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
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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:
Radiation
Oncologist
Medical
Physicist
Radiation
Therapist
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
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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
Stereotactic Radiotherapy
• Stereotactic Radiosurgery
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•
•
•
•
Alternative to surgery
Ablative paradigm
Single Fraction
Targets < 2 cm
Margins ≈ 0
• Stereotactic Body
Radiotherapy
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•
•
•
•
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 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
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
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
SBRT
Jaffray, 2011
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:
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• Precision
• Accuracy
• Conformality
Not Accurate, not Precise
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Accurate, not Precise
Precise, not Accurate
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
Target
Delivery
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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1
1
What you can
achieve now
Conventional Therapy
(add margin)
SBRT
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Accuracy and Precision
• ICRU 62 Definitions
Accuracy and Precision
• Why be accurate?
•𝑉 =
4
𝜋𝑟 3
3
Verellen, Nat. Rev. Can. 2009
• 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
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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
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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
Disher, PMB 2012
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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 highenergy 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.
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Physics of Lung Dose
• Lateral Disequilibrium
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
• Small tumours in lung absorb less dose from these
effects. Problem is worse at higher energies
• 1cm tumour, 3cm field:
Disher, PMB 2012
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Physics of Lung Dose
• Calculating dose in low density
4MV
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
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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Motion
Low / 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
SBRT
Give the right dose to
the right place.*
Give the right dose to
the right place.**
*if your not sure, add a safety margin
**if your not sure, don’t do it
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
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THANK YOU
Fiona Hegi-Johnson
Tomas Kron
Sean White