2. • A voxel represents a
value on a regular
grid in three-
dimensional space.
• Voxel is a combination
of "volume" and
"pixel" where pixel is a
combination of
"picture" and
"element"
3. Intro
• The display and analysis of dose distribution
Displaying dose
– Colour scales
– Dose banding
– Interactivity
Dose volume histograms
– Types
– Calculation and interpretation
– Problems and pitfalls
5. Methods of displaying dose
• 1. Iso-dose contours
Sets of closed contours linking voxels of equal dose
• 2. Colour wash
The coding of CT and dose in the same voxel
through the modulation of both intensity (CT) and
colour (dose)
• 3. Iso-dose surfaces
The shaded surface (pseudo-3d) representation of
a particular dose level and selected VOIs
6.
7.
8. Intro
• To form a DVH for any 3D object, one looks at the dose value
for each voxel in the object and forms a histogram,
counting the number of voxels that receive each different
dose level
• Because the volume of each voxel is known, the volume of the
organ receiving each dose level is known.
• Both the volume (vertical) and dose (horizontal) axes can be
displayed in absolute terms (as cubic centimeters [cc] or
Gray [Gy]) or in relative terms (% volume or % dose),
depending on how the planner wants to analyze the results.
9.
10. • DVHs are displayed in :
• cumulative,
• and differential
11. The differential histogram
• The generic form of any histogram, displaying the
volume of the organ that receives dose within each
dose bin (1% or 0.5 to 1 Gy is a typical dose bin
width).
• It is useful for display of the dose-to-target volumes,
because one can easily visualize the minimum dose,
the maximum dose, and the dose most
representative of the dose to the entire target volume
12. • This type is necessary in order to appropriately
compare DVHs formed with different dose bin
sizes, because the volume contained in any dose
bin changes as the dose bin size changes.
• The differential DVH plots (1/Dbin) *DV/DD, so
different differential DVHs can be compared even if
their dose bin sizes are different
15. Cumulative DVH
• Volumes receiving at least a given dose
value are plotted.
• The cumulative DVH integrates the direct
histogram, so it always begins at 100% (100%
of the organ receives at least 0 dose), and
ends at the maximum dose
19. Problems and Pitfalls
1. DVHs are insensitive to small ‘hot’ and ‘cold’
spots
2. The shape of a DVH alone can be misleading
3. DVHs can only be calculated for defined VOIs
4. DVHs throw away all spatial information
20. 1) Insensitiveness to small ‘hot’ and ‘cold’ spots
Consider
comparative
DVHs from
competing
plans
• From DVHs,
plan 2
appears to be
the best
…apart from
apparently
inignificant
increase in high
dose to posterior
fossa
• Posterior fossa
25. Visual assessment of dose distributions
• The most direct and informative representation of a
treatment plan available - however….
• 3-D dose distributions are large and
cumbersome and difficult to analyse quantitatively
• User interactivity is essential to extract the most
information from dose distributions (slice
selection/multi slice display, dose banding, dose
querying etc).
26. • Provide a succinct and quantitative method of
representing 3-d dose within selected VOI’s
-however…
• DVH’s should only be used in conjunction with careful
visual analysis of 3-d dose distributions
• In particular, care should be taken when analysing
large volumes using DVH’s
• DVH’s should always be assessed in conjunction with
dose-volume statistics.
27. Assessing and ranking a plan—Dose Based Scoring
Possible score functions
1.Visual assessment of the 3-d dose distribution
2. Visual assessment of DVHs
3. Quantitative analysis of dose distributions (conformity
index, homogeneity index etc)
4. Quantitative analysis of DVHs
(max, min, dose-to-volume etc)
