CMD Treatment Planning & Dosimetry 2 ā Questions and Answers
Question 1: Which dose calculation algorithm accounts for electron transport and is most accurate in heterogeneous tissues such as lung?
- Pencil beam convolution
- Collapsed cone convolution
- Anisotropic analytical algorithm (AAA)
- Monte Carlo simulation (Correct answer)
Correct answer: Monte Carlo simulation
Monte Carlo simulation explicitly models individual particle interactions including electron transport, making it the most accurate algorithm in heterogeneous media like lung.
Question 2: In IMRT planning, what is the primary purpose of using a dose-volume histogram (DVH)?
- To visualize isodose lines on CT slices
- To summarize dose distribution statistics for structures (Correct answer)
- To calculate monitor units for each beam
- To determine the optimal gantry angles
Correct answer: To summarize dose distribution statistics for structures
A DVH compresses 3D dose distribution data into a 2D graph showing the volume of a structure receiving at least a given dose, enabling quantitative plan evaluation.
Question 3: A patient's treatment plan shows a dose gradient of 10 Gy/cm across the spinal cord. What is the primary concern with this scenario?
- Increased skin toxicity risk
- Uncertainty in cord dose due to setup errors (Correct answer)
- Increased monitor units required
- Higher likelihood of secondary malignancy
Correct answer: Uncertainty in cord dose due to setup errors
Steep dose gradients near critical structures mean small setup errors can translate into large dose differences, making cord dose uncertain and potentially unsafe.
Question 4: When using the equivalent square field method, what is the equivalent square for a 6 cm Ć 12 cm rectangular field?
- 7.0 cm
- 8.0 cm (Correct answer)
- 9.0 cm
- 10.0 cm
Correct answer: 8.0 cm
The equivalent square is calculated as 2(aĆb)/(a+b) = 2(6Ć12)/(6+12) = 144/18 = 8.0 cm.
Question 5: In proton therapy, the sharp distal dose fall-off beyond the Bragg peak creates a clinical challenge known as:
- Range uncertainty (Correct answer)
- Lateral penumbra widening
- Neutron activation of tissues
- Radiobiological effectiveness reduction
Correct answer: Range uncertainty
Range uncertainty in proton therapy means the actual Bragg peak location may differ from planned, potentially delivering high dose beyond the tumor or underdosing the target.
Question 6: Which planning objective best describes the concept of 'dose conformity' in stereotactic radiosurgery (SRS)?
- Minimizing the volume of tissue receiving any dose
- Matching the high-dose region closely to the target volume (Correct answer)
- Maximizing dose homogeneity within the PTV
- Reducing beam-on time to limit neutron dose
Correct answer: Matching the high-dose region closely to the target volume
Dose conformity in SRS describes how well the prescription isodose surface conforms to the shape of the target, sparing surrounding normal tissue.
Question 7: During VMAT planning, which parameter directly controls the trade-off between plan quality and delivery efficiency?
- Number of isocenters
- Maximum dose rate
- Degree of MLC modulation allowed (Correct answer)
- Couch rotation angle
Correct answer: Degree of MLC modulation allowed
Restricting MLC modulation in VMAT limits the complexity of leaf patterns per control point, reducing delivery time but potentially compromising plan quality.
CMD Treatment Planning & Dosimetry 2
Which dose calculation algorithm accounts for electron transport and is most accurate in heterogeneous tissues such as lung?