Review of IMRT and VMAT Planning Flashcards
(25 cards)
Definition of IMRT
Traditional IMRT is delivered using static segments (e.g., step and shoot) or dynamic segments (e.g., DMLC)
Definition of VMAT
A type of rotational IMRT where the MLC and gantry move while the radiation beam is on
IMRT Planning Concepts
- Modulates MLC to shape the 2D beam profile on a fixed angle beam
- Multiple Beams (non-opposing)
- Segments are used
- Beams can be different energies
VMAT Planning Concepts
- Modulates MLC’s, gantry speed and dose rate during beam on to shape 3D dose distribution
- Changing gantry speed and dose rate achieves variable MU per degree during delivery
- No. of arcs depend on complexity of plan
- Partial arcs, non-coplanar arcs can be used
3DCRT Planning Optimisation Strategies
- Altering collimator angles
- Altering wedges (EDW’s)
- Altering beam weights
- Altering margins
- Altering beam attributes if necessary
IMRT Planning Optimisation
- Automated, iterative optimisation techniques
- IMRT prescriptions define the constraints, priorities, and dose objectives for OAR’s and target volumes
What is a target objective?
- A desired treatment goal
What is a constraint and what is it’s purpose?
- A boundary/limit on the plan
- Constraints decrease the speed of the optimisation as they restrict solutions available to the optimisation algorithm
Example of Dose Shaping ROI’s
In IMRT/VMAT dose shaping ROIs are used (e.g. ROI Rings)
What is a conflict in planning and how can it be overcome?
- A conflict is occurs when a volume is under the influence of opposing constraints
- ROI Expansion/Contraction to remove overlaps between ROI’s
-> then allows for dose objectives to be applied to each
individual ROI
Primary Benefits of Inverse Planning - IMRT/VMAT (compared to Forward Planning)
- Better target coverage
- Better quality of life
- Greater OAR sparing
- Improvement in departmental workflow
Summary of ICRU 83
- Emphasis on statistics (homogeneity, conformity, biological metrics)
- No longer use ICRU ref point
- Prescription and reporting with dose volume specifications
ICRU 83 Dose Volume Histogram Metric Specifications
- Dmedian = D50%
- Prescriptive Value = e.g. D95%
- Near Minimum = D98%
- Near Maximum = D2%
What is a fluence map?
- Modulated beam’s radiation fluence as a visual illustration in beam’s eye view in colour or in grayscale
Abbreviations in VMAT Arc Direction
CCW = Counter-Clockwise
CW = Clockwise
Constant Dose Rate (CDR) in VMAT Planning
- Normal VMAT plan requires the LINAC to deliver variable dose rate beam output along with variable gantry speeds
- CDR VMAT plans have a constant MU/degree throughout the sequence
Why should the collimator angle be offset in VMAT Planning?
- Minimise the streaking effects as a result of the interleaf leakage
- Interleaf leakage is still present, but will be scattered
- 5 degree collimator offset is acceptable
What is the difference between Qualitative Evaluation and Quantitative Evaluation?
- Qualitative Evaluation = Slice by Slice Evaluation
- Quantitative Evaluation = Use Metrics (HI Index) (CI Index)
Homogeneity Index (HI)
- An objective tool to analyse the uniformity of dose distribution in the target volume
- Directly calculated from the statistics of the DVH
- HI = (D2%-D98%) / D50%
D2% = Dose received by 2% of the PTV volume
D98% - Dose received by 98% of the PTV volume
D50% = Median absorbed dose received by 50% of the volume
RTOG Conformity Index (CI)
- Relation between the volume of the reference dose (VRI) and the target volume (TV)
- CI = VRI/TV
How to find VRI in the RTOG Conformity Index?
- Convert 95% isodose line into the contour
- Check the volume of the 95% isodose structure relative to the volume of the target structure
Limitation of the conformity index (CI)
- The CI index alone cannot provide any practical information
- With this method alone, compliance with the treatment plan can only be assess by visualisation of CT sections and DVH’s
Biological Metrics
- Treatment plans can also be evaluated by biological metrics
- TCP
- NTCP
ICRU62 information
3DCRT
Ref point positioning
Dmax, Dmean, Dmin, Dref
Dose prescription
