,CHAPTER LIST
Part I
Chapter 1: Physics and chemistry of radiation absorption
Chapter 2: Molecular mechanisms of DNA and chromosome damage and
repair
Chapter 3: Cell survival curves
Chapter 4: Radiosensitivity and cell age in the mitotic cycle
Chapter 5: Fractionated radiation and the dose-rate effect
Chapter 6: Oxygen effect and reoxygenation
Chapter 7: Linear energy transfer and relative biologic effectiveness
Chapter 8: Acute radiation syndrome
Chapter 9: Medical countermeasures to radiation exposure
Chapter 10: Radiation carcinogenesis
Chapter 11: Heritable effects of radiation
Chapter 12: Effects of radiation on the embryo and fetus
Chapter 13: Radiation cataractogenesis
Chapter 14: Radiologic terrorism
Chapter 15: Doses and risks in diagnostic radiology, interventional radiology
and cardiology, and nuclear medicine
Chapter 16: Radiation protection
Part II
Chapter 17: Molecular techniques in radiobiology
Chapter 18: Cancer biology
Chapter 19: Dose-response relationships for model normal tissues
Chapter 20: Clinical response of normal tissues
Chapter 21: Model tumor systems
Chapter 22: Cell, tissue, and tumor kinetics
Chapter 23: Time, dose, and fractionation in radiotherapy
Chapter 24: Retreatment after radiotherapy: the possibilities and the perils
Chapter 25: Alternative radiation modalities
Chapter 26: The biology and exploitation of tumor hypoxia
Chapter 27: Chemotherapeutic agents from the perspective of the radiation
biologist
Chapter 28: Hyperthermia
,Chapter 1: Physics and Chemistry of Radiation
Absorption
Questions
1. Which of the following best explains why X-rays and gamma rays are classified
as indirectly ionizing radiation?
A) They carry a charge and directly disrupt covalent bonds in DNA
B) They deposit energy exclusively via the Compton effect
C) They produce fast electrons that in turn cause ionization of cellular molecules
D) They interact primarily with the nucleus of target atoms
E) They have insufficient energy to ionize water molecules
✔ Answer: C
Rationale: X-rays and gamma rays are uncharged electromagnetic radiations.
They do not directly ionize matter; instead, they interact with matter (via
photoelectric effect, Compton scattering, or pair production) to liberate fast
electrons. These secondary electrons are the actual agents of ionization and
biological damage. This indirect mechanism means energy is deposited less
locally than with charged-particle radiation, a concept central to Hall & Giaccia
Chapter 1.
2. A radiation physicist states that 'most radiation-induced biological damage is
chemical in origin.' The mechanism that best supports this statement is:
A) Direct strand breakage of DNA by gamma ray photons
B) Radiolysis of intracellular water producing hydroxyl radicals
C) Activation of lysosomal enzymes by absorbed dose
D) Nuclear membrane disruption by secondary electrons
E) Mitochondrial uncoupling due to reactive nitrogen species
✔ Answer: B
,Rationale: Because cells are ~70–80% water, the majority of ionizing events occur
in water molecules, generating highly reactive hydroxyl radicals (•OH). These
diffuse to DNA and cause strand breaks — the principal mechanism of indirect
action. Hall & Giaccia emphasize that approximately two-thirds of X-ray-induced
DNA damage occurs via this indirect (radical-mediated) pathway.
3. The mean energy required to produce one ion pair in air (the W-value) is
approximately:
A) 5 eV
B) 15 eV
C) 34 eV
D) 100 eV
E) 340 eV
✔ Answer: C
Rationale: The W-value for air is approximately 34 eV per ion pair. Although
individual ionization events require only ~10–15 eV, much energy is lost to
excitation (non-ionizing interactions), raising the average energy expenditure per
ion pair. This value is important for dosimetry and for understanding the relative
efficiency of different radiation types.
4. Compared with low-LET radiation, high-LET radiation (e.g., alpha particles)
differs in which of the following ways when producing DNA damage?
A) It produces fewer double-strand breaks per unit dose
B) It induces damage that is more amenable to enzymatic repair
C) It deposits energy in dense, discrete clusters along its track
D) It is more effective per unit dose due to a greater indirect effect
E) It has a lower probability of causing lethal chromosomal aberrations
✔ Answer: C
Rationale: High-LET particles (alpha, fast neutrons, heavy ions) deposit energy in
densely ionized tracks, creating clustered DNA lesions — multiple adjacent DNA
strand breaks and base damages within one or two helical turns. These complex
lesions are far harder to repair faithfully than the isolated lesions produced by
low-LET sparsely ionizing radiation. The high-LET direct effect dominates, and
hydroxyl radical scavenging is less relevant.
5. At diagnostic X-ray energies (25–150 keV), the dominant interaction between
photons and soft tissue is:
, A) Photoelectric effect
B) Compton scattering
C) Pair production
D) Rayleigh scattering
E) Nuclear excitation
✔ Answer: B
Rationale: At the photon energies used in diagnostic radiology (roughly 30–150
keV in soft tissue), Compton scattering predominates. In Compton interactions,
the incoming photon ejects an outer-shell electron and is deflected with reduced
energy. Photoelectric absorption dominates at lower energies and with higher-Z
materials (bone, contrast agents). Pair production requires photon energies >1.02
MeV and is relevant only in megavoltage radiotherapy.
6. Which physical parameter most directly determines the rate of energy
deposition along the track of a charged particle in tissue?
A) Photon wavelength
B) Electron binding energy of target atoms
C) Linear energy transfer (LET)
D) Absorbed dose in gray
E) Mean free path of secondary electrons
✔ Answer: C
Rationale: LET (linear energy transfer) quantifies the energy deposited per unit
track length (keV/μm) by a charged particle in a medium. It directly governs the
spatial density of ionizations and hence the biological effectiveness. LET bridges
radiation physics and radiobiology and is central to understanding RBE
differences between radiation types.
7. A 5-MeV alpha particle and a 200-keV X-ray photon deliver the same absorbed
dose (in Gy) to a cell culture. Which statement is most accurate?
A) They will produce identical biological effects because absorbed dose is equal
B) The alpha particle will produce greater biological damage due to its higher LET
C) The X-ray will produce greater biological damage due to its longer range
D) The alpha particle will produce less damage because it travels more slowly
E) Biological effects depend only on dose, not on radiation quality
✔ Answer: B
, Rationale: Equal absorbed doses do not imply equal biological effect. The alpha
particle (high-LET, ~80–100 keV/μm) deposits energy in dense clusters, producing
complex, poorly repairable DNA lesions. The RBE of alpha particles relative to
reference X-rays is approximately 10–20 for cell killing. This concept — the same
dose from different radiation types produces different effects — is foundational
to the entire discipline of radiobiology.
8. In the photoelectric effect, the probability of interaction is proportional to:
A) Z/E
B) Z²/E
C) Z³/E³
D) Z/E³
E) Z²/E²
✔ Answer: C
Rationale: The cross-section for photoelectric absorption is proportional to Z³/E³
(atomic number cubed divided by photon energy cubed). This steep Z-
dependence explains why high-Z materials like iodine (Z=53) and barium (Z=56)
are effective contrast agents, and why bone (with calcium, Z=20) attenuates more
than soft tissue at low energies. The energy dependence explains why
photoelectric interactions dominate only at lower photon energies.
9. TRUE or FALSE: Free radical intermediates produced by radiolysis of water are
sufficiently long-lived to diffuse from the site of their formation to DNA and cause
strand breaks.
✔ Answer: TRUE
Rationale: Hydroxyl radicals (•OH), despite a half-life of ~10⁻⁹ seconds in pure
water, can diffuse ~2–3 nm in cells — sufficient to reach DNA from nearby water
molecules. In cells, however, the presence of radical scavengers (glutathione,
thiols) reduces their effective diffusion distance. This spatial relationship between
radical formation and DNA is the mechanistic basis for the indirect action of
radiation and for the radioprotective effect of thiol compounds.
10. TRUE or FALSE: The absorbed dose (in gray) is an adequate and complete
descriptor of radiation risk to biological systems when comparing radiation types
of different LET.
✔ Answer: FALSE
Part I
Chapter 1: Physics and chemistry of radiation absorption
Chapter 2: Molecular mechanisms of DNA and chromosome damage and
repair
Chapter 3: Cell survival curves
Chapter 4: Radiosensitivity and cell age in the mitotic cycle
Chapter 5: Fractionated radiation and the dose-rate effect
Chapter 6: Oxygen effect and reoxygenation
Chapter 7: Linear energy transfer and relative biologic effectiveness
Chapter 8: Acute radiation syndrome
Chapter 9: Medical countermeasures to radiation exposure
Chapter 10: Radiation carcinogenesis
Chapter 11: Heritable effects of radiation
Chapter 12: Effects of radiation on the embryo and fetus
Chapter 13: Radiation cataractogenesis
Chapter 14: Radiologic terrorism
Chapter 15: Doses and risks in diagnostic radiology, interventional radiology
and cardiology, and nuclear medicine
Chapter 16: Radiation protection
Part II
Chapter 17: Molecular techniques in radiobiology
Chapter 18: Cancer biology
Chapter 19: Dose-response relationships for model normal tissues
Chapter 20: Clinical response of normal tissues
Chapter 21: Model tumor systems
Chapter 22: Cell, tissue, and tumor kinetics
Chapter 23: Time, dose, and fractionation in radiotherapy
Chapter 24: Retreatment after radiotherapy: the possibilities and the perils
Chapter 25: Alternative radiation modalities
Chapter 26: The biology and exploitation of tumor hypoxia
Chapter 27: Chemotherapeutic agents from the perspective of the radiation
biologist
Chapter 28: Hyperthermia
,Chapter 1: Physics and Chemistry of Radiation
Absorption
Questions
1. Which of the following best explains why X-rays and gamma rays are classified
as indirectly ionizing radiation?
A) They carry a charge and directly disrupt covalent bonds in DNA
B) They deposit energy exclusively via the Compton effect
C) They produce fast electrons that in turn cause ionization of cellular molecules
D) They interact primarily with the nucleus of target atoms
E) They have insufficient energy to ionize water molecules
✔ Answer: C
Rationale: X-rays and gamma rays are uncharged electromagnetic radiations.
They do not directly ionize matter; instead, they interact with matter (via
photoelectric effect, Compton scattering, or pair production) to liberate fast
electrons. These secondary electrons are the actual agents of ionization and
biological damage. This indirect mechanism means energy is deposited less
locally than with charged-particle radiation, a concept central to Hall & Giaccia
Chapter 1.
2. A radiation physicist states that 'most radiation-induced biological damage is
chemical in origin.' The mechanism that best supports this statement is:
A) Direct strand breakage of DNA by gamma ray photons
B) Radiolysis of intracellular water producing hydroxyl radicals
C) Activation of lysosomal enzymes by absorbed dose
D) Nuclear membrane disruption by secondary electrons
E) Mitochondrial uncoupling due to reactive nitrogen species
✔ Answer: B
,Rationale: Because cells are ~70–80% water, the majority of ionizing events occur
in water molecules, generating highly reactive hydroxyl radicals (•OH). These
diffuse to DNA and cause strand breaks — the principal mechanism of indirect
action. Hall & Giaccia emphasize that approximately two-thirds of X-ray-induced
DNA damage occurs via this indirect (radical-mediated) pathway.
3. The mean energy required to produce one ion pair in air (the W-value) is
approximately:
A) 5 eV
B) 15 eV
C) 34 eV
D) 100 eV
E) 340 eV
✔ Answer: C
Rationale: The W-value for air is approximately 34 eV per ion pair. Although
individual ionization events require only ~10–15 eV, much energy is lost to
excitation (non-ionizing interactions), raising the average energy expenditure per
ion pair. This value is important for dosimetry and for understanding the relative
efficiency of different radiation types.
4. Compared with low-LET radiation, high-LET radiation (e.g., alpha particles)
differs in which of the following ways when producing DNA damage?
A) It produces fewer double-strand breaks per unit dose
B) It induces damage that is more amenable to enzymatic repair
C) It deposits energy in dense, discrete clusters along its track
D) It is more effective per unit dose due to a greater indirect effect
E) It has a lower probability of causing lethal chromosomal aberrations
✔ Answer: C
Rationale: High-LET particles (alpha, fast neutrons, heavy ions) deposit energy in
densely ionized tracks, creating clustered DNA lesions — multiple adjacent DNA
strand breaks and base damages within one or two helical turns. These complex
lesions are far harder to repair faithfully than the isolated lesions produced by
low-LET sparsely ionizing radiation. The high-LET direct effect dominates, and
hydroxyl radical scavenging is less relevant.
5. At diagnostic X-ray energies (25–150 keV), the dominant interaction between
photons and soft tissue is:
, A) Photoelectric effect
B) Compton scattering
C) Pair production
D) Rayleigh scattering
E) Nuclear excitation
✔ Answer: B
Rationale: At the photon energies used in diagnostic radiology (roughly 30–150
keV in soft tissue), Compton scattering predominates. In Compton interactions,
the incoming photon ejects an outer-shell electron and is deflected with reduced
energy. Photoelectric absorption dominates at lower energies and with higher-Z
materials (bone, contrast agents). Pair production requires photon energies >1.02
MeV and is relevant only in megavoltage radiotherapy.
6. Which physical parameter most directly determines the rate of energy
deposition along the track of a charged particle in tissue?
A) Photon wavelength
B) Electron binding energy of target atoms
C) Linear energy transfer (LET)
D) Absorbed dose in gray
E) Mean free path of secondary electrons
✔ Answer: C
Rationale: LET (linear energy transfer) quantifies the energy deposited per unit
track length (keV/μm) by a charged particle in a medium. It directly governs the
spatial density of ionizations and hence the biological effectiveness. LET bridges
radiation physics and radiobiology and is central to understanding RBE
differences between radiation types.
7. A 5-MeV alpha particle and a 200-keV X-ray photon deliver the same absorbed
dose (in Gy) to a cell culture. Which statement is most accurate?
A) They will produce identical biological effects because absorbed dose is equal
B) The alpha particle will produce greater biological damage due to its higher LET
C) The X-ray will produce greater biological damage due to its longer range
D) The alpha particle will produce less damage because it travels more slowly
E) Biological effects depend only on dose, not on radiation quality
✔ Answer: B
, Rationale: Equal absorbed doses do not imply equal biological effect. The alpha
particle (high-LET, ~80–100 keV/μm) deposits energy in dense clusters, producing
complex, poorly repairable DNA lesions. The RBE of alpha particles relative to
reference X-rays is approximately 10–20 for cell killing. This concept — the same
dose from different radiation types produces different effects — is foundational
to the entire discipline of radiobiology.
8. In the photoelectric effect, the probability of interaction is proportional to:
A) Z/E
B) Z²/E
C) Z³/E³
D) Z/E³
E) Z²/E²
✔ Answer: C
Rationale: The cross-section for photoelectric absorption is proportional to Z³/E³
(atomic number cubed divided by photon energy cubed). This steep Z-
dependence explains why high-Z materials like iodine (Z=53) and barium (Z=56)
are effective contrast agents, and why bone (with calcium, Z=20) attenuates more
than soft tissue at low energies. The energy dependence explains why
photoelectric interactions dominate only at lower photon energies.
9. TRUE or FALSE: Free radical intermediates produced by radiolysis of water are
sufficiently long-lived to diffuse from the site of their formation to DNA and cause
strand breaks.
✔ Answer: TRUE
Rationale: Hydroxyl radicals (•OH), despite a half-life of ~10⁻⁹ seconds in pure
water, can diffuse ~2–3 nm in cells — sufficient to reach DNA from nearby water
molecules. In cells, however, the presence of radical scavengers (glutathione,
thiols) reduces their effective diffusion distance. This spatial relationship between
radical formation and DNA is the mechanistic basis for the indirect action of
radiation and for the radioprotective effect of thiol compounds.
10. TRUE or FALSE: The absorbed dose (in gray) is an adequate and complete
descriptor of radiation risk to biological systems when comparing radiation types
of different LET.
✔ Answer: FALSE
