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4D CT in Radiation
Therapy
Parminder S. Basran, PhD
Dept. Medical Physics
BCCA-Vancouver Island Centre
Outline
• The PTV problem
– ITV, MTV
• Various solutions
• Our solution
• Protocol
ICRU standard definitions for
margins in radiation therapy
GTV
CTV
ITV
PTV
PTV margin assessment
• Requires compartmentalizing
“independent” sources of
displacement errors
– Combine errors in quadr...
PTV margin components
- Inter-fraction motion: i.e., motion between
fractions
- Primarily a function of the immobilization...
PTV: Intra-fraction motion
- Intra-fraction motion
- motion during the course of treatment
- Major factors determine the e...
PTV: Intra-fraction motion
due to Breathing
Large magnitude of displacement errors for
lung, liver, upper GI disease : MTV...
Breathing Motion:
Target Localization (current approach)
GTV
CTV
PTV = CTV + standard margin
Breathing Motion:
normal tissue toxicity
• Growing evidence that significant portions
of lung can be spared if the margin
...
Breathing Motion:
Target Localization
• Ample evidence that the PTV margin
allotted for respiratory motion may be
severely...
Breathing Motion:
Target Localization
• Methods for assessing the PTV
– Fluoroscopy and CT
– CT scans at exhale, inhale an...
Breathing Motion:
What is 4-D CT?
• CT simulation provides a volumetric dataset
for dose calculation
• 4D CT provides volu...
Assume a simple moving target
What is 4-D CT?
GTV
25%
50%
75%
100%
Snap-shots of motion at
0, 25, 50, 75 and 100%
What is 4-D CT?
What is 4-D CT?
• A longer than conventional CT scan (about 5X
longer, ~ 40 seconds)
• Slightly more dose , ~ extra 6 cGy ...
What is 4-D CT?
• Assumptions
– Block motion correlates with lung motion
• Highly dependent on the patients pulmonary stat...
Result
• 4D CT will result in a number of CT
image sets that can be used for PTV
definition
Question:
Do you want to conto...
4D CT at VIC
• 4D CT will result in a number of CT
image sets that can be used for MTV
definition
• One can then fuse all ...
Question:
• Do you want to contour on all 10
datasets to define you PTVs?
Answer
• No because I can contour on a MIP
instead of all of the different phase-
related images
Question
• What is a MIP?
• Maximum Intensity Projection
MIPs
• Use the fact that in lung, higher
density tumors will appear to move
against a lower density lung
0% 25% 50% 75% = ...
5 different phases
MIP =
MIPs
• Will provide a ‘blurred’ image of the
target
• Ignores surrounding tissues, and will
therefore
– Make lung volumes ...
Some examples: Case 1
Conventional MIP
Case 1
Conventional MIP
Case 1
Conventional MIP
Case 2
Conventional MIP
Case 2
Conventional MIP
Case 3
Conventional MIP
Case 4
Conventional MIP
Case 5
Conventional MIP
MIPs
Pros Cons
- 1 dataset to contour the GTV
(ITV), not 10
-Difficult to define ITV for tumors
near chestwall, mediastinu...
Proposal
When should I use 4D CT?
• Non-palliative cases
• When lung toxicity is a concern
• When the target is close to t...
Suggested Workflow
• CT Simulation
– Set-up for RPM scan
– Perform a standard CT
– Perform a retrospective 4DCT w/ RPM
– M...
Suggested Workflow
• Advantage Workstation
– Reconstruct CT images using 10 phases
– Create the MIP AND Average CT
– For S...
Suggested Workflow
• Eclipse
– There will be at least 2 CT datasets
• MIP
– Contour GTV_MIP
• Planning CT
– Contour GTV
– ...
Once we are
comfortable with
the AVERAGE
CT datasets for
normal tissue
contouring, drop
the conventional
CT and just
perfo...
Conclusions
• MIPs are a simple means of collapsing the 4D data
into a single data set
• There are some shortcomings
• MIP...
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4dct (2012)

Four Dimensional Computed Tomography in Radiation Therapy - basic principles and implementation in a radiation oncology setting

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4dct (2012)

  1. 1. 4D CT in Radiation Therapy Parminder S. Basran, PhD Dept. Medical Physics BCCA-Vancouver Island Centre
  2. 2. Outline • The PTV problem – ITV, MTV • Various solutions • Our solution • Protocol
  3. 3. ICRU standard definitions for margins in radiation therapy GTV CTV ITV PTV
  4. 4. PTV margin assessment • Requires compartmentalizing “independent” sources of displacement errors – Combine errors in quadrature – ‘Recipe’ based (ex: VanHerk formulas)
  5. 5. PTV margin components - Inter-fraction motion: i.e., motion between fractions - Primarily a function of the immobilization and localization methodology - Other considerations: patient preparation (ex: bowel status), other confounding factors during treatment (ex: degrading pulmonary status) - Intra-fraction motion - Primarily a function of the extent of the target motion during the course of treatment
  6. 6. PTV: Intra-fraction motion - Intra-fraction motion - motion during the course of treatment - Major factors determine the extent of target motion during treatment - Voluntary motion - Involuntary motion (done without intention) - Breathing motion - Gastro-intestinal motion - Cardiac motion and pulsatile motion - Coughing, sneezing, swallowing, twitches, etc.,
  7. 7. PTV: Intra-fraction motion due to Breathing Large magnitude of displacement errors for lung, liver, upper GI disease : MTV Somewhat periodic/regular Confounds the treatment two key ways - localization of the target - Irradiation of the surrounding normal tissue (i.e., lung)
  8. 8. Breathing Motion: Target Localization (current approach) GTV CTV PTV = CTV + standard margin
  9. 9. Breathing Motion: normal tissue toxicity • Growing evidence that significant portions of lung can be spared if the margin associated with respiratory motion not included in the PTV definition GTV CTV PTV = CTV + standard margin
  10. 10. Breathing Motion: Target Localization • Ample evidence that the PTV margin allotted for respiratory motion may be severely under- or over- estimated if one simply allots a standard ‘1 cm’ margin for the PTV GTV CTV PTV = CTV + standard margin
  11. 11. Breathing Motion: Target Localization • Methods for assessing the PTV – Fluoroscopy and CT – CT scans at exhale, inhale and normal respiration • CT1 RPM Gating System • DIBH LSB – 4D CT • New CT scanner
  12. 12. Breathing Motion: What is 4-D CT? • CT simulation provides a volumetric dataset for dose calculation • 4D CT provides volumetric datasets for assessing target motion due to breathing
  13. 13. Assume a simple moving target What is 4-D CT? GTV 25% 50% 75% 100% Snap-shots of motion at 0, 25, 50, 75 and 100%
  14. 14. What is 4-D CT?
  15. 15. What is 4-D CT? • A longer than conventional CT scan (about 5X longer, ~ 40 seconds) • Slightly more dose , ~ extra 6 cGy (versus ~2 cGy for the first scan) • Requires a signal to sort out the phases of the breathing cycle (use the RPM gating block) • Provides up to 10 different phases, or sets of CT images (divided in roughly equal parts of the breathing cycle)
  16. 16. What is 4-D CT? • Assumptions – Block motion correlates with lung motion • Highly dependent on the patients pulmonary status – The (reconstructed) images provide an accurate measure of the lung motion – The respiratory motion exhibited during the scan is representative of that during treatment • Not quantified, but seems reasonable for those patients whose pulmonary status has not changed significantly
  17. 17. Result • 4D CT will result in a number of CT image sets that can be used for PTV definition Question: Do you want to contour on all 10 datasets to define you PTVs?
  18. 18. 4D CT at VIC • 4D CT will result in a number of CT image sets that can be used for MTV definition • One can then fuse all 10 data sets with the original planning CT for MTV definition
  19. 19. Question: • Do you want to contour on all 10 datasets to define you PTVs?
  20. 20. Answer • No because I can contour on a MIP instead of all of the different phase- related images
  21. 21. Question • What is a MIP? • Maximum Intensity Projection
  22. 22. MIPs • Use the fact that in lung, higher density tumors will appear to move against a lower density lung 0% 25% 50% 75% = phases of motion GTV 25% 50% 75% 100%
  23. 23. 5 different phases MIP =
  24. 24. MIPs • Will provide a ‘blurred’ image of the target • Ignores surrounding tissues, and will therefore – Make lung volumes smaller than reality (due to chest wall and heart motion) – Make external contours larger than reality – ‘blur’ vessels in the lung
  25. 25. Some examples: Case 1 Conventional MIP
  26. 26. Case 1 Conventional MIP
  27. 27. Case 1 Conventional MIP
  28. 28. Case 2 Conventional MIP
  29. 29. Case 2 Conventional MIP
  30. 30. Case 3 Conventional MIP
  31. 31. Case 4 Conventional MIP
  32. 32. Case 5 Conventional MIP
  33. 33. MIPs Pros Cons - 1 dataset to contour the GTV (ITV), not 10 -Difficult to define ITV for tumors near chestwall, mediastinum structures and diaphragm - Captures 3D range of motion, as opposed to just exhale/inhale - Artificially increases all CT values (hence standard window/level will result in brighter images) - Great for lung tumors against background lung, and liver nodes against normal liver - Provides data for only 1 of many structures that need contouring
  34. 34. Proposal When should I use 4D CT? • Non-palliative cases • When lung toxicity is a concern • When the target is close to the diaphragm • When the target is not in the apex ?
  35. 35. Suggested Workflow • CT Simulation – Set-up for RPM scan – Perform a standard CT – Perform a retrospective 4DCT w/ RPM – Mark-up patient – Start 4DCT analysis …
  36. 36. Suggested Workflow • Advantage Workstation – Reconstruct CT images using 10 phases – Create the MIP AND Average CT – For SBRT cases, also export 0 and 50% phases – Export 2-4 datasets into Eclipse – Start contouring …
  37. 37. Suggested Workflow • Eclipse – There will be at least 2 CT datasets • MIP – Contour GTV_MIP • Planning CT – Contour GTV – All other normal tissues – May be used for planning planning purposes • Average CT – Contour GTV – All other normal tissue – May be used for planning purposes
  38. 38. Once we are comfortable with the AVERAGE CT datasets for normal tissue contouring, drop the conventional CT and just perform a 4DCT scan
  39. 39. Conclusions • MIPs are a simple means of collapsing the 4D data into a single data set • There are some shortcomings • MIPs along with the 0% (rest exhale) and 50% (inhale) phase images will be available for planning purposes • Audio coaching… some data to suggest better quality images…

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