LET and RBE:
How is RBE related to LET?
As LET increases, the survival curve slope increases and initial shoulder decreases • RBE increases with LET up to about ~100 keV/µm
What occurs to the LET of charged particles with increasing energy?
As the E increases the LET decreases
What occurs to the OER with increasing RBE?
As the OER declines, RBE increases until an LET of ~100 keV/µm is reached
What occurs to the RBE after 100keV/um?
Beyond this value for the LET, the RBE again falls to lower values.
What occurs to the OER with increasing LET?
Decreases with increasing LET above ~30 keV/µm, and reaches unity by an LET of ~160 keV µm
What happens to the RBE for neutrons when the dose is fractionated?
Fractionating the dose increases the RBE for neutrons, not because it increases the damage done by neutrons, but because it decreases the effect of x-rays
What is the optimal LET and why?
• At ~100 keV/µm (~5 MeV α) greatest RBE; producing most biological effect per unit dose separation between ionizing events ~ the diameter of DNA double helix highest probability of double strand break per unit dose
What does it mean with cells have a photo survival curve with a small shoulder?
curve have small neutron RBE values
What is the charge of one ion?
1.6 x 10-19 Coulomb
How is RBE calculated?
2 standards: 250 kVp x rays; 60Co γ rays -for example: -LD50 for 250 kVp x-rays = 6 Gy (the standard) -LD50 for 2 MeV neutrons = 3 Gy (the test radiation) -thus, the RBE for 2 MeV neutrons is 2
What are the units for LET?
LET is the average energy locally imparted (deposited) per unit track length (keV/µm)
What is the pattern for LET of photons?
LET of photons tends to increase with Energy except for very high energies
What are examples of Low and High LET radiations?
LOW: x-rays, gamma rays and betas HIGH: alphas, betas, protons and neutrons
What is LET?
Linear energy transfer which is the energy transferred per unit length of the track
How does RBE vary?
RBE also varies depending on tissue type and biological endpoint
Explain radiation weighting factor:
RBE is too specific for use in radiation protection • Considering differences in biological effectiveness for different radiations, the RBE concept is simplified by using the radiation weighting factor
What does it mean with cells have a photo survival curve with a large shoulder?
indicating that they can incur and repair a large amount of sublethal radiation damage, show a large RBE for neutrons
What other factors does RBE Depend on?
radiation quality biological endpoint biological system choice of radiation "standard" radiation dose and dose rate number of dose fractions (& dose per fraction)
What is relative biological effectiveness?
•Relates biological effect to a "standard" -needed because equal energy deposition events (doses) from different radiations do not produce equal effects in biological systems
Explain the energy deposition pathways for alphas and betas?
A: Generally straight with concentrated E deposition B: Paths are random, E deposition more dispersed
What are strengths of ranges for different radiations?
Alpha range is short, then beta and then gamma range is large
How much E is needed for one ionisation?
Approximately 33.7 eV of energy is needed for one ionisation.
What is RBE defined as?
-RBE is defined as the ratio of the standard dose to the test dose required for equal biological effect
What occurs to RBE between 10-100keV/um?
Between 10 and 100 keV/ µm, the RBE increases rapidly with increasing LET and reaches the maximum at about 100 keV µm.
