, Chapter 1: Physics and Chemistry of Radiation
Absorption
How ionizing radiation interacts with matter and biological molecules; primary
ionizations and free radical formation
Question 1
A 200 keV X-ray photon enters soft tissue and ejects an inner-shell electron
through the photoelectric effect. What secondary process is most likely to follow
this initial ionization event?
A. Emission of a characteristic X-ray from the atom
B. Production of bremsstrahlung radiation by the ejected electron
C. Direct double-strand DNA breakage
D. Release of free radicals from water radiolysis
✅ Correct Answer: A. Emission of a characteristic X-ray from the atom
Rationale:
Photoelectric absorption removes an inner-shell electron, leaving a vacancy. This
is usually filled by an outer-shell electron, releasing a characteristic X-ray photon
or an Auger electron. While free radicals may form later from indirect effects, the
immediate consequence of photoelectric interaction is characteristic radiation.
Keywords: Photoelectric effect, characteristic X-ray, ionization, photon
interaction
Question 2
During external beam therapy, a patient receives exposure to 1 MeV photons.
Which interaction dominates in soft tissue at this energy?
A. Coherent scattering
B. Photoelectric absorption
,C. Compton scattering
D. Pair production
✅ Correct Answer: C. Compton scattering
Rationale:
At 1 MeV, Compton scattering is the predominant interaction in soft tissue due to
the relative abundance of loosely bound outer-shell electrons. Photoelectric effect
dominates at lower energies (<50 keV) and pair production dominates at energies
>1.022 MeV.
Keywords: Compton scattering, photon interactions, energy dependence
Question 3
A proton beam deposits most of its energy at the end of its path, producing dense
ionizations. What is the name of this phenomenon?
A. Linear energy transfer (LET)
B. Bragg peak
C. Auger cascade
D. Compton shoulder
✅ Correct Answer: B. Bragg peak
Rationale:
The Bragg peak describes the sharp increase in energy deposition near the end of a
charged particle’s track, leading to highly localized ionizations. This property
makes proton therapy advantageous in sparing surrounding normal tissues.
Keywords: Bragg peak, proton therapy, energy deposition
Question 4
If a 100 eV photon ionizes a water molecule, what is the approximate number of
ion pairs produced, assuming 33 eV is required per ion pair?
,A. 1
B. 2
C. 3
D. 4
✅ Correct Answer: C. 3
Rationale:
On average, 33 eV is needed to produce one ion pair in water. Thus, 100 eV / 33
eV ≈ 3 ion pairs. The excess energy may go into excitation rather than ionization.
Keywords: Ion pair, energy deposition, water radiolysis
Question 5
In radiation therapy, indirect DNA damage is primarily mediated by what free
radical species formed during water radiolysis?
A. OH• (hydroxyl radical)
B. H• (hydrogen radical)
C. O₂•− (superoxide radical)
D. HO₂• (hydroperoxyl radical)
✅ Correct Answer: A. OH• (hydroxyl radical)
Rationale:
The hydroxyl radical is the most reactive and abundant free radical formed from
water radiolysis. It readily reacts with DNA bases and sugars, leading to strand
breaks and oxidative modifications.
Keywords: Hydroxyl radical, water radiolysis, indirect DNA damage
Question 6
A 6 MV X-ray beam is used for therapy. Which photon interaction contributes the
most to dose deposition in deep soft tissues at this energy?
,A. Photoelectric absorption
B. Coherent scattering
C. Compton scattering
D. Pair production
✅ Correct Answer: C. Compton scattering
Rationale:
At megavoltage energies typical of therapy (1–10 MeV), Compton scattering
dominates in soft tissue. Pair production becomes important only above ~10 MeV,
while photoelectric absorption is negligible at these energies.
Keywords: Megavoltage, Compton scattering, therapy beams
Question 7
When ionizing radiation produces clustered DNA damage, which mechanism most
likely explains the high biological lethality of such lesions?
A. Single nucleotide excision is highly efficient
B. Free radicals cannot diffuse in the nucleus
C. Repair pathways are overwhelmed by complexity
D. Chromosomes immediately fragment
✅ Correct Answer: C. Repair pathways are overwhelmed by complexity
Rationale:
Clustered DNA damage sites (multiple lesions within 1–2 helical turns) are
difficult for repair enzymes to process. This increases the likelihood of misrepair or
double-strand breaks, leading to high cytotoxicity.
Keywords: Clustered DNA damage, repair complexity, lethality
Question 8
A patient is treated with carbon ion therapy. Compared to photons, why does
carbon ion radiation produce a greater proportion of direct DNA damage?
, A. Lower penetration depth
B. Higher linear energy transfer (LET)
C. Reduced oxygen dependence
D. Greater production of characteristic X-rays
✅ Correct Answer: B. Higher linear energy transfer (LET)
Rationale:
High-LET radiation such as carbon ions deposits energy densely along its track,
causing direct ionizations within DNA. In contrast, low-LET radiation (photons,
electrons) relies more on indirect free radical-mediated effects.
Keywords: High LET, carbon ions, direct DNA damage
Question 9
A diagnostic radiology resident calculates the probability of photoelectric
absorption in bone compared to soft tissue at 40 keV. Which factor most influences
the difference?
A. Mass density
B. Effective atomic number (Z)
C. Water content
D. Electron binding energy
✅ Correct Answer: B. Effective atomic number (Z)
Rationale:
The photoelectric effect probability scales approximately with Z³ and inversely
with E³ (photon energy). Bone has a higher effective Z than soft tissue, explaining
its greater photoelectric absorption.
Keywords: Photoelectric effect, Z³ dependence, bone imaging
Question 10
Absorption
How ionizing radiation interacts with matter and biological molecules; primary
ionizations and free radical formation
Question 1
A 200 keV X-ray photon enters soft tissue and ejects an inner-shell electron
through the photoelectric effect. What secondary process is most likely to follow
this initial ionization event?
A. Emission of a characteristic X-ray from the atom
B. Production of bremsstrahlung radiation by the ejected electron
C. Direct double-strand DNA breakage
D. Release of free radicals from water radiolysis
✅ Correct Answer: A. Emission of a characteristic X-ray from the atom
Rationale:
Photoelectric absorption removes an inner-shell electron, leaving a vacancy. This
is usually filled by an outer-shell electron, releasing a characteristic X-ray photon
or an Auger electron. While free radicals may form later from indirect effects, the
immediate consequence of photoelectric interaction is characteristic radiation.
Keywords: Photoelectric effect, characteristic X-ray, ionization, photon
interaction
Question 2
During external beam therapy, a patient receives exposure to 1 MeV photons.
Which interaction dominates in soft tissue at this energy?
A. Coherent scattering
B. Photoelectric absorption
,C. Compton scattering
D. Pair production
✅ Correct Answer: C. Compton scattering
Rationale:
At 1 MeV, Compton scattering is the predominant interaction in soft tissue due to
the relative abundance of loosely bound outer-shell electrons. Photoelectric effect
dominates at lower energies (<50 keV) and pair production dominates at energies
>1.022 MeV.
Keywords: Compton scattering, photon interactions, energy dependence
Question 3
A proton beam deposits most of its energy at the end of its path, producing dense
ionizations. What is the name of this phenomenon?
A. Linear energy transfer (LET)
B. Bragg peak
C. Auger cascade
D. Compton shoulder
✅ Correct Answer: B. Bragg peak
Rationale:
The Bragg peak describes the sharp increase in energy deposition near the end of a
charged particle’s track, leading to highly localized ionizations. This property
makes proton therapy advantageous in sparing surrounding normal tissues.
Keywords: Bragg peak, proton therapy, energy deposition
Question 4
If a 100 eV photon ionizes a water molecule, what is the approximate number of
ion pairs produced, assuming 33 eV is required per ion pair?
,A. 1
B. 2
C. 3
D. 4
✅ Correct Answer: C. 3
Rationale:
On average, 33 eV is needed to produce one ion pair in water. Thus, 100 eV / 33
eV ≈ 3 ion pairs. The excess energy may go into excitation rather than ionization.
Keywords: Ion pair, energy deposition, water radiolysis
Question 5
In radiation therapy, indirect DNA damage is primarily mediated by what free
radical species formed during water radiolysis?
A. OH• (hydroxyl radical)
B. H• (hydrogen radical)
C. O₂•− (superoxide radical)
D. HO₂• (hydroperoxyl radical)
✅ Correct Answer: A. OH• (hydroxyl radical)
Rationale:
The hydroxyl radical is the most reactive and abundant free radical formed from
water radiolysis. It readily reacts with DNA bases and sugars, leading to strand
breaks and oxidative modifications.
Keywords: Hydroxyl radical, water radiolysis, indirect DNA damage
Question 6
A 6 MV X-ray beam is used for therapy. Which photon interaction contributes the
most to dose deposition in deep soft tissues at this energy?
,A. Photoelectric absorption
B. Coherent scattering
C. Compton scattering
D. Pair production
✅ Correct Answer: C. Compton scattering
Rationale:
At megavoltage energies typical of therapy (1–10 MeV), Compton scattering
dominates in soft tissue. Pair production becomes important only above ~10 MeV,
while photoelectric absorption is negligible at these energies.
Keywords: Megavoltage, Compton scattering, therapy beams
Question 7
When ionizing radiation produces clustered DNA damage, which mechanism most
likely explains the high biological lethality of such lesions?
A. Single nucleotide excision is highly efficient
B. Free radicals cannot diffuse in the nucleus
C. Repair pathways are overwhelmed by complexity
D. Chromosomes immediately fragment
✅ Correct Answer: C. Repair pathways are overwhelmed by complexity
Rationale:
Clustered DNA damage sites (multiple lesions within 1–2 helical turns) are
difficult for repair enzymes to process. This increases the likelihood of misrepair or
double-strand breaks, leading to high cytotoxicity.
Keywords: Clustered DNA damage, repair complexity, lethality
Question 8
A patient is treated with carbon ion therapy. Compared to photons, why does
carbon ion radiation produce a greater proportion of direct DNA damage?
, A. Lower penetration depth
B. Higher linear energy transfer (LET)
C. Reduced oxygen dependence
D. Greater production of characteristic X-rays
✅ Correct Answer: B. Higher linear energy transfer (LET)
Rationale:
High-LET radiation such as carbon ions deposits energy densely along its track,
causing direct ionizations within DNA. In contrast, low-LET radiation (photons,
electrons) relies more on indirect free radical-mediated effects.
Keywords: High LET, carbon ions, direct DNA damage
Question 9
A diagnostic radiology resident calculates the probability of photoelectric
absorption in bone compared to soft tissue at 40 keV. Which factor most influences
the difference?
A. Mass density
B. Effective atomic number (Z)
C. Water content
D. Electron binding energy
✅ Correct Answer: B. Effective atomic number (Z)
Rationale:
The photoelectric effect probability scales approximately with Z³ and inversely
with E³ (photon energy). Bone has a higher effective Z than soft tissue, explaining
its greater photoelectric absorption.
Keywords: Photoelectric effect, Z³ dependence, bone imaging
Question 10
