Lecture 2 IMRT & VMAT

Question 1

In IMRT, beam intensity manipulated to address the __________________.

Answer 1

Objective functions

Question 2

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

Answer 2

Beamlets; intensities

Question 3

The resultant modulated beam fluence is calculated by ________________.

Answer 3

Inverse approach

Question 4

What are the reasons for the use of IMRT?

Answer 4

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

Question 5

What are the methods to deliver IMRT?

Answer 5

• 3d compensator
• MLCs
• tomotherapy
• VMAT

Question 6

What is the difference between segmented MLC and dynamic MLC?

Answer 6

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

Question 7

What parameters can be modulate in VMAT?

Answer 7

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

Question 8

What is inverse planning?

Answer 8

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

Question 9

What is upper objective?

Answer 9

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

Question 10

What is lower objectives?

Answer 10

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

Question 11

What is normal tissue objective?

Answer 11

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

Question 12

What is GEUD?

Answer 12

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

Question 13

What are the different types of gEUD

Answer 13

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.

Question 14

What is the Range of biological parameter

Answer 14

-40 to 40

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

Question 15

Pros and cons of using gEUD?

Answer 15

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

Question 16

What are priorities?

Answer 16

= 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

Question 17

What is the problem of increasing priorities too high?

Answer 17

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

Priorities are balanced against fluence map smoothness

Question 18

What are the uncertainties in IMRT?

Answer 18

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

Question 19

What are risks of IMRT?

Answer 19

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

Question 20

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

Answer 20

Problem: very high hotspots

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

Question 21

What is fluence editing?

Answer 21

Reduce unnecessary fluctuations in intensity distribution

Only available in fixed beam IMRT

Question 22

What are the rapid arc delivery constraints ?

Answer 22

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

Question 23

What is the MLC span limit in VMAT?

Answer 23

15cm

Suggest to keep field width less that 18-17cm

Question 24

What do we need rotate collimator in VMAT planning?

Answer 24

To minimise problem of interleaf leakage

Question 25

What is NTO?

Answer 25

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

Question 26

What is butterfly objective?

Answer 26

To indicate the NTO starting point

Question 27

What is avoidance sector?

Answer 27

Selectively beam off for certain segment
Dose rate keep 0 during those segments

Question 28

What are some limitation and constraints when applying avoidance sector ?

Answer 28

Max 2 sectors
Min 15 degree sector length
Min 15 degree apart the 2 avoidance sector

Question 29

Difference between old and new PRO?

Answer 29

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

Question 30

What is jaw tracking ?

Answer 30

= Main jaw follows the MMLC shape during gantry rotation

• Reduce leakage through large areas of closed MLC
• useful in case of multiple small targets

Question 31

What is inhomogeneity correction and air cavity correction?

Answer 31

Finer resolution in the internal dose calculation grid during outmigration when air equivalent densities identified

Question 32

What is the use of intermediate dose calculation?

Answer 32

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