L 7: LET and RBE Flashcards
LET
- Energy transferred per unit length of the track keV/µm
- LET = avg Energy/avg distance.
- Higher the energy, lower the LET
- It is the average energy deposited along the linear track.
LET Ranking
Boards Question
Correct ranking of LET: 20MeV photons<50KeV x-rays<20MeV alpha<250KeV alpha.
* LET for Co-60 = 0.2
* 250Kv x-rays = 2.0
* 10 MeV protons = 4.7
* 150MeV protons = 0.5
Neuttrons/alpha particles = very very high = 166
- Quality of RT first and then energy
How to calculate LET
- Track average—divide track into equal lengths, calculate energy deposited in each and then average it.
- Energy average—divide track into equal energy increments and average the lengths of track over which these energy increments are averaged
- For gamma-rays/monoenergetic particles: 2 methods give same result
For neutrons, energy average is most accurate. - For a type of particle: increase energy, decrease LET, decrease biological
effectiveness
RBE
Relative biological effectiveness
- Absorbed dose measures in Gy.
- Dose—measure energy absorbed/unit mass of tissue.
- Equal doses do not produce equal biological effects for different types of radiation.
- High LET radiations tend to cause exponential survival
curves. - 1Gy neutrons»»>1Gy gamma rays for biological effects
- RBE varies with different tissues and endpoints
- RBE increase as dose decrease.
LD-50
- Mean lethal dose
Board Question
Imp curve
- Avg separation of ionizination events is 20A which is the diameter of the DNA helix—results in 2 cleavages of the DNA.
*
RBE is influenced by what factors
Board Question
- Radiation quality (LET, type of radiation and energy)
- Radiation dose
- Number of dose fractions
- Dose rate
- Biological system/endpoint—higher for tissues with SLDR than those that don’t have it
Relationship of OER and LET
- Curve starts at 1 not 0.
- OER decreases with increasing LET until about 200 KeV/um.
Survival factor
SF
- In 9/20 cell lines, neutrons gave lower cell survival than photons when comparing.
- Photon SF2 with neutron SF.68
Imp picture
Radiation weighting factor
WR
- Radiation Weighting Factor (WR) allows for rapid comparisons of particular doses/qualities of radiation; sometimes Q (Quality Factor) is used to mean Weighting Factor.
- measured in Sv
- see the picture for problems solving (will be asked in boards).
- WR are chosen by the ICRP.
equivalent dose
It is the quantity produced by multiplying the dose by the weighting factor.
Units = Sv
Summary
- LET is the amount of energy deposited per unit length of track.
- RBE is the relative ability of a radiation of a particular quality to cause damage; all RBE values are related to x-rays.
- RBE is influenced by LET, dose, dose-rate, biological system that is used as an endpoint, number of fractions
Increase in LET and survival curve
- Survival curve becomes steeper
- Shoulder becomes progressively smaller.
- Between 10-100KeV, RBE increases rapidly as LET increases
Most biologically effective LET
It is when there is a coincidence between the diameter of the DNA helix and the average separation of ionizing events.
Low dose rate
facilitates the cells to progress through the cell cycle.
ICRU 89 brachy rates
LDR: 0.4-1Gy/hr
MDR: 1Gy -12Gy/hr
HDR: >12Gy/hr
Type of imaging contributing to high total effective dose
CT
Contributes to 50% of dose from medical radiation
Nuclear medicine procedures that has high total collective effective dose
Cardiac procedures
SABR dose per fx
> 8Gy
Types of SRT
- SRS
- FSRT
- SBRT
Hyperfractionation
- Dividing total dose over more fractions.
- Treatment time is kept same as conventional
- Increased acute effects but reduced late effects
Collective effective dose
Sum of all doses to all individuals in a population.
Unit = manSv, Person-Sv
Sv is unit for
- Equivalent Dose
- Effective Dose
Tissue weighting factor
- It is a relatie measure of the risk of stochastic effects that might result from irradiation of that specific tissue
- Lowest weighting factor = Brain
0.12 = Stomach, colon, lung, bone marrow, breast
0.08 = gonads
0.04 = Urinary bladder, esophagus, liver, thyroid
0.01 = Bone surface, brain, salivary glands
Collective effective equivalent dose
It represents the product of the effective doses times the number of persons exposed.
Effective dose
It is the sum of weighted tissue equivalent for all organs and tissues within the body
Equivalent Dose
It is the average dose absorbed within a single organ that is weighted for radiation quality.
Total effective dose equivalent
- It is the sum of the effective dose equivalent and the committed effective dose equivalent.
- It is the best measure to determine the excess increased risk of secondary malignancy from diagnostic radiation.
