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CSB049 - Planning 2 > Review of IMRT and VMAT Planning > Flashcards

Review of IMRT and VMAT Planning Flashcards

1
Q

Definition of IMRT

A

Traditional IMRT is delivered using static segments (e.g., step and shoot) or dynamic segments (e.g., DMLC)

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2
Q

Definition of VMAT

A

A type of rotational IMRT where the MLC and gantry move while the radiation beam is on

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3
Q

IMRT Planning Concepts

A
  • 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
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4
Q

VMAT Planning Concepts

A
  • 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
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5
Q

3DCRT Planning Optimisation Strategies

A
  • Altering collimator angles
  • Altering wedges (EDW’s)
  • Altering beam weights
  • Altering margins
  • Altering beam attributes if necessary
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6
Q

IMRT Planning Optimisation

A
  • Automated, iterative optimisation techniques
  • IMRT prescriptions define the constraints, priorities, and dose objectives for OAR’s and target volumes
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7
Q

What is a target objective?

A
  • A desired treatment goal
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8
Q

What is a constraint and what is it’s purpose?

A
  • A boundary/limit on the plan
  • Constraints decrease the speed of the optimisation as they restrict solutions available to the optimisation algorithm
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9
Q

Example of Dose Shaping ROI’s

A

In IMRT/VMAT dose shaping ROIs are used (e.g. ROI Rings)

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10
Q

What is a conflict in planning and how can it be overcome?

A
  • 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
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11
Q

Primary Benefits of Inverse Planning - IMRT/VMAT (compared to Forward Planning)

A
  • Better target coverage
  • Better quality of life
  • Greater OAR sparing
  • Improvement in departmental workflow
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12
Q

Summary of ICRU 83

A
  • Emphasis on statistics (homogeneity, conformity, biological metrics)
  • No longer use ICRU ref point
  • Prescription and reporting with dose volume specifications
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13
Q

ICRU 83 Dose Volume Histogram Metric Specifications

A
  • Dmedian = D50%
  • Prescriptive Value = e.g. D95%
  • Near Minimum = D98%
  • Near Maximum = D2%
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14
Q

What is a fluence map?

A
  • Modulated beam’s radiation fluence as a visual illustration in beam’s eye view in colour or in grayscale
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15
Q

Abbreviations in VMAT Arc Direction

A

CCW = Counter-Clockwise
CW = Clockwise

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16
Q

Constant Dose Rate (CDR) in VMAT Planning

A
  • 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
17
Q

Why should the collimator angle be offset in VMAT Planning?

A
  • 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
18
Q

What is the difference between Qualitative Evaluation and Quantitative Evaluation?

A
  • Qualitative Evaluation = Slice by Slice Evaluation
  • Quantitative Evaluation = Use Metrics (HI Index) (CI Index)
19
Q

Homogeneity Index (HI)

A
  • 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

20
Q

RTOG Conformity Index (CI)

A
  • Relation between the volume of the reference dose (VRI) and the target volume (TV)
  • CI = VRI/TV
21
Q

How to find VRI in the RTOG Conformity Index?

A
  • Convert 95% isodose line into the contour
  • Check the volume of the 95% isodose structure relative to the volume of the target structure
22
Q

Limitation of the conformity index (CI)

A
  • 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
23
Q

Biological Metrics

A
  • Treatment plans can also be evaluated by biological metrics
  • TCP
  • NTCP
24
Q

ICRU62 information

A

3DCRT
Ref point positioning
Dmax, Dmean, Dmin, Dref
Dose prescription

25
Q

What is EUD

A

Dose if given uniformly to a ROI will produce the same biological response as the heterogenous dose distribution for the ROI
A<1 for target structures, increases cold spots
A=1 for parallel structures, considers hot and cold spots equally
A>1 for serial structures, reduces hot spots

CSB049 - Planning 2 (8 decks)

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