Lecture 2 IMRT & VMAT Flashcards
In IMRT, beam intensity manipulated to address the __________________.
Objective functions
A radiation beam is divided into _________, each ____________ are independently determined.
Beamlets; intensities
The resultant modulated beam fluence is calculated by ________________.
Inverse approach
What are the reasons for the use of IMRT?
- Clinical dose escalation
- better spare OARs
- able to improve tumor control without increasing the risk of side effects - OAR dose reduction
- reduce treatment side effects without decreasing local tumor control
What are the methods to deliver IMRT?
- 3d compensator
- MLCs
- tomotherapy
- VMAT
What is the difference between segmented MLC and dynamic MLC?
Segmented MLC = collimator leafs is constant during irradiation and changes between irradiation
Dynamic MLC = collimator leafs shape changes during irradiation.
What parameters can be modulate in VMAT?
- dose rate
- gantry speed
- gantry angles
- MLC
- collimator angles (Elekta only)
What is inverse planning?
Planner specifies the desired dose distribution, and the computer calculates the required beam intensities to best meet the specified dose distribution
What is upper objective?
No more than 0% of the structure may receiving more than ___ Gy
What is lower objectives?
At least 100% of the structure must receive at least ___ Gy
What is normal tissue objective?
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
What is GEUD?
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
What are the different types of gEUD
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.
What is the Range of biological parameter
-40 to 40
Tumour: -40 to -1
OAR: 1 to -40
Pros and cons of using gEUD?
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
What are priorities?
= 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
What is the problem of increasing priorities too high?
May results in a very ‘choppy’ fluence pattern, harder for the hardware to deliver
Priorities are balanced against fluence map smoothness
What are the uncertainties in IMRT?
- MLC transmission and scattering
- motion interplay effects
- small fields dose accuracy
What are risks of IMRT?
- reduced margin lead to geographic misses of disease
- larger volume being exposed by low dose
- inaccuracies lead to harm to adjacent structures
What is problem and the solution if PTV is near or outside the patient outline
Problem: very high hotspots
Solution: add bonus / remove the part of the PTV close to patient’s outline
What is fluence editing?
Reduce unnecessary fluctuations in intensity distribution
Only available in fixed beam IMRT
What are the rapid arc delivery constraints ?
Gantry speed: 6 degree per sec
Dose rate: 600/2400
Dose/degree: 20 / 80 MU per degree
MLC speed: 2.5 cm/s
What is the MLC span limit in VMAT?
15cm
Suggest to keep field width less that 18-17cm
What do we need rotate collimator in VMAT planning?
To minimise problem of interleaf leakage
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
What is butterfly objective?
To indicate the NTO starting point
What is avoidance sector?
Selectively beam off for certain segment
Dose rate keep 0 during those segments
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
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
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
What is inhomogeneity correction and air cavity correction?
Finer resolution in the internal dose calculation grid during outmigration when air equivalent densities identified
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
