RBE and LET

Question 1

How well does RBE model late effects?

Answer 1

It is only measured by acute effects, the impact on late effects is poorly understood

Question 2

How does cell radiosensitivity affect RBE?

Answer 2

RBE increases with radioresistant cells (more room to overwhelm repair capacity)

Question 3

How does fraction size affect RBE?

Answer 3

RBE is greatest at small fraction size where there is more repair capacity

Question 4

How does LET affect RBE?

Answer 4

RBE increases with escalating LET until a peak of 100 keV/um

Question 5

What is the peak LET of max RBE? Why?

Answer 5

RBE maxes out at an LET of 100 keV/um. This corresponds to 1 ion pair every 2nm, which is the diameter of DNA. This is maximally effective – as you increase ionization events beyond this, you can’t inactivate the DNA any more, so any increased deposition of dose does not make the cell more dead.

Question 6

What is the relationship of OER and LET?

Answer 6

As LET increases, OER decreases. For <1 keV/um, OER = 2.5 - 3.5; for >100 keV/um, OER = 1

Question 7

What is the LET for photons? protons? neutrons? carbon ions?

Answer 7

photons = 0.2-2 keV/um
protons = 5
neutrons = 100
heavy ions = 200-1000 keV/um