IMRT & VMAT Flashcards
What can 3D planning be described as?
Static Fields and very basic field shaping with modulation through wedges and blocks
What can Arc therapy be described as?
Rotational fields, that are a constant shape, Additional shaping can be added as an additional arc and not within the same arc
What fluence?
Number of photons entering a cross-section area of a sphere
What is fluence rate?
The rate of fluence per unit time
What is energy fluence?
Sum of all energies of all the photons that enter the cross-section
Energy Fluence rate
Energy fluence per unit time, also known as energy flux density or intensity
What does IMRT stand for?
Intensity Modulated Radiation Therapy
What is IMRT?
A treatment in which NONUNIFORM fluences are delivered to a patient from different directions to optimize composite dose
What is significant about the beam arrangement in IMRT?
Many beams of varying intensities are utilized
How are beamlet intensities determined?
based on the optimizer to calculate intensity of the beams in relation to meet the dose constraints
How are beamlets given in a sense of field shape?
The beamlets are given in segments, with moving MLCs
What is inverse planning?
Planning process, in which, you give the optimization a desired result with certain criteria to fulfill and an output is made. For example, you give the computer system the desire regimen, and dose constraints and the plan is built in reverse of that goal.
What can IMRT treatment delivery be described as?
Heterogenous treatment deliver intensity distribution delivered by MLC collimation
What three characteristics of Forward planning does the user define?
Geometry, Collimation, Fluence
What two characteristics of inverse planning do the user define?
Geometry, Dosimetry criteria and desired weighting
IMRT can NOT be forward planned. True or False?
False, it can be forward planned
What portions of planning does the optimization algorithm define in inverse planning?
Collimation and beam fluence
What is the analytic method of IMRT planning?
Desired dose distribution is inverted by using a back projection algorithm
What is the iterative method of IMRT?
Beamlets weight are adjusted in order to minimize the value of a cost function, deviating from the desired goal
What is gradient descent?
Faster, much more gradient descent towards a goal. Downside is the process can get stuck in a local minima before being able to actually achieve the goal
What is simulated annealing?
Slower process, however system accept some higher cost in pursuit of a global minimum, acceptance decreases exponentially as process goes on
What is a cost function?
Can be described as the necessary cost to produce a specific thing. In regards to radiotherapy planning, you made add a beamlet but its going to cost certain dose to structures, while also costing you less weighting from other beamlets
How does cost function work in the optimization algorithm?
Once in the optimizer, you will have all structures and be able to assign priority of goal achievement in the optimization process
What is Step and Shoot?
Step and Shoot IMRT, is treatment that is given with the gantry in different stationary positions with a fixed collimator opening for that set gantry position
How do you describe the intensity levels of radiation being given during a step and shoot technique?
Discrete and there is fixed output
What is sliding window IMRT?
Sliding Window IMRT, is the treatment technique that involves a stationary gantry position and continuously moving MLC collimation to modulate the intensity.
How do you describe the intensity levels of radiation being given during a sliding window technique?
Continuous Intensity levels
What happens if cost function is unacceptable?
Dose constraints may have to be changed
What is the initial first step of optimization?
Entering the prescription and Dose constraints
What is the 2nd step in the optimization process?
Optimization based on the cost function
What is the 3rd step of optimization process?
Intensity modulated fields and fluence patterns
What is the 4th step of the optimzation process?
Leaf sequencing
What is the final step in the optimization process?
Dose delivery
Why would it not be optimal to continuously add 3D fields for treatment instead of using IMRT?
In terms of 3DCRT, continuously adding 3D static fields, would only increase treatment time and set-up to get the desired conformality, while utilizing IMRT can give the conformality in an ARC, which cuts down on treatment times (Known as diminished return)
Describe past-pointing and why is utilized?
Past-pointing is the process in which the AN ARC is skipped and the isocenter is adjusted to the the skipped portion to shift high-dosage areas
What is the rule of thumb for past pointing?
Isocenter should be placed 2-2.5cm from the midpoint of the tumor
If your PTV is at a depth of 5 cm, what would be the optimal depth to place the isocenter to shift the 100% isodose line to the ptv?
7cm
What is the equation to find Gantry Speed?
Monitor Units/ Degrees within the arc (Mu/Degree)
What is the equation for finding the degrees within an arc?
Monitor Unit/ (MU/Degree)
How do you determine the finishing angle of the arc, if you are given the rotation orientation?
MU/ (MU/Degree) to achieve the degrees of the arc, however you need to differentiate if you need to add or subtract your outputs from the starting angle to achieve the finishing angle
What is VMAT?
Volume Modulated Arc Therapy
What is the major drawback of VMAT in comparison to IMRT?
VMAT gives a higher volume of low dose throughout the patient
What is Varians VMAT called? Philips VMAT? Elekta VMAT?
Eclipse RapidArc, Pinnacle SmartArc, and Monaco VMAT
What are some advantages of VMAT?
Shorter treatment time, compared to static IMRT, Continuous gantry movement, Changing MLC/DR/ and Rotation Speed, Increased sparing of normal tissue, Lower MU output= scatter dose= lower chance of secondary malignancy
What occurs if two different dose constraints are too close ?
A high dose gradient occurs, which is difficult to achieve
What happens if a Organ dose tolerance is changed after plan has been optimized?
The plan must be re-run and re-optimized to account for the changes
What is important to the success of IMRT/VMAT planning as a planner?
Experience with the system, and also the trial-and-error approach to planning
In terms of Contours, what should the planner consider before getting into IMRT/VMAT planning?
The planner should consider the challenges that the contours might bring. while also making adjustments to constraints or new structures, to be aid the optimizer in achieving the desired goal
What must the collimator be rotated to minimize hot spots due to interleaf leakage?
30 degrees
What are clinical goals?
User inputs and templates that are utilized as plan evaluators, can be provided by rad onc, and/or dosimetrist for plan optimzation
What is the benefit of Clinical goals?
It can be used to provide explanation for certain outcomes, while also providing that explanation without having to affect the dose distribution, allowing editing and input of new clinical goals to be placed without manipulation
What is the benefit of VMAT Breast
Limited risk of position errors, increased reproducibility (Monoisocentric, No shifts and IGRT), Increase homogeneity, Cosmetic improvement
What is the downside of VMAT Breast?
Higher integral dose, More sensitive to positioning errors and tissue changes due to modulation, lower optimization process in comparison to other VMAT plans
What is the standard flash that is given for breast patients?
2cm but can be larger depending on breast, added to the lateral and anterior aspect of the patient for breast changes and 2 cm inferiorly for patient anatomy positioning
