Intensity Modulated Radiation Therapy: Treatment Planning Techniques - - PowerPoint PPT Presentation

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Intensity Modulated Radiation Therapy: Treatment Planning Techniques - - PowerPoint PPT Presentation

Jul 12, 2023 111 likes •764 views

Intensity Modulated Radiation Therapy: Treatment Planning Techniques ICPT School on Medical Physics for Radiation Therapy Justus Adamson PhD Assistant Professor Department of Radiation Oncology Duke University Medical Center IMRT Treatment

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Intensity Modulated Radiation Therapy: Treatment Planning Techniques ICPT School on Medical Physics for Radiation Therapy Justus Adamson PhD

Assistant Professor Department of Radiation Oncology Duke University Medical Center

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IMRT Treatment Planning Techniques: Today’s Overview

  • Treatment chain & implications for successful IMRT

treatment planning

  • Case study: Head and Neck
  • Case study: Prostate

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IMRT Treatment Planning Process

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Simulation Contouring (MD & Dosimetrist)

Prescription & Dosimetric Constraints (MD)

Set Beam Geometry Select Optimization Criteria: target & organ constraints & weights Optimize Fluence Calculate MLC motion (leaf sequence) Calculate Dose

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IMRT Treatment Planning Process

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Simulation Contouring (MD & Dosimetrist)

Prescription & Dosimetric Constraints (MD)

Set Beam Geometry Select Optimization Criteria: target & organ constraints & weights Optimize Fluence Calculate MLC motion (leaf sequence) Calculate Dose

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Implications for successful IMRT Treatment Planning: Simulation

  • Better immobilization

= smaller CTV to PTV margins

  • Poor immobilization =

larger margins -> can negate conformality benefit of IMRT

  • Patient comfort:

longer treatment times for IMRT

– Can the patient remain in this position for the full treatment?

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CT Simulation Setup Examples:

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laser location Marked (often fiducials placed for CT) Immobilization details noted

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IMRT Treatment Planning Process

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Simulation Contouring (MD & Dosimetrist)

Prescription & Dosimetric Constraints (MD)

Set Beam Geometry Select Optimization Criteria: target & organ constraints & weights Optimize Fluence Calculate MLC motion (leaf sequence) Calculate Dose

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Implications for successful IMRT Treatment Planning: Contouring

Application of RT: Prostate 8

The IMRT plan is only as good as the contours! Accurate contours are more important for IMRT than 3D because inverse

  • ptimization tailors the

dose to them

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Implications for successful IMRT Treatment Planning: Contouring

Application of RT: Prostate 9

Verify contours especially in areas where PTV and OARs What effect will a small erroneous pixel in the PTV have?

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Implications for successful IMRT Treatment Planning: Contouring

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May be useful to create separate structures in

  • verlap regions (PTV-OAR, OAR-PTV &

OARPTV)

PTV OAR PTV-OAR OAR-PTV PTVOAR

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Optimization Structures

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IMRT Treatment Planning Process

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Simulation Contouring (MD & Dosimetrist)

Prescription & Dosimetric Constraints (MD)

Set Beam Geometry Select Optimization Criteria: target & organ constraints & weights Optimize Fluence Calculate MLC motion (leaf sequence) Calculate Dose

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Implications for successful IMRT Treatment Planning: Beam Geometry

  • Typically 5-12 equi-

spaced beams

– Provides degrees of freedom for the inverse

  • ptimization
  • Isocenter placed near

center of PTV

13 f RT: Head & Neck

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Implications for successful IMRT Treatment Planning: Beam Geometry

  • Jaws can be set

automatically or manually

  • Examples when

jaws should be manually fixed:

– avoid going through shoulders – avoid OARs with very stringent dose criteria

14 f RT: Head & Neck

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Implications for successful IMRT Treatment Planning: Beam Geometry

  • Some tables have

adjustable support bars with high attenuation!

  • Take care to make

sure the beam doesn’t enter through them

  • Otherwise, the

inverse

  • ptimization may

force high fluence through them

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AAPM Task Group 176, “Dosimetric effects caused by couch tops and immobilization devices” (2014)

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IMRT Treatment Planning Process

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Simulation Contouring (MD & Dosimetrist)

Prescription & Dosimetric Constraints (MD)

Set Beam Geometry Select Optimization Criteria: target & organ constraints & weights Optimize Fluence Calculate MLC motion (leaf sequence) Calculate Dose

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Implications for successful IMRT Treatment Planning: Setting Optimization Criteria

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Tumor Control Normal Tissue Complication

Compared to 3D, IMRT may provide: dose escalation and/or decreased normal tissue complications

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Normal Tissue Tolerances

  • Derived from various sources:

– Animal irradiation experiments – Analysis of radiotherapy patients

  • Quantitative Analyses of Normal Tissue Effects

in the Clinic (QUANTEC)

– Recent compilation of relationship between complication and dose / volume.

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Optimization Criteria

  • DVH based & mean dose criteria
  • Normal tissue constraint(s)
  • Fluence smoothing
  • Biological optimization criteria
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DVH based optimization criteria

  • Most common

criteria for inverse

  • ptimization
  • Weightings are

relative, no need to

  • verstress

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Normal tissue optimization criteria

  • Penalize all volume
  • utside the PTV
  • Cost is defined as a

function of distance from the PTV

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Fluence smoothing

  • Smooth fluence =

– <monitor units – <leakage – More robust dose distribution (less susceptible to motion)

  • Some inverse

planning systems allow for criteria to encourage smoother fluence

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Biological Optimization Criteria

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Biological Optimization Criteria

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Biological Optimization Criteria

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Biological Optimization Criteria

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Biological Constraints: Summary

  • Controls entire DVH rather than a single point

– Multiple OAR DV constraints may be replaced with a single EUD constraint with appropriate parameters

  • Biological constraints for target control cold spots->

equivalent to DVH based minimum dose constraint

  • Biological constraints do not control target maximum

dose- large dose heterogeneities for standard IMRT have no track record (except SRS, brachy, & SIB) and should be avoided

  • DVH & isodose lines should still be used for plan

analysis

  • EUD generic numbers:

– Parallel organ: a=1 – Serial organ: a=8

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IMRT Treatment Planning Process

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Simulation Contouring (MD & Dosimetrist)

Prescription & Dosimetric Constraints (MD)

Set Beam Geometry Select Optimization Criteria: target & organ constraints & weights Optimize Fluence Calculate MLC motion (leaf sequence) Calculate Dose

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Inverse Planning: Optimization (Eclipse)

dosimetric criteria dosimetric criteria & dose volume histogram beam fluence

  • bjective function

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penalty to smooth fluence normal tissue

  • ptimization constraint
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IMRT Treatment Planning Process

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Simulation Contouring (MD & Dosimetrist)

Prescription & Dosimetric Constraints (MD)

Set Beam Geometry Select Optimization Criteria: target & organ constraints & weights Optimize Fluence Calculate MLC motion (leaf sequence) Calculate Dose

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Implications for successful IMRT Treatment Planning: Calculating the leaf sequence

  • When fluence is optimized, some differences may

exist between ideal and actual fluence

  • More segments-> better agreement between DVH

during optimization & final dose calculation

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IMRT Treatment Planning Process

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Simulation Contouring (MD & Dosimetrist)

Prescription & Dosimetric Constraints (MD)

Set Beam Geometry Select Optimization Criteria: target & organ constraints & weights Optimize Fluence Calculate MLC motion (leaf sequence) Calculate Dose

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Dose Calculation

  • Dose can be modified further by:

– Dose renormalization – Fluence painting – Re-optimization

  • Make sure dose grid is appropriate for the amount of

dose falloff that is expected

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Example Case: Head and Neck

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Planned Treatment Volume: Primary Volume vs. Nodal Extension

35 Application of RT: Head & Neck

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Example 1: GTV-> CTV->PTV

36 Application of RT: Head & Neck

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37 Application of RT: Head & Neck

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38 Application of RT: Head & Neck

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39 Application of RT: Head & Neck

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Example: GTV (Primary & Nodes)->CTV->PTV

40 Application of RT: Head & Neck

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41 Application of RT: Head & Neck

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42 Application of RT: Head & Neck

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43 Application of RT: Head & Neck

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Nearby Normal Tissues

Lungs Esophagus Oral Cavity Brainstem Mandible Larynx Pharynx Parotid Glands Spinal Cord

44 Application of RT: Head & Neck

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Normal Tissue Tolerances

Larynx: Parotids:

45 Application of RT: Head & Neck

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Normal Tissue Tolerances

Lung: Spinal Cord:

46 Application of RT: Head & Neck

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Spinal Cord Pharynx Parotids PTV Oral Cavity Mandible

47 Application of RT: Head & Neck

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Historical (3D) Treatment Technique

48 Application of RT: Head & Neck

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Historical (3D) Treatment Technique: Isocenter Placement

isocenter

49 Application of RT: Head & Neck

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Historical (3D) Treatment Technique

50 Application of RT: Head & Neck

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Historical (3D) Treatment Technique

Prescribed Dose = 44Gy

51 Application of RT: Head & Neck

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3D Boost to 60Gy

52 Application of RT: Head & Neck

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3D IMRT 3D IMRT

53 Application of RT: Head & Neck

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3D vs IMRT

3D IMRT 3D IMRT

54 Application of RT: Head & Neck

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PTV DVH: 3D vs IMRT

55 Application of RT: Head & Neck

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Spinal Cord DVH: 3D vs IMRT

56 Application of RT: Head & Neck

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Larynx DVH: 3D vs IMRT

Mean dose: 3D: 53Gy IMRT: 26Gy

57 Application of RT: Head & Neck

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Parotid DVH: 3D vs IMRT

58 Application of RT: Head & Neck

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Some comments on IMRT

  • Better conformity -> may be easier to miss the target ?!

– Potentially a significant problem – First get the margins correct, then implement IMRT

  • Beam selection can be non-intuitive
  • Tendency to use more beams not less !
  • Typical MUs for an IMRT plan are 3-5 times higher

– Tendency to use lower energy (reduce neutron)

  • Tendency to ‘over-stress’ IMRT planning

– Give the optimization a consistent set of objectives – Avoid extreme weighting etc

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Summary of IMRT Advantages

  • Ability to produce

remarkably conformal dose distributions

  • Dose escalation

(improvement in local control)

  • Decreased dose to

surrounding tissues (reduction in complications) Disadvantages

  • Planning is labor intensive
  • Extended delivery time

(typically)

  • Danger of being too

conformal

  • Generally more

inhomogeneous dose distribution

  • Increased MU→ increased

whole body dose & increased room shielding

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References

  • INTENSITY-MODULATED RADIOTHERAPY:

CURRENT STATUS AND ISSUES OF INTEREST, Int. J. Radiation Oncology Biol. Phys., Vol. 51, No. 4, pp. 880–914, 2001

  • Optimized Planning Using Physical Objectives and

Constraints, Thomas Bortfield, Seminars in Radiation Oncology, Vol 9, No 1 (January), 1999:pfl 20-34

  • Image Guided Radiation Therapy (IGRT) Technologies for

Radiation Therapy Localization and Delivery, Int J Radiation Oncol Biol Phys, Vol. 87, No. 1, pp. 33e45, 2013

  • Image-guided radiotherapy: rationale, benefits, and limitations,

Lancet Oncol 2006; 7: 848–58

  • Planning in the IGRT Context: Closing the Loop, Semin

Radiat Oncol 17:268-277

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References:

  • ESTRO Guidebook 9: GUIDELINES FOR THE

VERIFICATION OF IMRT (2008)

  • AAPM:

– Report 82: Guidance document on delivery, treatment planning, and clinical implementation of IMRT: Report of the IMRT subcommittee of the AAPM radiation therapy committee (2003) – TG119: IMRT commissioning: Multiple institution planning and dosimetry comparisons (2009) – TG120: Dosimetry tools and techniques for IMRT (2011)

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Thank you!

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