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Dose volume histogram

Dose Volume Histogram

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Dose volume histogram

  1. 1. Dose-Volume Histogram Dr Sasikumar Sambasivam
  2. 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. 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
  4. 4. Displaying dose
  5. 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. 6. 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.
  7. 7. • DVHs are displayed in : • cumulative, • and differential
  8. 8. 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
  9. 9. • 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
  10. 10. Differential DVH
  11. 11. 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
  12. 12. Cumulative DVH
  13. 13. 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
  14. 14. 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
  15. 15. 2) The shape of a DVH alone can be misleading
  16. 16. 3. DVHs can only be calculated for defined VOIs.
  17. 17. 4. DVHs throw away all spatial information
  18. 18. 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).
  19. 19. • 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.
  20. 20. 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)
  21. 21. Thank you.
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Dose Volume Histogram

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