Lecture 2 IMRT & VMAT Flashcards

1
Q

In IMRT, beam intensity manipulated to address the __________________.

A

Objective functions

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2
Q

A radiation beam is divided into _________, each ____________ are independently determined.

A

Beamlets; intensities

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3
Q

The resultant modulated beam fluence is calculated by ________________.

A

Inverse approach

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4
Q

What are the reasons for the use of IMRT?

A
  1. Clinical dose escalation
    - better spare OARs
    - able to improve tumor control without increasing the risk of side effects
  2. OAR dose reduction
    - reduce treatment side effects without decreasing local tumor control
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5
Q

What are the methods to deliver IMRT?

A
  • 3d compensator
  • MLCs
  • tomotherapy
  • VMAT
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6
Q

What is the difference between segmented MLC and dynamic MLC?

A

Segmented MLC = collimator leafs is constant during irradiation and changes between irradiation
Dynamic MLC = collimator leafs shape changes during irradiation.

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7
Q

What parameters can be modulate in VMAT?

A
  • dose rate
  • gantry speed
  • gantry angles
  • MLC
  • collimator angles (Elekta only)
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8
Q

What is inverse planning?

A

Planner specifies the desired dose distribution, and the computer calculates the required beam intensities to best meet the specified dose distribution

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9
Q

What is upper objective?

A

No more than 0% of the structure may receiving more than ___ Gy

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10
Q

What is lower objectives?

A

At least 100% of the structure must receive at least ___ Gy

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11
Q

What is normal tissue objective?

A

To take into account the decrease in dose levels as the distance from targets is increased

To limit the dose level and prevent hotspot outside PTV

To obtain a sharp dose falloff

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12
Q

What is GEUD?

A

Generalised equivalent uniform dose

Account for the biological response, according to the delivered dose distribution in that organ

The uniform dose distribution that gives a biological effect equivalent to that of a given heterogeneous dose distribution

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13
Q

What are the different types of gEUD

A

Upper gEUD = maximum equivalent uniform dose value a structure or OAR may receive

Lower gEUD = minimum equivalent uniform dose value that a target structure must receive (similar to DCH lower objectives)

Target gEUD = the exact equivalent uniform dose value that a target structure must receive.

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14
Q

What is the Range of biological parameter

A

-40 to 40

Tumour: -40 to -1
OAR: 1 to -40

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15
Q

Pros and cons of using gEUD?

A

Pros : useful for OAR dose control, less objectives needed to obtain similar results

Cons:
- limited number of parameters controlled by planners
- less room for fine tuning
- solely rely on target gEUD can result in non-uniform dose

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16
Q

What are priorities?

A

= the importance of an objective function
- inactive when all constraints are satisfied
- the priority score tell the optimiser how to balance off between them if some are not satisfied

17
Q

What is the problem of increasing priorities too high?

A

May results in a very ‘choppy’ fluence pattern, harder for the hardware to deliver

Priorities are balanced against fluence map smoothness

18
Q

What are the uncertainties in IMRT?

A
  • MLC transmission and scattering
  • motion interplay effects
  • small fields dose accuracy
19
Q

What are risks of IMRT?

A
  • reduced margin lead to geographic misses of disease
  • larger volume being exposed by low dose
  • inaccuracies lead to harm to adjacent structures
20
Q

What is problem and the solution if PTV is near or outside the patient outline

A

Problem: very high hotspots

Solution: add bonus / remove the part of the PTV close to patient’s outline

21
Q

What is fluence editing?

A

Reduce unnecessary fluctuations in intensity distribution

Only available in fixed beam IMRT

22
Q

What are the rapid arc delivery constraints ?

A

Gantry speed: 6 degree per sec
Dose rate: 600/2400
Dose/degree: 20 / 80 MU per degree
MLC speed: 2.5 cm/s

23
Q

What is the MLC span limit in VMAT?

A

15cm

Suggest to keep field width less that 18-17cm

24
Q

What do we need rotate collimator in VMAT planning?

A

To minimise problem of interleaf leakage

25
What is NTO?
Normal tissue objective - limit dose and prevent hotspot in healthy tissue - obtain steep dose gradient falloff around PTV - 100% corresponds to the lowest upper objectives of PTV
26
What is butterfly objective?
To indicate the NTO starting point
27
What is avoidance sector?
Selectively beam off for certain segment Dose rate keep 0 during those segments
28
What are some limitation and constraints when applying avoidance sector ?
Max 2 sectors Min 15 degree sector length Min 15 degree apart the 2 avoidance sector
29
Difference between old and new PRO?
PRO2 - 5 phases - control points increased from 10 to 177 from 1st to 5th phase PRO3 - 4 phases - all 178 control points are generated from 1st phase
30
What is jaw tracking ?
= Main jaw follows the MMLC shape during gantry rotation - Reduce leakage through large areas of closed MLC - useful in case of multiple small targets
31
What is inhomogeneity correction and air cavity correction?
Finer resolution in the internal dose calculation grid during outmigration when air equivalent densities identified
32
What is the use of intermediate dose calculation?
Multi-resolution pencil beam photon dose calculation algorithm (MRDC) is less accurate when accounting for tissue inhomogeneity A second tun optimisation using the dose distribution obtained with the final algorithm as reference dose