+𝜷d^2=𝜶(nd)(1+d/𝜶/𝜷)
for early responding tissues 𝜶/𝜷
10Gy
for late responding tissues 𝜶/𝜷
3Gy
example
30 fractions of 2 Gy for 6 weeks (5 days a week): E/𝜶=nd(1+d/𝜶/𝜷)=60(1+2/10)=72Gy for early responding and 60(1+2/3)=100 for late
Committed Effective Dose
Committed equivalent dose to individual organs or tissue resulting from the
intake of a radionuclide, multiplied by the appropriate WT and then summed.
Committed Equivalent Dose
In the case of irradiation from internally deposited radionuclides: the integral
over 50 years of the Equivalent Dose in a given tissue (Sv). (equal to annual equivalent dose for short-lived and greater for long-lived isotopes)
Somatic Effects
related to body health of an irradiated person
Acute effect
Radiation sickness (nausea, vomiting, secondary infections due to depletion of
white blood cells)
• Death that occurs
within minutes or up to several months
LD50(30) for human
3.5-4.5 Sv. Can be raised to 7 with antibiotics
Tolerance dose
an estimated dose that might cause injury over a certain period of time
TD5/5
5% chance of injury occurring over the next 5 years
Prodromal syndrome
nausea, vomiting, etc
Latent period
few hours to a few weeks. People report feeling better
Manifest illness
The patient feels worse after the latent period.
Recovery or death
may last for years. If a death hasn’t occurred in a few months then it is likely that the subject will recover
At very high doses (> 20 Gy)
the collapse of the central
nervous system and the cardiovascular system that leads to shock
and prompt death
~100 Gy of gamma-rays results
in death in 24 to 48 hours
at dose 10 Gy
Gastrointestinal Syndrome, leads to death in 3-10 days. Cells responsible for the absorption of water and electrolytes from the gastrointestinal tract are being killed.nausea, vomiting, loss of appetite, diarrhea, inanition
Hematopoietic Syndrome (Bone Marrow Syndrome) – 2.5-5 Gy
Signs: bleeding, infections, and anemia
• Death 30-60 days
Radionecrosis, deep ulceration
> 50 Gy
Erythema; distinguishable from thermal burn; minutes to weeks postexposure,
depending on dose
6 Gy:
Cataract
deterministic effect with a threshold may be linear - no threshold. An annual limit of 15 rem (0.15 Sv) to the eye
Deterministic Effects
Always has threshold dose
below which effects not
observed, above which severity is proportional to the dose.
Stochastic Effects
The severity of stochastic effects
is independent of dose
