Treatment Plan Validation Flashcards
What comes under plan validation?
Correct plan?
Deliverable plan?
What are qualitative and quantitative plan analysis examples?
Qualitative: dose distributions, DVHs
Quantitative: dose statistics, figures of merit, plan complexity
How do we establish if plan is correct?
Checking - might have check report (warnings, successful checks all listed)
Plan comparison - have we met goals, is cord dose low enough? Have we met prescription?
Why might we not just look at dose distribution?
Rationale for best is subjective, a lot of data in a 3D dose distribution which is hard to qualitatively assess - need quantitative meaurements
What are some basic comparators?
Dose stats - min, max, median, mean, STDev
Now use near max, near min (dose to 1 or 2% or dose to 99%, 98%)
Why is outlining so important for DVHs?
Has big impact on the volume % affected, only contour a small volume of rectum, dose to that looks like large %
Could specify dose to cc
What are levels of dose reporting?
Level 1 - minimum standard. For simple treatments - dose on CAX and 2D DD known
Level 2 - for treatments using computational dosimetry and 3D imaging, VOIs are defined and DVHs computed. QA in place.
Level 3 - optional R&D reporting, includes techniques not yet established such as TCP or NTCP
What are figures of merit?
Values produced to attempt to make plan assessment of treatment plans objective
What are some examples of figures of merit?
Conformality
Homogeneity
Unified
Homogeneity index
Measure of homogeneity
Ratio of max dose to prescribed (only shows level of overdosing)
Maximum minus minimum normalised to median (0 would suggest homogeneous)
Conformity index
Single parameter to describe conformity of dose to target
Ratio of treated volume to PTV
Unified dosimetry index
Single metric to describe plan quality
Takes into account coverage, conformity, homogeneity, dose gradient index
How do we check correct dose?
Independent MU checks (RadCalc)
Validation by measurement
What are some methods of doing validation by measurement?
Point doses (0D)
Detector arrays (1,2D)
EPIDS (2D)
GafC filme (2D)
Gel dosimetry (3D)
What is gamma index?
Objective evaluation of dose at a point.
Ellipsoid representing acceptance criteria, gamma value less than 1 if position can be found in sphere that has DD in tolerance
What do we consider for plan deliverability?
The plan complexity
The plan robustness
What are some factors in plan complexity?
ALPO
Modulation factor
MU/Gy
What is plan robustness?
Different plans might have same dose at tend but different uncertainty levels in relation to positioning/internal anatomy variability.
TPSs now letting users model these uncertainties
3 factors for plan validation
Plan parameters
Calculation
Measurement
What is function of contouring/voluming features?
Represents, defines TV and OAR volumes and relevant planning margins to account for uncertainties
Allows planning using IMRT
Enables assessment of complex dose distributions
What are possible sources of error in contouring and volume growing options
Definition of 3D structure from 2D contour
Capping of structures
Method of defining volume growing algorithm
Discretising continuous contour drawn in 2D slices onto 3D matrix/voxels
How could accuracy of contouring and volume growing be assessed?
Define structures of known volume
Validate calculation of dimensions and volume of
structure representation.
Grow structure by a range of margins, evaluate the dimensional and volumetric accuracy of the resulting structures
Describe use of DVH in planning process
2D graphical representations of 3D dose distribution
Can analyse dose-volume distribution of structures and evaluate different DDs
Contain no spatial information, need to use with isodose distribution
Suggest 3 sources of uncertainty in DVH data
Sampling of dose distribution
Sampling of volumes
Structure set voxel inclusion/exclusion criteria
Propose method to validate accuracy of DVH functionality
Create structures of known volume
Produce known DD
Independently calculate DVH stats, compare with those from TPS
