Treatment Plan Validation

Question 1

What are the requirements of the beam choices in a RT plan?

Answer 1

Minimise normal tissue exposure
Avoid critical normal tissue
Weight and shape beams to target

Question 2

What are the aims of a plan?

Answer 2

Get prescribed dose to the target
Reasonable dose distribution achieved
Doesn’t overdose normal tissue

Question 3

What are the considerations for the dose distribution assessment?

Answer 3

Min, mean, median, max dose
Standard deviation
IMRT: D1%, D2%, D98%, D99%

Question 4

How are the dose distributions compared?

Answer 4

2D schematic

DVHs - requires consistent outlining and specification of dose to cc

Question 5

What are the important considerations for parallel and serial organs?

Answer 5

Parallel: dose to a volume
Series: Max dose

Question 6

What are the ICRU documents for dose reporting?

Answer 6

50 - photons
62 - supplement to 50
71 - electrons
78 - protons
83 - IMRT

Question 7

What is level 1 dose reporting?

Answer 7

Minimum standard - report dose to CAX and know 2D dose distribution

Question 8

What is level 2 dose reporting?

Answer 8

For treatments using computational dosimetry and 3D imaging
Use VOIs from CT or MRI and report th dose distribution including heterogeneity correction, and DVHs
Needs complete QA program to ensure prescribed dose is delivered

Question 9

What is level 3 reporting?

Answer 9

Optional R&D reporting

For techniques where reporting criteria not yet established ie TCP, NTCP, EUD

Question 10

What is the uniformity dosimetry index?

Answer 10

UDI = (PI(between 1 and 4) . Wk (abs(1-DIk)+0.1)) . 10^4
Can be bench marked against clinician’s rankings
Includes coverage index = PTVpi/PTV
Conformity index = DVpi/PTV
Homogeneity index = Dmax/Dpi
Dose gradient index = DVpi/DVhpi

Question 11

What is the homogeneity index?

Answer 11

ratio of max dose to prescribed dose

Or min-max mormalised to median

Question 12

What is the conformity index?

Answer 12

Parameter to describe conformity of dose to the target

Ratio of treated volume to the PTV

Question 13

How can the dose to be delivered be checked?

Answer 13

Independent MU calculation

Patient specific QA using point doses, detector arrays, EPIDs, gafachromic film, gel dosimetry

Question 14

What is gamma analysis?

Answer 14

There is an ellipsoid of tolerance values around a point of specified dose. If the dose to the point being measured is on that ellipsoid then the gamma analysis is passed.

Question 15

What is the equation for the gamma analysis?

Answer 15

GAMMA=((DTA/CDTA)^2 + (DD/CDD)^2)^0.5

Question 16

How does the gamma analysis allow the assessment of homogeneous and step dose gradient regions?

Answer 16

The dose to agreement term allows for large dose difference in high dose gradients to still be a pass - due to a small shift
The dose difference allows for small differences in dose in a homogeneous region which may fail the DTA criteria to pass

Question 17

What needs to be checked to ensure the plan is deliverable?

Answer 17

The Average Leaf Pair Opening, modulation factor, MU/Gy

Question 18

Why does plan robustness need to be checked?

Answer 18

Different plans can give same physical dose, so need to look at uncertainties in the plan to decide which is best