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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.

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1. The physics behind SBRT

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  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

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