Linear Energy Transfer Flashcards
Concerning RBE, OER, and LET, which of the following statements is TRUE?
A. Maximum cell killing per dose delivered occurs at an LET corresponding to approximately 1000 keV/µm
B. RBE changes the most over the LET range of 0.1 to 10 keV/µm
C. The relationship between OER and LET is bell-shaped
D. RBE decreases with increasing LET above about 100 keV/µm
E. OER increases with LET
D
RBE decreases with increasing LET above approximately 100 keV/µm. This is thought to be due to the “overkill” effect in which many more ionizations (and damage) are produced in a cell traversed by a very high LET particle than are minimally necessary to kill it, thereby “wasting” some of the energy.
Maximum cell killing occurs at an LET of approximately 100 keV/µm, not 1000 keV/µm (Answer Choice A).
RBE shows the greatest changes for LET values between roughly 20 and 100 keV/µm (Answer Choice B).
OER decreases slowly with increasing LET for low LET values, but falls rapidly after LET exceeds about 60 keV/µm and, therefore, does not follow a bell-shaped curve (Answer Choices C and E).
Which of the following statements is CORRECT? Compared with damage from low LET radiation, damage from high LET radiation:
A. Is reduced to a greater extent in the presence of sulfhydryl compounds
B. Shows more potentially lethal damage recovery
C. Exhibits a greater OER
D. Is less subject to split-dose recovery
E. Shows greater sparing when the irradiation is given at a low dose rate
D
There is little or no split-dose recovery following high LET radiation exposure because the single dose survival curves for high LET radiations have little or no shoulder. There is also little or no potentially lethal damage recovery, oxygen effect or radioprotection afforded by the presence of sulphydryl compounds. Delivery of a radiation dose at a low dose rate leads to less sparing for a high LET radiation compared with a low LET radiation.
Which of the following statements concerning RBE is TRUE? The RBE:
A. Is lower for neutrons than for protons over the therapeutic energy range
B. Is greater for high LET particles in hypoxic cells as compared to oxygenated cells of the same type
C. Is diminished for carbon ions when delivered over several fractions as compared to a single dose
D. Is greatest for heavy charged particles at the beginning of the particle track
E. Increases for MeV alpha-particles with increasing dose
B
Relative Biological Effectiveness (RBE) is defined as:
Dose of Reference Radiation (250 keV X-Rays) /
Dose of Test Radiation to give the same biological effect
The reference radiation for calculation of RBE is low LET radiation, such as 250 keV X-rays or Co-60.
The dose of the reference radiation that will achieve the same level of cell killing as high LET particles in hypoxic cells will be greater because there is little to no oxygen effect for high LET radiation (Answer Choice B).
The RBE is greater for neutrons than it is for protons in the therapeutic energy range because the high energy protons used in radiotherapy are of a relatively low LET and therefore possess an RBE of approximately 1.1 (Answer Choice A).
The RBE for carbon ions, or any other type of high LET radiation, is greater for fractionated irradiation compared with an acute exposure due to the substantial sparing exhibited with reference X-rays with fractionation (Answer Choice C).
The RBE for charged particles is low at the beginning of the particle track and greatest near the end of the track, in the Bragg peak region (Answer Choice D).
RBE does show a fractionation dependence; it decreases with increasing fraction size. The RBE for 4 MeV alpha particles will decrease with increasing dose because there is more sublethal damage repair with low-LET X-rays at lower doses, and therefore more survival compared with high-LET radiation (Answer Choice E).
Which of the following pairs of radiation type and approximate LET value is CORRECT?
A. 150 MeV protons – 0.5 keV/μm
B. 1 GeV Fe ions – 20 keV/μm
C. 60-Co y-rays – 15 keV/μm
D. 2.5 MeV a-particles – 5 keV/μm
E. 250 kV X-rays – 10 keV/μm
A
150 MeV protons have an LET of approximately 0.5 keV/μm. 1 GeV Fe ions, 60Co y-rays, 2.5 MeV a-particles and 250 kV X-rays have LET values of approximately 143, 0.2, 166, and 2 keV/μm, respectively.
Which of the following statements concerning LET is INCORRECT?
A. LET is proportional to charge density of a medium
B. LET is proportional to charge (squared) of the particle moving through a medium
C. LET is inversely proportional to speed (squared) of the particle
D. LET is inversely proportional to mass of the particle moving through a medium
E. LET is related to density of ionizations along the particle’s track
D
LET is a measure of local energy deposition along a track of medium. It is inversely proportional to the energy of a given charged particle. The local transfer of energy to medium is more probable at lower energies.
Which statement concerning the linear energy transfer (LET) is CORRECT?
A. LET is equal to the energy transferred by ionizing radiation to soft tissue per unit mass of soft tissue
B. LET is equal to the number of ion pairs formed per unit track length
C. Once a photon transfers all its energy to an electron, the LET is that of the electron
D. LET is the quotient of the average energy that a particle lost in causing ionization to the average distance it travels between two consecutive ionizations
E. The track average method and the energy average method for calculating LET give different numerical values for therapy protons in soft tissue
D
LET is the quotient of the average energy that a particle lost in causing ionization to the average distance it travels between two consecutive ionizations. Photons, such as 250 KV X-rays, in passing through tissue produce no ionizations directly but only by setting in motion atomic electrons of tissue molecules. Electrons set in motion by incident photons have a broad energy distribution which is dissipated in tracts with LET ranging from about 0.4 to 40 keV/μm. Radiation therapy high energy photons can generate neutrons with energy between 0.1 to 2 MeV through photon interactions with nuclei of high atomic number materials that constitute the linac head and collimator systems. These neutrons in passing through tissue also produce no ionization directly but by setting protons in motion by knock on collisions with hydrogen nuclei of the cellular water molecules. Protons set in motion by photoneutrons dissipate energy over a range of LET up to about 90 keV/μm. Answer choice E mainly pertains to neutrons, not protons, where the average method and the energy method for calculating LET give significantly different numbers.
How many ion clusters are formed by 55 keV/μm silicon ion along a 1 μm segment of the ion trajectory through the cell nucleus? Assume silicon ion irradiation with the beam parallel to a cellular monolayer and that ion clusters are uniformly spaced along the silicon ion track
A. 0.5 cluster every 1 μm or 1 cluster every 2 μm
B. 5.5 clusters every 1 μm
C. 500 clusters every 1 μm
D. 5,500 clusters every 1 μm
E. 55,000 clusters every 1 μm
C
On average, the formation of a three-ion cluster requires dissipation of 110 eV. Therefore,
(55 kev/μm) * (1000 eV/1 keV) * (1 ion cluster/110 eV) = (500 ion clusters/ μm)
or 1 cluster every 20 Ǻ (1 μm = 10,000 Ǻ). This spacing of ion clusters along the silicon ion track corresponds to a 20 Ǻ diameter of the DNA helix.
