Hall 7 - LET & RBE

Question 1

What is LET?

Answer 1

Linear energy transfer

The average energy deposited locally per unit length of an ionizing particle’s track

*keV/μm

Question 2

What is the unit for LET?

Answer 2

keV/μm

Question 3

What method of calculating LET involves dividing up the track into equal lengths and averaging?

Answer 3

Track method

Question 4

What method of calculating LET involves dividing up the track into equal energies and averaging?

Answer 4

Energy method

Question 5

What method of calculating LET works better for neutrons?

Answer 5

Energy method

Question 6

LET is ______ to the charge density of the medium.

Answer 6

Proportional

Question 7

LET is ______ to the charge of the particle.

Answer 7

Proportional

Question 8

LET is ______ to the speed of the particle.

Answer 8

Inversely proportional

Question 9

LET is ______ to the energy of the particle.

Answer 9

Inversely proportional

*Higher energy = lower LET and lower RBE

Question 10

What is RBE?

Answer 10

= dose of reference RT / dose of RT of interest

To produce equal biological effect

Question 11

What is the reference RT when referring to RBE?

Answer 11

Low LET photons
- 250 keV x-rays
- Co-60 gamma rays

Question 12

How many ionizations per unit mass in 1 Gy?

Answer 12

1 Gy = 1 J/kg = 2x10^17 ionizations/kg = 10^6 ionizations per mammalian cell

Question 13

When is the RBE for high-LET higher?

Answer 13

1. Lower doses
2. More fractionated treatment

*Since no prominent shoulder (less SLD repair)

Question 14

The dose rate effect is more pronounced with _____ LET/RBE.

Answer 14

Low
(due to repeating shoulder with each dose/fraction)

Question 15

How does hypoxia affect RBE for high LET?

Answer 15

RBE for high LET is higher under hypoxia vs aerated conditions

*Anything that affects the radiosensitivity of cells to reference RT (x-rays) differently than to RT of interest (high LET) will affect RBE of RT of interest

Question 16

As LET increases, what happens to the survival curve?

Answer 16

1. Becomes steeper
2. Shoulder becomes progressively smaller

Question 17

What is the optimal LET to produce peak RBE?

Answer 17

100 keV/μm

*Average separation between ionization events is ~2nm (diameter of DNA helix), so DSBs can be most efficiently produced by a single particle track

*Also where OER drops off (more direct action)

Question 18

Why does RBE decrease for LET >100 keV/μm?

Answer 18

Overkill effect
Ionization events are too close together (energy is wasted)

Question 19

LET _____ with increasing energy.

Answer 19

Decreases!

250 kV x-rays = 2 keV/μm
Co-60 gamma rays = 0.2 keV/μm
150 MeV protons = 0.5 keV/μm
2.5 MeV alpha particles = 166 keV/μm

Question 20

The formation of a 3 ion cluster requires how much energy?

Answer 20

110 eV