IMRT Flashcards
Define IMRT
intensity modulated RT
- uses mlcs to create non-uniform fluences for any fields to create uniform dose distribution within the target that spares critical structures simultaneously
- assigns non uniform intensities to beamlets/rays
difference between 3DCRT and IMRT
3d -target coverage using uniform beam
imrt - uses same tools as 3d but intensities are diff within each beam
beamlet
segments of a beam
forward planning vs inverse planning
forward - design shapes manually
inverse - begin with desired dose
x5 steps inverse planning
- contour structures, add margins, create opti structures
- choose number of configurations of beams/arcs
- define prescription/optimization objectives
- computer optimization
- plan evaluation
reducing irradiated volume outside target allows? x4
- higher dose to tumour vol
- decreased OAR, and risks
- large fields and boosts can be integrated into a plan
- sharper fall off beyond PTV
IMRT initial disadvantages x5
- specialized equipment
- non-intuitive intensity maps
- involves lots of QA**
- longer tx times
- more wear and tear on machine
IMRT process (flowchart) x10 points x3 categoires
IMAGING AND PREPLANNING
-immobilization, CT sim/imaging, target organ delineation
OPTIMIZATION
-specifiy objective function and beam, optimization, dose calc
QA AND DELIVERY
leaf sequence generation, dosimetric verification of beams, patient set up, i/ex vivo dosimetry
Opti-structures
- structures that are made due to competing objectives
- for example if theres overlap between 2 structures that causes competing objectives the overlap is integrated into one of the structures
NTO
normal tissue objective
-designed to increase conformity of prescription isodoses - eliminates dose dumping
Target and OAR objectives
-objectives in the form of DVH metrics (max/min/mean dose, dose volume objectives)
Fluence objectives
-objectives designed to eliminate unnecessary fluence modulation
clinical objectives
-parameter to characterize the dose distribution if the computer is to find the best plan
characteristics of a good plan x4
- adequate tumour dose homogeneity
- normal structures are spared
- deliverable by machine
- possible to QA
objective function and cost function
- both are measurements of quality of a plan
- cost function - measure of how much the actual plan deviates from the desired plan
step 4 of IMRT categories x2
fluence based optimization
direct aperture optimization
Direct Aperture optimization
where the beamlet apertures and weights (ie beam-on times) are optimized simultaneously
step 4 optimization flow chart
- select the initial beamlet weights
- compute dose distributions
- determine updated factors
- recompute dose distribution
- acceptable? yes or no
if no go back to determining factors
fluence based optimization x2 steps
- find the optimal fluence
- convert optimal fluence to deliverable fluence
minimization methods; stochastic vs deterministic
stochastic: elements of randomness (same conditions may not mean same solution); can escape the local minima to find global minima
deterministic: same initial conditions = same solution; downhill techniques; gets trapped in minima
interactive inverse planning
- where planners job is to figure out what objectives and weights will lead to optimal beams rather than trying to find the optimal beams directly through DVH
- it allows for the planner to see how adjustments are affecting the optimization process
step and shoot/static mlc
- leaves do not move when beam is on
- leaf motion and the radiation are executed sequentially
dynamic MLC
- leaves move when the beam is on
- leaves motion and radiation are controlled separately and can be executed simultaneously
advantages of static mlcs x5
- easy to understand
- easy to resume and interuppt
- relatively simple linac control system
- can verify individual segments
- fewer MUs than DMLC
