LET & RBE

Question 1

what is a dose

Answer 1

amount of energy a patient receives

Question 2

what is the prescribed dose

Answer 2

amount of radiation in terms of the absorbed dose

Question 3

what is a gray

Answer 3

physical quantity of radiation

Question 4

does short distances have a low or high LET

Answer 4

HIGH LET

Question 5

what is the RBE

Answer 5

Relative biological effectiveness used for comparing different radiations by introducing a given biological effect

Question 6

how can RBE be calculated

Answer 6

the ratio of dose which produces the same biological effect

Question 7

what is the optimum LET

Answer 7

100 KeV/um

Question 8

what types of LET do neutrons and alpha particles have?

Answer 8

neutron = high
alpha = even higher LET (doesn’t penetrate far)

Question 9

what is LD50

Answer 9

Lethal Dose which kills 50% of all cells

Question 10

What was the ratio for killing plants with neutrons and x-rays

Answer 10

LD50 X-rays = 6Gy
LD50 neutrons = 4Gy

1.5 : 1 therefore neutrons are 50% more effective

Question 11

what is classed as wasted energy

Answer 11

any high LET energy which is undeposited at the DNA

Question 12

what is LET

Answer 12

energy deposited per unit track

Question 13

what is more effective at cell kill 1 Gy of high or low LET

Answer 13

high

Question 14

what manner does high LET deposit at

Answer 14

a small and controlled manner

Question 15

what doses are found with low LET x-rays

Answer 15

high doses
as LET increases, the curve becomes more steep

Question 16

what sort of damage is low LET

Answer 16

indirect

Question 17

what damage is high LET

Answer 17

direct

Question 18

link high LET with RBE

Answer 18

it increases the RBE to a max, which then drops due to over kills

Question 19

what factors affect RBE

Answer 19

• biological system or endpoint
• type of radiation and energy
• magnetic or particulate
• charged or uncharged
• dose level and number of fractions
• dose rate (low doses are more significant )

Question 20

what proportions are DSB found at

Answer 20

low proportions

Question 21

describe the RBE and neutrons relationship

Answer 21

• neutrons vary with dose per fraction
• fractionated regime for neutrons is greater than a single exposure, due to smaller doses
• RBE is larger for smaller doses
• neutrons are more efficient at doses per fraction
• this is reduced when the number of fractions increases
• RBE is larger at low dose rates than an acute exposure because of the n effectiveness decreasing with dose rate to a smaller extent than gamma and x-rays