Treatment Plan Validation

Question 1

What comes under plan validation?

Answer 1

Correct plan?
Deliverable plan?

Question 2

What are qualitative and quantitative plan analysis examples?

Answer 2

Qualitative: dose distributions, DVHs
Quantitative: dose statistics, figures of merit, plan complexity

Question 3

How do we establish if plan is correct?

Answer 3

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?

Question 4

Why might we not just look at dose distribution?

Answer 4

Rationale for best is subjective, a lot of data in a 3D dose distribution which is hard to qualitatively assess - need quantitative meaurements

Question 5

What are some basic comparators?

Answer 5

Dose stats - min, max, median, mean, STDev

Now use near max, near min (dose to 1 or 2% or dose to 99%, 98%)

Question 6

Why is outlining so important for DVHs?

Answer 6

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

Question 7

What are levels of dose reporting?

Answer 7

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

Question 8

What are figures of merit?

Answer 8

Values produced to attempt to make plan assessment of treatment plans objective

Question 9

What are some examples of figures of merit?

Answer 9

Conformality
Homogeneity
Unified

Question 10

Homogeneity index

Answer 10

Measure of homogeneity
Ratio of max dose to prescribed (only shows level of overdosing)
Maximum minus minimum normalised to median (0 would suggest homogeneous)

Question 11

Conformity index

Answer 11

Single parameter to describe conformity of dose to target
Ratio of treated volume to PTV

Question 12

Unified dosimetry index

Answer 12

Single metric to describe plan quality
Takes into account coverage, conformity, homogeneity, dose gradient index

Question 13

How do we check correct dose?

Answer 13

Independent MU checks (RadCalc)
Validation by measurement

Question 14

What are some methods of doing validation by measurement?

Answer 14

Point doses (0D)
Detector arrays (1,2D)
EPIDS (2D)
GafC filme (2D)
Gel dosimetry (3D)

Question 15

What is gamma index?

Answer 15

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

Question 16

What do we consider for plan deliverability?

Answer 16

The plan complexity
The plan robustness

Question 17

What are some factors in plan complexity?

Answer 17

ALPO
Modulation factor
MU/Gy

Question 18

What is plan robustness?

Answer 18

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

Question 19

3 factors for plan validation

Answer 19

Plan parameters
Calculation
Measurement

Question 20

What is function of contouring/voluming features?

Answer 20

Represents, defines TV and OAR volumes and relevant planning margins to account for uncertainties
Allows planning using IMRT
Enables assessment of complex dose distributions

Question 21

What are possible sources of error in contouring and volume growing options

Answer 21

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

Question 22

How could accuracy of contouring and volume growing be assessed?

Answer 22

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

Question 23

Describe use of DVH in planning process

Answer 23

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

Question 24

Suggest 3 sources of uncertainty in DVH data

Answer 24

Sampling of dose distribution
Sampling of volumes
Structure set voxel inclusion/exclusion criteria

Question 25

Propose method to validate accuracy of DVH functionality

Answer 25

Create structures of known volume
Produce known DD
Independently calculate DVH stats, compare with those from TPS