L 7: LET and RBE

Question 1

LET

Answer 1

• 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.

Question 2

LET Ranking

Boards Question

Answer 2

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

Question 3

How to calculate LET

Answer 3

1. Track average—divide track into equal lengths, calculate energy deposited in each and then average it.
2. 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

Question 4

RBE
Relative biological effectiveness

Answer 4

• 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.

Question 5

LD-50

Answer 5

• Mean lethal dose

Question 6

Board Question
Imp curve

Answer 6

• Avg separation of ionizination events is 20A which is the diameter of the DNA helix—results in 2 cleavages of the DNA.
  *

Question 7

RBE is influenced by what factors

Board Question

Answer 7

• 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

Question 8

Relationship of OER and LET

Answer 8

• Curve starts at 1 not 0.
• OER decreases with increasing LET until about 200 KeV/um.

Question 9

Survival factor
SF

Answer 9

• In 9/20 cell lines, neutrons gave lower cell survival than photons when comparing.
• Photon SF2 with neutron SF.68

Question 10

Imp picture

Answer 10

Question 11

Radiation weighting factor
WR

Answer 11

• 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.

Question 12

equivalent dose

Answer 12

It is the quantity produced by multiplying the dose by the weighting factor.
Units = Sv

Question 13

Summary

Answer 13

• 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

Question 14

Increase in LET and survival curve

Answer 14

• Survival curve becomes steeper
• Shoulder becomes progressively smaller.
• Between 10-100KeV, RBE increases rapidly as LET increases

Question 15

Most biologically effective LET

Answer 15

It is when there is a coincidence between the diameter of the DNA helix and the average separation of ionizing events.

Question 16

Low dose rate

Answer 16

facilitates the cells to progress through the cell cycle.

Question 17

ICRU 89 brachy rates

Answer 17

LDR: 0.4-1Gy/hr
MDR: 1Gy -12Gy/hr
HDR: >12Gy/hr

Question 18

Type of imaging contributing to high total effective dose

Answer 18

CT
Contributes to 50% of dose from medical radiation

Question 19

Nuclear medicine procedures that has high total collective effective dose

Answer 19

Cardiac procedures

Question 20

SABR dose per fx

Answer 20

> 8Gy

Question 21

Types of SRT

Answer 21

1. SRS
2. FSRT
3. SBRT

Question 22

Hyperfractionation

Answer 22

• Dividing total dose over more fractions.
• Treatment time is kept same as conventional
• Increased acute effects but reduced late effects

Question 23

Collective effective dose

Answer 23

Sum of all doses to all individuals in a population.
Unit = manSv, Person-Sv

Question 24

Sv is unit for

Answer 24

• Equivalent Dose
• Effective Dose

Question 25

Tissue weighting factor

Answer 25

* 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

Question 26

Collective effective equivalent dose

Answer 26

It represents the product of the effective doses times the number of persons exposed.

Question 27

Effective dose

Answer 27

It is the sum of weighted tissue equivalent for all organs and tissues within the body

Question 28

Equivalent Dose

Answer 28

It is the average dose absorbed within a single organ that is weighted for radiation quality.

Question 29

Total effective dose equivalent

Answer 29

* 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**.