ASTRO Questions Flashcards
What is the approximate maximum diffusion distance of oxygen from a
normally-oxygenated capillary through a typical respiring tissue?
200 um (0.2 mm)
Modifiers of Cell Survival: Oxygen Effect
Estimate of hypoxic fraction?
surviving fraction hypoxic/surviving fraction oxygenated
Modifiers of Cell Survival: Oxygen Effect
What is Km for radiosensitivity?
Oxygen concentration 0.5-1% (3-8 mmHg)
The oxygen concentration at which cellular radiosensitivity is halfway between the fully aerobic and fully hypoxic response
Modifiers of Cell Survival: Oxygen Effect
The most dramatic change in radiation sensitivity occurs over which range of oxygen tension (in units of mm Hg or Torr)?
0-30 mm Hg (Torr).
Cells irradiated under an oxygen partial pressure at the low end of this range are maximally radioresistant, whereas irradiation at 30 mm Hg oxygen results in near maximum radiosensitization
Modifiers of Cell Survival: Oxygen Effect
Relationship between OER, LET and RBE?
When LET increases, OER decreases.
When LET increases, RBE increases until reaching a maximum at approximately 100 keV/µm, and then decreases.
Modifiers of Cell Survival: Oxygen Effect
OER values obtained for high energy protons used in radiotherapy are
similar to those measured for X-rays. True or false?
True
Since the high energy protons used in radiotherapy have an LET similar
to that of X-rays, their OER values are also similar.
Modifiers of Cell Survival: Oxygen Effect
What oxygen partial pressure is needed for full radiosensitization?
2-3%
Modifiers of Cell Survival: Oxygen Effect
What is OER?
The OER is defined as the ratio of the radiation dose needed to cause a
certain biological effect in hypoxic cells divided by the dose needed to
produce the same effect in aerated cells
Modifiers of Cell Survival: Oxygen Effect
Why the tumor is more radiosensitive in aerated conditions?
The increased cell killing resulting from irradiation in the presence of
oxygen is thought to be the result of increased radical damage and damage
fixation by oxygen. The initial number of ionizations produced by
radiation in the aerated and hypoxic cells would be the same
Modifiers of Cell Survival: Oxygen Effect
For single, large radiation doses delivered at a high dose rate, the ratio of
the OER for X-rays divided by the OER for 15-MeV neutrons is
approximately
2
Since the X-ray OER is typically about 3 and the OER for 15 MeV
neutrons is about 1.6, the ratio of the OERs is about 2
Modifiers of Cell Survival: Oxygen Effect
What does hypoxia do to gene transcription?
Exposure of cells to hypoxia, as in other stress situations, leads to changes
in expression of a number of stress genes, many of which are responsive
to the transcription factor, hypoxia-inducible factor-1a (HIF1A).
Modifiers of Cell Survival: Oxygen Effect
What is the primary mechanism that oxygen acts as a radiosensitizer?
through its ability to “fix” radiation-induced DNA damage
Modifiers of Cell Survival: Oxygen Effect
What are the processes that an incident photon can undergo based on its energy?
Photoelectric absorption: The photon’s energy is absorbed by an electron in the metal.
Compton scattering: The photon collides with a target at rest, and a new photon emerges at an angle. The target recoils, carrying away some of the incident energy.
Electron-positron pair production: The incident photon can undergo this process.
Radiation interaction with matter
What is the primary product of water radiolysis and how much of the indirect damage is attributed to it?
OH* (hydroxyl radical)
65-75%
Radiation interaction with matter
The approximate minimum photon energy required to cause ionization of
a water molecule is:
10-25 eV
On average, about 25 eV is required to create an ion pair in water,
although the minimum energy needed to eject an electron is only 12.6 eV
Radiation interaction with matter
Which of the following X-ray interactions with matter is most important
for producing high-contrast diagnostic radiographs?
Photoelectric effect
Radiation interaction with matter
What is the photon energy range for photoelectric, Compton, and triplet production?
100 KeV - Compton
10 KeV - photoelectric
1-2 MeV - triplet production
Radiation interaction with matter
The lifetime of an OH* radical is approximately:
10^-9 second
Radiation interaction with matter
Annihilation photons always have an energy of …. MeV each in pair production.
Annihilation photons always have an energy of 0.511 MeV each, which
is equal to the rest energy of the positron and electron.
Radiation interaction with matter
What results from the recombination of the initial water radiolysis products?
Water
Radiation of the pure water results in short-lived free radicals:
H2O -> OH (45%) + H(10%) + e-aq
These will react with DNA or each other
OH + H -> H2O
e-aq + e-aq +2 H2O -> H2 + 2 OH-
OH + OH -> H2O2
H + H -> H2
Radiation interaction with matter
Molecular mechanisms of DNA damage
For survival fraction of 37% (to kill 63% of cells), how many double strand breaks or pyrimidine dimers are needed?
40 DSBs
1,000,000 pyrimidine dimers
Molecular mechanisms of DNA damage
Which assay would be the most appropriate to use for quantitative measurement of DNA double-strand breaks (DSBs) in cells immediately following exposure to ionizing radiation?
Neutral comet assay
Molecular mechanisms of DNA damage
What does alkaline elution assay measure?
measure single-strand breaks and some base
damages
Molecular mechanisms of DNA damage
