CMD Treatment Simulation & Immobilization 2 — Questions and Answers
Question 1: Which type of CT acquisition is used to capture and characterize respiratory tumor motion during thoracic simulation?
- Standard helical CT
- 4D-CT (four-dimensional CT) (Correct answer)
- Cone-beam CT
- Fluoroscopic simulation
Correct answer: 4D-CT (four-dimensional CT)
4D-CT acquires images sorted by respiratory phase, enabling visualization of tumor motion throughout the full breathing cycle for lung and liver treatments.
Question 2: What does the Internal Target Volume (ITV) account for in radiation therapy planning?
- Setup uncertainty between treatment fractions
- Internal organ motion due to physiological processes such as respiration (Correct answer)
- Microscopic tumor extension beyond the visible gross tumor
- Patient weight changes occurring during a treatment course
Correct answer: Internal organ motion due to physiological processes such as respiration
The ITV encompasses the range of motion of the GTV/CTV caused by internal physiological movement, most commonly respiratory motion in thoracic and abdominal treatments.
Question 3: A Digitally Reconstructed Radiograph (DRR) is generated from:
- Portal imaging acquired during treatment delivery
- CT simulation data using ray-tracing algorithms through the 3D volume (Correct answer)
- MRI simulation data only
- Fluoroscopic images acquired at time of simulation
Correct answer: CT simulation data using ray-tracing algorithms through the 3D volume
DRRs are synthetic radiographs computed by tracing rays through CT simulation volumetric data, used as reference images for image-guided patient setup verification.
Question 4: What is the primary advantage of vacuum-lock (vac-lock) bags compared to other immobilization devices?
- They provide rigid immobilization equivalent to stereotactic frames
- They conform to any body contour and harden into custom-fitted, reproducible positioning aids (Correct answer)
- They have significantly lower fabrication cost than all alternatives
- They have no effect on CT imaging quality or HU values
Correct answer: They conform to any body contour and harden into custom-fitted, reproducible positioning aids
Vacuum bags conform to the patient's body surface and become rigid when air is evacuated, providing comfortable, custom-fitted immobilization adaptable to any treatment site.
Question 5: When using respiratory gating for lung SBRT, the treatment beam is delivered during:
- The entire respiratory cycle to ensure full tumor coverage
- A defined phase or amplitude range of the respiratory cycle (Correct answer)
- Only when the patient voluntarily holds their breath indefinitely
- The CT scan acquisition period only
Correct answer: A defined phase or amplitude range of the respiratory cycle
The gating window is a specific respiratory phase or amplitude range during which the beam is active, reducing the required PTV margin by treating only when the tumor is in a reproducible position.
Question 6: What is the primary purpose of multi-modality image registration during radiation therapy simulation?
- To reduce total imaging dose delivered to the patient
- To combine anatomical and functional information from multiple imaging modalities for improved target and OAR delineation (Correct answer)
- To improve native CT scan spatial resolution
- To eliminate the need for physical patient simulation
Correct answer: To combine anatomical and functional information from multiple imaging modalities for improved target and OAR delineation
Image registration overlays MRI, PET, or other modality data onto the CT simulation dataset, leveraging complementary information to improve the accuracy of target volume and organ-at-risk delineation.
Question 7: How does the Planning Target Volume (PTV) relate to the Clinical Target Volume (CTV)?
- PTV and CTV are geometrically identical volumes
- PTV is a subset contained entirely within the CTV
- PTV adds a margin around the CTV to account for setup uncertainty and residual organ motion (Correct answer)
- PTV is defined independently without reference to the CTV
Correct answer: PTV adds a margin around the CTV to account for setup uncertainty and residual organ motion
The PTV margin expands around the CTV to ensure the CTV receives the prescribed dose despite geometric uncertainties including setup errors and residual internal motion during treatment.
CMD Treatment Simulation & Immobilization 2
Which type of CT acquisition is used to capture and characterize respiratory tumor motion during thoracic simulation?