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Motion in Radiotherapy

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  1. Motion in Radiotherapy Martijn Engelsman

  2. Contents • What is motion ? • Why is motion important ? • Motion in practice • Qualitative impact of motion • Motion management • Motion in charged particle therapy

  3. What is motion ?

  4. Motion in radiotherapy • Aim of radiotherapy • Deliver maximum dose to tumor cells and minimum dose to surrounding normal tissues • “Motion” • Anything that may lead to a mismatch between the intended and actual location of delivered radiation dose

  5. Radiotherapy treatment process • Diagnosis • Patient immobilization • Imaging (CT-scan) • Target delineation • Treatment plan design • Treatment delivery (35 fractions) • Patient follow-up

  6. Why is motion important ?

  7. GTV (Gross Tumor Volume):  = 5 cm, V = 65 cm3 CTV (Clinical Target Volume):  = 6 cm, V = 113 cm3 PTV (Planning Target Volume):  = 8 cm, V = 268 cm3 High dose region PTV concept (1) (ICRU 50 and 62)

  8. GTV CTV PTV High Dose PTV concept (2) • Margin from GTV to CTV • Typically 5 mm or patient and tumor specific • Improved by: • Better imaging • Physician training • Margin from CTV to PTV • Typically 5 to 10 mm • Tumor location specific • Improved by: • Motion management • Smart treatment planning

  9. Example source of motion 35 Fractions = 35 times patient setup www.pi-medical.gr

  10. Sources of motion • Patient setup • Patient breathing / coughing • Patient heart-beat • Patient discomfort • Target delineation inaccuracies • Non-representative CT-scan • Target deformation / growth / shrinkage • Etc., etc. etc.

  11. Subdivision of motion • Systematic versus Random • Inter-fractional versus Intra-fractional • Treatment Preparation versus Treatment Execution • Less commonly used

  12. Systematic versus Random • Systematic • Same error for all fractions (possibly even all patients). • Random • Unpredictable. Day to day variations around a mean. • Known but neither • Breathing, heartbeat

  13. Setup errors for three patients y Beam’s Eye View x

  14. Setup errors for a single patient Random (x) Random (y) Systematic (y) Systematic (x)

  15. Inter-fractional versus Intra-fractional • Inter-fractional • Variation between fractions • Intra-fractional • Variation within a fraction

  16. Always systematic Systematic and/or random Treatment preparation versus treatment execution • Patient immobilization • CT-scan • Target delineation • Treatment plan design • Treatment delivery (35 fractions) Treatment preparation Treatment execution

  17. Motion in practice

  18. Target delineation Steenbakkers et al. Radiother Oncol. 2005; 77:182-90

  19. y x Patient setup

  20. Target deformation / motion 1/3 Bladder Target

  21. Target deformation / motion 2/3 Bladder Target

  22. Target deformation / motion 3/3 • Patient immobilization • CT-scan • Target delineation • Treatment plan design • Treatment delivery (35 fractions)

  23. ” Breathing motion Movie by John Wolfgang

  24. Qualitative impact of motion

  25. Almost least Most Least Let’s “prove” it Importance of motion Raise your hand to vote • Breathing motion / heart beat • Systematic errors • Random errors

  26. GTV CTV PTV High Dose Simulation parameters (1) To enhance the visible effect of motion: High dose conformed to CTV GTV CTV High Dose

  27. CTV 100 95 % 90 GTV CTV High Dose 80 Dose (% of prescribed dose) 70 60 Parallel opposed beams 50 Direction of motion -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 distance from beam axis (mm) Simulation parameters (2)

  28. DVH reduction into: • Tumor Control Probability (TCP) • Assumption: homogeneous irradiation of the CTV to 84 Gy results in a TCP = 50 %

  29. Typical motion: Tumor motion and tumor control probability

  30. Almost least Most Least Importance of motion Therefore … • Breathing motion / heart beat • Systematic errors • Random errors

  31. Why are systematic errors worse ? Random errors / breathing blurs the cumulative dose distribution dose Systematic errors shift the cumulative dose distribution CTV Slide by M. van Herk

  32. In other words… • Systematic errors • Same part of the tumor always underdosed • Random errors / Breathing motion / heart beat • Multiple parts of the tumor underdosed part of the time, correctly dosed most of the time But don’t forget: Breathing motion and heart beat can have systematic effects on target delineation

  33. Motion management

  34. Radiotherapy treatment process • Patient immobilization • CT-scanning • Target delineation • Treatment plan design • Treatment delivery

  35. Patient immobilization Leg pillow Intra-cranial mask www.sinmed.com GTC frame Breast board www.sinmed.com www.massgeneral.og

  36. Benefits of immobilization • Reproducible patient setup • Limits intra-fraction motion

  37. Radiotherapy treatment process • Patient immobilization • CT-scanning • Target delineation • Treatment plan design • Treatment delivery

  38. CT-scanning • Multiple CT-scans prior to treatment planning • Reduces geometric miss compared to single CT-scan • 4D-CT scanning • Extent of breathing motion • Determine representative tumor position • See lecture “Advances in imaging for therapy”

  39. Radiotherapy treatment process • Patient immobilization • CT-scanning • Target delineation • Treatment plan design • Treatment delivery

  40. Target delineation • Multi-modality imaging • CT-scan, MRI, PET, etc. • Physician training and inter-collegial verification • Improved drawing tools and auto-delineation

  41. Radiotherapy treatment process • Patient immobilization • CT-scanning • Target delineation • Treatment plan design • Treatment delivery

  42. Treatment plan design • Choice of beam angles • e.g. parallel to target motion • Smart treatment planning • Robust optimization • IMRT • See, e.g., lecture “Optimization with motion and uncertainties”

  43. Radiotherapy treatment process • Patient immobilization • CT-scanning • Target delineation • Treatment plan design • Treatment delivery

  44. Magnitude of motion in treatment delivery • Systematic setup error • Laser: S = 3 mm • Bony anatomy: S = 2 mm • Cone-beam CT: S = 1 mm • Random setup errors • s = 3 mm • Breathing motion • Up to 30 mm peak-to-peak • Typically 10 mm peak-to-peak • Tumor delineation • See next slide

  45. Tumor delineation • 22 Patients with lung cancer • 11 Radiation oncologists from 5 institutions • Comparison to median target surface 5? Steenbakkers et al. Radiother Oncol. 2005; 77:182-90

  46. Motion management

  47. Motion management for setup errors • Portal imaging