CMD Clinical Oncology & Treatment Techniques 2 — Questions and Answers
Question 1: The dose-volume histogram (DVH) type that shows the percentage of a structure volume receiving at least a given dose is called:
- Cumulative (integral) DVH (Correct answer)
- Differential DVH
- Absolute DVH only
- Serial DVH
Correct answer: Cumulative (integral) DVH
The cumulative DVH plots the fractional volume of a structure receiving at least a specified dose and is the standard format for evaluating treatment plans against dose-volume constraints.
Question 2: Which organ at risk constraint is commonly applied for the spinal cord in radical radiation therapy to prevent radiation myelopathy?
- Maximum point dose ≤ 45–50 Gy in conventional fractionation (Correct answer)
- Mean dose < 30 Gy
- V20 < 35%
- D98% < 40 Gy
Correct answer: Maximum point dose ≤ 45–50 Gy in conventional fractionation
The spinal cord maximum dose is typically limited to 45–50 Gy in conventional fractionation to maintain the risk of radiation myelopathy below 0.2%.
Question 3: In prostate cancer treatment planning, which normal tissue constraint is used to reduce the risk of rectal bleeding?
- Rectal V70 < 20% (volume of rectum receiving 70 Gy) (Correct answer)
- Bladder V80 < 15%
- Femoral head Dmax < 60 Gy
- Penile bulb mean dose < 30 Gy
Correct answer: Rectal V70 < 20% (volume of rectum receiving 70 Gy)
Limiting the rectal V70 to less than 20% reduces the risk of late rectal toxicity, including rectal bleeding, following definitive prostate radiotherapy.
Question 4: The electron beam depth dose characteristic known as the 'practical range' (Rp) is used to:
- Estimate the maximum depth of electron beam penetration in tissue (Correct answer)
- Identify dmax for the electron beam
- Calculate the penumbra width
- Determine the isocenter depth
Correct answer: Estimate the maximum depth of electron beam penetration in tissue
The practical range Rp represents the depth beyond which the electron dose falls to near zero and is used to select appropriate electron beam energy for target coverage.
Question 5: Which beam arrangement is used in total body irradiation (TBI) to achieve dose uniformity across the entire patient?
- Bilateral extended-SSD fields with compensators or lung blocks (Correct answer)
- Anterior-posterior (AP/PA) fields at standard SSD
- IMRT with multiple coplanar fields
- Single lateral field
Correct answer: Bilateral extended-SSD fields with compensators or lung blocks
TBI uses extended SSD bilateral fields with tissue compensators and partial lung blocks to achieve ±10% dose homogeneity across the heterogeneous patient anatomy.
Question 6: In brachytherapy, HDR (High Dose Rate) is defined as a dose rate greater than:
- 12 Gy/hour (Correct answer)
- 2 Gy/hour
- 0.4 Gy/hour
- 5 Gy/hour
Correct answer: 12 Gy/hour
Per ICRU Report 38, HDR brachytherapy is defined as dose rates exceeding 12 Gy/hour (0.2 Gy/min), typically delivered with Ir-192 sources in outpatient fractionated treatments.
CMD Clinical Oncology & Treatment Techniques 2
The dose-volume histogram (DVH) type that shows the percentage of a structure volume receiving at least a given dose is called: