Planning + MU calcs Flashcards
Name 3 types of IMRT
- Multiple static field delivery
- Dynamic delivery
- Serial tomotherapy
- Helical tomotherapy
- Intensity modulated arc therapy
- Volumetric modulated arc therapy
What are the two types of planning technique?
Forward planning
Inverse planning
Briefly describe forward planning
The planner has target parameters to reach and has to optimise the plan through MLCs, wedges, beam arrangements etc. to reach an acceptable plan
Briefly describe inverse planning
The planner defines objectives and constraints
Beamlet weights or segment shapes and weights are computed
Dose calculated
If the requirements are met then can accept plan or go on for further optimisation
What are the advantages of inverse planning compared to forward planning?
It is more consistent between planners
Necessary for more complex techniques
Allows for complex anatomy and modulations
Complex delivery can take place
What are some advantages of forward planning?
It is useful for simple IMRT techniques
Simple modulations
Extension of conformal RT
What is the aim of optimisation?
To meet a cost function
How is a cost function calculated and optimised?
Based on deviation from objectives set by planner
Plan parameters are adjusted
Doses and cost function re-calculated
Define EUD
Equivalent Uniform Dose
The uniform dose distribution which would have the same TCP or NTCP
it is a way of expressing the radiobiological effect of a dose distribution in a single number
What is the formula for EUD?
EUD = (1/nSUM(d_i)^a)^1/a
a = tissue-specific parameter
How is the EUD used in a TPS to manipulate dose distributions?
By changing the ‘a’ value
Increasing ‘a’ will prioritise high doses, decreasing ‘a’ will prioritise low doses
What may be used for plan analysis?
DVH
Radiobiological models - NTCP/TCP/EUD/BED
Plan criteria
How might a plan be edited if a patient has a hip prosthesis?
Not enter through the prosthesis
Less concerned about exiting
What must be verified for IMRT treatments?
Transfer of RT plan
- Fluence, dose distribution
Dose
- to phantom, to pt
What methods of plan verification are there?
Solid water phantom with single point ion chamber
Solid water phantom with film
Use of a 2D ion-chamber array
EPID patient QA
What is gamma analysis?
Distance to agreement (DTA) Dose difference (DD)
A measure of acceptability which is the multidimensional difference between the measurement and calculation points in the dose and physical distance, scaled as a fraction of the acceptance criteria.
Why are manual/independent MU/dose checks done?
To check the dose calculation that was done in the TPS.
What is the difference in calculation between isocentric and fixed SSD manual MU calculations?
isocentric uses TPR
Fixed SSD uses PDD
What is the equation for the isocentric calculation?
MU = D/(D0 x Sc x MTPR x WF x TF x OAR x SAD factor)
MTPR = Sp x TPR
SAD factor = ((SSD0 + d0)/SPD)^2 = (SAD/(SSD+d))^2
What is the tolerance for the difference between manual and TPS calculated MU?
+/- 5%
What is the equation for the equivalent field size, r0?
r0 = 4A/P
What would using equivalent depth in water do to the results?
Improve the accuracy of calculations
What is the equation for a fixed SSD calculation?
MU = 100xD/(D0 x Sc x Sp x PDD x WF x TF x OAR x SSD factor)
SSD factor = ((SAD0 + dmax)/(SSD + dmax))^2
What do the normalisation conditions of the fixed SSD calculation assume?
Maximum dose is delivered at dmax
