1 / 48

1. The physics behind SBRT

1. The physics behind SBRT. RT in Oz Roles and responsibility of the physicist Achieving accuracy, precision and conformality. jeffrey.barber@swahs.health.nsw.gov.au. Jeffrey Barber, Medical Physicist Nepean Cancer Care Centre IAEA RAS6065, Singapore Dec 2012. SBRT in Australia.

erna
Télécharger la présentation

1. The physics behind SBRT

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 1. The physics behind SBRT RT in Oz Roles and responsibility of the physicist Achieving accuracy, precision and conformality jeffrey.barber@swahs.health.nsw.gov.au Jeffrey Barber, Medical Physicist Nepean Cancer Care Centre IAEA RAS6065, Singapore Dec 2012

  2. SBRT in Australia Planning For the Best,2011 www.radiationoncology.com.au • Population 22.6M • ~120,000 new cancer cases/year • 38% receive radiotherapy • 260 Rad Oncs • 200 Physicists • 1400 Therapists

  3. SBRT in Australia Planning For the Best,2011 www.radiationoncology.com.au • 40+ Rad Onc Centres • 170 linacs • 70 with CBCT or similar • 40 with respiratory monitoring • 4000 IMRT cases (10%) • 325 SBRT/SRT cases • ≥ 4 centres Spine SBRT • ≥ 5 centres Lung SBRT • Multi-centre trials

  4. Roles in SBRT (Australia) • Australia has 3 professions delivering treatments within Radiation Oncology: • Radiation • Oncologist • Medical • Physicist • Radiation • Therapist

  5. Physicist’s Role (Australia) • Commission equipment and planning systems • Maintain equipment (treatment, image guidance, simulation) • Review simulation and assess motion • Consult on treatment options, parameters • Verify the plan is achievable both dosimetrically and geometrically • Perform measurements to verify, on phantoms and in vivo • Monitor treatment procedure looking for deviations from the plan • Review treatment results in aggregate to improve processes

  6. Radiation Therapist Role (Australia) • Perform simulation, assess motion, immobilisation • Generate treatment plan • OAR contouring • Beam placement • Optimisation • Set up patients for treatment • Perform image guidance with RO • Deliver treatment and monitor

  7. Useful References • ASTRO White Paper (full report) • AAPM TG-101 Report on SBRT • UK Royal College Radiologists report BFCO(08)5 On target: Ensuring geometric accuracy in radiotherapy, 2008

  8. Stereotactic Radiotherapy • Stereotactic Radiosurgery • Alternative to surgery • Ablative paradigm • Single Fraction • Targets < 2 cm • Margins ≈ 0 • Stereotactic Body Radiotherapy • Alternative to surgery? • Ablative paradigm • Few Fractions • Targets < 5 cm • Margins < 5 mm

  9. Stereotactic Radiotherapy • SBRT has origins in Stereotactic Radiosurgery (SRS) • Initial programs used the same immobilisation, localisation and beam planning/delivery • Dose prescription methods comes from Radiosurgery • In the last 10 years, SBRT has changed with the use of IGRT • The gap between modern radiotherapy and stereotactic radiotherapy is closing NOT MY EXPERTISE

  10. Stereotactic Body Radiotherapy • SBRT looks a lot like conventional linac radiotherapy • But it requires: • excellent immobilisation • high level Image Guidance • high precision delivery • relatively small field sizes • many beams or arcs • motion management

  11. Are you ready for SBRT?

  12. Are you ready for SBRT? • Linac meeting TG-142 SRS/SBRT QA spec • 4DCT or other motion assessment simulation • PET/MR fusion capabilities • Motion Management solution • Planning System Modelling and Verification • Image Guidance for treatment setup and motion assessment • Patient Specific QA • Staff training and protocols

  13. A Physicist’s Thoughts • (example from Siyong Kim, Mayo Clinic Florida) • You want to treat opposed pair. AP or Lats? • Choose AP at first (arbitrary) • But there is couch transmission. Lats. • But there is an OAR laterally. AP. • But the target is likely to move A/P. Lats • But the gantry is known to flex/sag laterally… There can be a lot of thought and compromise in even simple beam arrangements

  14. The Physics behind SBRT (1) • Equipment Accuracy • Isocentre – Gantry, Collimator, Couch • MLC and jaws • Couch remote movement • Immobilisation • Motion Management • At simulation, in the planning system, at treatment

  15. The Physics behind SBRT (2) • Image Guidance • Coincidence of imaging and treatment beam • Spatial integrity of imaging systems (Sim and Tx) • Beam modelling and dosimetry • Heterogeneous dose calculation • Small field dosimetry • Build up effects • Lateral disequilibrium

  16. Accuracy and Precision • Competing Priorities in SBRT: • Precision of equipment (conformal dose) • Accuracy of process (right dose, right place) • Minimise uncertainty during treatment

  17. Accuracy and Precision SBRT Jaffray, 2011

  18. Accuracy and Precision

  19. Accuracy and Precision • Many sources of uncertainty/error Remeijer, 2010

  20. Accuracy and Precision • Physicists quantify everything • But does that mean we get it right? • There is always some error • How do these errors effect treatment?

  21. 1 Accurate, not Precise Accuracy and Precision • Definitions: • Precision • Accuracy • Conformality 1 Not Accurate, not Precise Precise, not Accurate Precise, Accurate

  22. Accuracy and Precision • What is the impact of errors? • Random Errors apply once per fraction • Blur the distribution • Systematic Errors apply once per patient • Shift the cumulative distribution Target Delivery

  23. Accuracy and Precision • What do you do if you can’t be precise and accurate? • Add margin • Internal Margin for internal error (what gating is for) • Setup Margin for setup changes (what CBCT is for) 1 1 1 What you can achieve now Conventional Therapy (add margin) SBRT

  24. Accuracy and Precision • ICRU 62 Definitions

  25. Accuracy and Precision • Why be accurate? • Assume a spherical target, 30mm diameter • No margin r=15mm V ~ 15cm3 • 5mm margin r=20mm V ~ 30cm3 2x • 10mm margin r=25mm V ~ 60cm3 4x Verellen, Nat. Rev. Can. 2009

  26. Dose Conformailty • Multiple non-overlapping beams • Converge on target concentrically • Coplanar or non-coplanar • No overlapping entry/exit • 9-12 linac beams • 1+ Arc beams • 200+ Cyberknife beams Urbanic, Wake Forest 2012

  27. Dose Conformality • Rule of thumb: Dose gradient ≥5%/mm • To obtain a highly conformal dose: • Sharp penumbra • Sufficient beam penetration • Small build-up region • Fine Beam shaping • Many beams • Individual beams <30% cumulative dose

  28. Dose Conformailty • Penumbra depends on: • Energy – higher E, wider penumbra • Source size – smaller source, smaller penumbra • Source – collimation distance • Collimation – target distance

  29. Physics of Lung Dose • Getting dose to be deposited in lung tumours is difficult • Low density region absorbs less dose • Build up and build down effects at interfaces (Electronic equilibrium) • Low density region spreads dose laterally (Lateral disequilibrium)

  30. Physics of Lung Dose • Build-up and build-down at interfaces as electron range changes

  31. Physics of Lung Dose • Low density region absorbs less dose as electron range changes • Build-down at lung entry • Secondary build-up at tissue re-entry • Higher dose at depth compared to homogeneous Disher, PMB 2012

  32. Physics of Lung Dose • Low density region spreads dose laterally (Lateral disequilibrium)

  33. Physics of Lung Dose • Lateral Disequilibrium • High energy photons interact with tissue to yield high-energy Compton electrons that travel up to several centimetres through tissue. • Some of these electrons deposit dose outside the photon beam. • As a result • the dose within the field is reduced • penumbra is broadened • dose outside the geometric field edge increases • This effect is more pronounced in low-density tissues such as lung, where Compton electrons can travel larger distances.

  34. Physics of Lung Dose • Lateral Disequilibrium Disher, PMB 2012

  35. Physics of Lung Dose • Small tumours in lung absorb less dose from these effects. Problem is worse at higher energies • 3cm tumour, 5cm field: Disher, PMB 2012

  36. Physics of Lung Dose • Small tumours in lung absorb less dose from these effects. Problem is worse at higher energies • 1cm tumour, 3cm field: Disher, PMB 2012

  37. Physics of Lung Dose 4MV • Calculating dose in low density regions (lung) is difficult • Calculating dose to small islands of water-density (tumour) in low density regions (lung) is even harder • Most algorithms over-estimate dose in lung 10MV 18MV Pencil Beam v Monte Carlo Profiles at d=10 and d=20, Knöös PMB 1995

  38. Accuracy and Precision • What matters in SBRT? • Position (geometric precision) • Localisation (geometric accuracy) • Dose (dosimetric accuracy)

  39. Accuracy and Precision • What can we do to minimise errors? • Before IGRT/SBRT • Determine precision of each step • Determine systematic accuracy • Estimate unknowns (patient internals) • Apply a margin to cover overall

  40. Accuracy and Precision • What can we do to minimise errors? • With IGRT/SBRT • Determine precision of each step • Determine systematic accuracy • Determine population based uncertainties per site • Image and correct for set up errors online • Apply a margin to cover residual errors and remaining unknowns

  41. Accuracy and Precision • SBRT requires greater accuracy and precision • Need to be sure of the end result • Look at the treatment as a system, not as a series of isolated systems

  42. Motion Low / Mackie 2011

  43. Motion • Inter-fraction: day-to-day • Weight loss/gain • Tumour growth/shrinkage • Intra-fraction: between getting on and getting off the bed • Cardiac motion • Respiratory motion • Bowel gas, bladder filling • Motion Assessment – is it a concern? • Motion Management – do you control it?

  44. Before you start… • More practice plans • More measurements in phantom • Planning guidelines • Staff training and practice • For trial participation, credentialing required

  45. Safety! • Serious consequences for getting it wrong in SBRT • The safety net of fractionation is removed • Understand the issues • Double check the data • Verify the whole process • When not to do SBRT? • If targets/sensitive organs can’t be localised • If systematic accuracy not enough for the necessary margin • If the patient is not stable for the duration of treatment

  46. How is SBRT different for Physicists? Conventional RT SBRT Give the right dose to the right place.** **if your not sure, don’t do it Give the right dose to the right place.* *if your not sure, add a safety margin

  47. Again, are you ready for SBRT? • Linac meeting TG-142 SRS/SBRT QA spec • 4DCT or other motion assessment simulation • PET/MR fusion capabilities • Motion Management solution • Planning System Modelling and Verification • Image Guidance for treatment setup and motion assessment • Patient Specific QA • Staff training and protocols

  48. Thank you Fiona Hegi-Johnson Tomas Kron Sean White

More Related