1. THE PHYSICS BEHIND SBRTRT in OzRoles and responsibility of the physicistAchieving accuracy, precision and conformality

Jeffrey Barber, Medical PhysicistNepean Cancer Care CentreIAEA RAS6065, Singapore Dec 2012

jeffrey.barber@swahs.health.nsw.gov.au

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SBRT in Australia• Population 22.6M• ~120,000 new cancer cases/year• 38% receive radiotherapy

• 260 Rad Oncs• 200 Physicists• 1400 Therapists

Planning For the Best,2011 www.radiationoncology.com.au

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SBRT in Australia• 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

Planning For the Best,2011 www.radiationoncology.com.au

Roles in SBRT (Australia)• Australia has 3 professions delivering treatments within Radiation Oncology:

RadiationOncologist

MedicalPhysicist

RadiationTherapist

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

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

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

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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

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

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

Are you ready for SBRT?

Are you ready for SBRT?Linac meeting TG-142 SRS/SBRT QA spec4DCT or other motion assessment simulationPET/MR fusion capabilitiesMotion Management solutionPlanning System Modelling and VerificationImage Guidance for treatment setup and motion assessmentPatient Specific QAStaff training and protocols

A Physicist’s Thoughts• (example from Siyong Kim, Mayo Clinic Florida)

• You want to treat opposed pair. AP or Lats?

•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

•Choose AP at first (arbitrary)

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

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

Accuracy and Precision• Competing Priorities in SBRT:

• Precision of equipment (conformal dose)

• Accuracy of process (right dose, right place)

• Minimise uncertainty during treatment

Accuracy and Precision

Jaffray, 2011

SBRT

Accuracy and Precision

Accuracy and Precision• Many sources of uncertainty/error

Remeijer, 2010

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?

Accuracy and Precision• Definitions:

• Precision

• Accuracy

• Conformality

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Accurate, not Precise Precise, not Accurate

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Not Accurate, not Precise

Precise, Accurate

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

TargetDelivery

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)

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What you can achieve now

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Conventional Therapy(add margin)

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SBRT

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Accuracy and Precision• ICRU 62 Definitions

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

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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

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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

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Dose Conformailty• Penumbra depends on:

• Energy – higher E, wider penumbra• Source size – smaller source, smaller penumbra• Source – collimation distance• Collimation – target distance

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)

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

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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

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Disher, PMB 2012

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

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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.

Physics of Lung Dose• Lateral Disequilibrium

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Disher, PMB 2012

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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

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Physics of Lung Dose

Disher, PMB 2012

• Small tumours in lung absorb less dose from these effects. Problem is worse at higher energies

• 1cm tumour, 3cm field:

Physics of Lung Dose• 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

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Pencil Beam v Monte Carlo Profiles at d=10 and d=20,Knöös PMB 1995

18MV

10MV

4MV

Accuracy and Precision• What matters in SBRT?

• Position (geometric precision)

• Localisation (geometric accuracy)

• Dose (dosimetric accuracy)

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

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

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

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MotionLow

/ Mackie 2011

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?

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

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

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How is SBRT different for Physicists?

Conventional RT

Give the right dose to the right place.*

*if your not sure, add a safety margin

SBRT

Give the right dose to the right place.**

**if your not sure, don’t do it

Again, are you ready for SBRT?Linac meeting TG-142 SRS/SBRT QA spec4DCT or other motion assessment simulationPET/MR fusion capabilitiesMotion Management solutionPlanning System Modelling and VerificationImage Guidance for treatment setup and motion

assessmentPatient Specific QAStaff training and protocols

THANK YOUFiona Hegi-JohnsonTomas KronSean White

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