Use of Volumetric Modulated Arc Therapy
for Intra and Extra Cranial Stereotactic
Radiosurgery
Part III – SmartArc & QA
Peter Balter, Ph.D.
Pinnacle SmartArc
• Clinically released in Ver 9
• Supports multiple linac
vendors
• New parts of photon
machine setup
• New SmartArc tab in IMRT
screen
SmartArc Optimization
• Optimized around the entire arc simultaneously
• Uses pinnacle DMPO framework
• Optimization includes “high quality” convolution
dose calculations (important in area of
disequilibrium ie Lung SBRT)
• Constraints are not allowed
Optimization approach
• SmartArc starts with
24o spacing around
the arc
• At each position
apertures are set to
the target and then
optimized fluences
are generated by
gradient search
Bzdusek, K, et al “Development and evaluation of an efficient approach to volumetric
arc therapy planning,” Med. Phys. 36, 6, June 2009
Optimization approach
• At each angle 2-4 segments are generated
• Largest 2 segments are chosen and the other 1-2 discarded
• The remain 2 are distributed to the left and the right (1/3 of the initial
spacing = 8o) in such a way as to minimize leaf travel
• The MLC settings at the original angles are filled in by creating new MLC
positions by linear interpolation of the distributed segments.
• This results in segments every 8o = 45 control points for Machine
Parameter optimization
• More interpolated segments are inserted to achieve the user-selected
final spacing (generally 4o) these are used for dose calculation but are not
independently optimized
Machine Characterization
Flexible settings to allow support of multi-vendor VMAT
Settings should come from linac vendor’s documentation
We set these values based on defualts in Eclipse (RapidArc),
these are more conservative than the example values in the
earlier referenced paper by Bzdusek (table 3).
Other important machine setting
• Maximum MU setting
– 999 MU limit may prevent
optimization for some beams
• Allow beam MU to exceed
maximum should be used
(any beams above maximum
must be split prior to export)
• Varian sells a >999 MU upgrade – important for SBRT
MOSAIQ Setup
Small changes in machine characterization
• Rapid Arc machine
• Conformal Arc Machine
• IMAT (Gantry Index) = YES • IMAT (Gantry Index) = Yes
• VMAT (MU Index) = YES • No VMAT section
(allows conformal arc and VMAT)
• Allows conformal arc only
SmartArc tab in IMRT parameters
• Arcs/beam
• Final gantry spacing
(deg)
• Start/Stop angle
• Compute
intermediate dose
• Compute final dose
• Maximum delivery
time
• Constrain leaf motion
Pinnacle Smartarc IMRT objectives
• Same interface we
know and love
• “Constrain” checkbox
cannot be used with
SmartArc
• Objectives can now
be adjusted during
optimization
Comparison SBRT Case
Example RTOG 0915 34Gy x 1
•
•
•
•
Clinical SBRT (used)
11 beams – 2 non coplanar
Planning time (3 hours)
Total MU: 8548
Delivery time: 23 minutes
SmartArc (Planning Study)
• 2 Arcs coplanar
• Planning Time (30 minutes)
• Total MU: 7969
• Estimated delivery time
12 minutes
Comparison: SBRT Case
Clinical IMRT Plan
SmartArc Plan
Comparison: SBRT Case
Clinical IMRT Plan
SmartArc Plan
Comparison: SBRT Case
Clinical IMRT Plan
SmartArc Plan
Comparison: SBRT Case
Solid: Clinical Plan
Dashed: SmartArc
Both plans meet the RTOG 0915 Guidelines for 34 Gy in 1 fx
Patient setup for VMAT SBRT treatment
• Direct Tissue targeting should be used
– CBCT can target soft tissue
• Can be used with BH gating
• Cannot be used (in current form) with
dynamic gating (but neither can RapidArc)
– Fiducials
• Can be used with traditional gating or BH
How does VMAT affect PTV margins
• Random and Systematic components:
– Systematic
• Alignment of kV(CBCT) and MV
isocenters ( 1 mm)
• Random sampling nature of
initial sim
PTV
CTV
GTV
– Random
• Interpretation of daily CT
vs planning CT
• Patient motion during treatment
• Deformation of tumors/organs
• Uncertainties in respiratory
management
ITV
How does VMAT affect PTV margins
• Only the patient motion component is
affected:
– Shorter time between imaging and
treatment should allow shrinking of
PTV margins
– No data to demonstrate this for
lung SBRT
• Patient’s more willing to comply if
treatment time is presented as shorter
• It is possible that simpler
immobilization (or no immobilization)
could be used – simplify simulation
process
• This needs to be studied further
Alignment accuracy for SBRT
• Alignment of Isocenter of setup modality vs
treatment beams must be known
• Initial Verification
– End to end test
• Monthly QA
– Alignment check
(winston-Lutz type test)
• Daily QA
– Check for gross changes
( 2 mm level)
End-to-end check
• A phantom is scanned, sent to TPS, planned, sent to
R&V, sent to Linac, Imaged, treated.
• RPC credentialing for RTOG SBRT protocols requires
this test.
• This should be done for all patient orientations used
(head-first supine, head first prone, etc).
• This should be done prior to first patient treatment
Some type of end-to-end check should be done for any
setup system being used for SBRT patients
Phantom designed to check congruence of isocenters: MV, kV, CBCT, Light Field, OGP. Enables
end-to-end testing using treatment conditions.
Align using kV / CBCT to center on cerrobend
dot. Place Gafchromic film and treat SRT beams.
Scan the film to ensure treatment isocenter
matched the setup imaging techniques
Charles Mayo, Ph.D
Monthly check should also be done to verify nothing has changed
BB
Radionics
target
pointer
Radionics BB on a stick used to
check MV, kV and CBCT
isocenter
Radiation isocenter is
determined using the the center
of a MLC created shape imaged
at multiple angles
Software automatically
determines the accuracy of
the kV and MV imager and
the CBCT.
1. Du W, Yang J, Luo D and Martel M. A simple method to quantify the coincidence between portal
image graticules and radiation field centers or radiation isocenter. Med Phys. 2010; 37: 2256-2263.
Daily QA
• Daily QA
– Look for gross changes due to sudden
failures (or unexpected maintenance)
– Should have achievable (2 mm) accuracy
while not significantly slowing down the
morning warm-up procedure
Pre-treatment
patient specific QA
– Record and Verify
• All parameters are correct for all
fields
– Imaging
• Integrity check of data in 3D-3D
matching system
• Verification of reference images
for 2D matching
– Dosimetry
• Measurement based IMRT QA done,
reviewed and signed by physicist and
attending before the first fraction
Per fraction QA
• Physicist present (at a minimum) first day
• Attending present for all fractions
– required for billing
• MV portal images for isocenter
verification can serve as a final check
– Verification at the 3-5 mm level
– Checks that
• Data in the R&V system for the
isocenter was correct
• Couch shifts were done correctly
• Patient did not move during after the
CBCT
• Not gross error occurred in the entire process
SBRT OAR Guidelines
SBRT is an ablative procedure: Care must be taken to control
normal tissue toxicity
OAR Guideline should be:
1) A function of the number of fractions
2) Based on published guidelines based
on heterogeneous calculations when
ever possible
3) Lower than traditional fractionation
4) Updated based on follow-up
Example: RTOG 0915
34 Gy x 1
12 Gy x 4
Change in fractionation can cut in half the dose constraints
Respiratory Management with VMAT
• Methods of respiratory motion management
– Dynamic gating
– Breath-hold gating
(either voluntary or ABC)
– ITV (treating the track of
tumor motion)
• Abdominal compression is a
special case of ITV as it
reduces rather than
eliminates motion
Breath-hold gating
• Current RapidArc machines
– Can perform CBCT under a
series (2-4) of breath-holds
– Can deliver RapidArc under a
series of BHs
• Probably not recommended by
the manufacturer
• We were not able to measure
any dose differences between an
uninterrupted delivery and one
started and stopped many times
• This should be done with caution
ITV treatments
• Margins expanded to cover
track of tumor motion
• Based on 4DCT
• Interplay affects are minimal
(as pointed out in previous
talk)
• Expected to be lower than
for traditional IMRT due to
• Slow gantry rotation
• Smooth motion of MLC
SAMS Question 1
Which method of respiratory motion management can be
currently implemented with Rapid Arc
20%
20%
20%
20%
20%
1.
2.
3.
4.
5.
Tumor tracking using MLCs
Tumor tracking using couch motions
Gating during normal respiration
Breath hold gating or 4DCT based ITV targeting
No motion management is possible
10
SAMS Question 1
•
1.
2.
3.
4.
5.
Which method of respiratory motion management can be
currently implemented with Rapid Arc
Tumor tracking using MLCs
Tumor tracking using couch motions
Gating during normal respiration
4DCT based ITV targeting or breath hold gating
No motion management is possible
Answer: 4) 4DCT based ITV targeting or breath hold gating
Wilko F.A.R. Verbakel, WF,” Rapid delivery of stereotactic
radiotherapy for peripheral lung tumors using volumetric intensitymodulated arcs.” Radiotherapy and Oncology 93 (2009) 122–124
and
Measurements done at MDACC (not yet published)
SAMS Question 2
How can VMAT affect the PTV margins used for IGRT
20%
4.
It may allow PTV reductions due to the shorter
delivery time
It may require PTVs to be expanded due to the
smearing of dose by the rotation
It does not affect PTV margins as PTVs are a
function of setup techniques and immobilization and
are included in the planning process
The rapid falloff allows smaller PTV margins
20%
5.
The rapid falloff requires larger PTV margins
20%
20%
20%
1.
2.
3.
10
SAMS Question 2
How can VMAT affect the PTV margins used for IGRT
1.
It may allow PTV reductions due to the shorter delivery
time
2. It may require PTVs to be expanded due to the
smearing of dose by the rotation
3. It does not affect PTV margins as PTVs are a function
of setup techniques and immobilization and are
included in the planning process
4. The rapid falloff allows smaller PTV margins
5. The rapid falloff requires larger PTV margins
Answer: 3) It does not affect PTV margins as PTVs are a
function of setup techniques and immobilization and
are included in the planning process
Sonke, J.J. et al “Frameless Stereotactic Body Radiotherapy for
Lung Cancer Using Four-Dimensional Cone Beam CT Guidance”,
Int J. Radiation Oncology Biol Phys, Vol 74, No 2, pp 567-574,
2009
SAMS Question 3
Planning guidelines for SBRT in the lung
20% 1. Are the same as for traditionally fractionated therapy
20%
20%
20%
20%
2.
3.
4.
5.
Don't need to be evaluated due to the rapid dose falloff
from VMAT
Should be taken from RTOG 0236 (The initial SBRT
lung trial)
Can be reduced when rapid arc is used for treatment
delivery
Should be taken from published data calculated with
heterogeneity corrections and with the same number
of fractions
10
SAMS Question 3
Planning guidelines SBRT in the lung
1.
Are the same as for traditionally fractionated therapy
2.
Don't need to be evaluated due to the rapid dose falloff from rapid arc
3.
Should be taken from RTOG 0236 (The initial SBRT lung trial)
4.
Can be reduced when rapid arc is used for treatment delivery
5.
Should be taken from published data calculated with heterogeneity
corrections and with the same number of fractions
Answer 5) Should be taken from published data calculated with heterogeneity
corrections and with the same number of fractions
•
Ref: Videtic, G.M, Singh, A.K., Chang, J.Y, Quynh-Thi, L, Parker,W, Olivier, K.R.,
Schild, S.E., Bae, K. “RTOG 0915: A RANDOMIZED PHASE II STUDY
COMPARING 2 STEREOTACTIC BODY RADIATIONTHERAPY (SBRT)
SCHEDULES FOR MEDICALLY INOPERABLE PATIENTS WITHSTAGE I
PERIPHERAL NON-SMALL CELL LUNG CANCER”, Radiation Therapy Oncology
Group, Philadelphia, PA, 2009
Conclusions (Senan & Verbakel)
• Fast lung SBRT in <6.5 mins delivery time
– Total linac time (+ CBCT set-up) 20 minutes
– Superior OAR sparing possible
– Less chance for intrafraction motion
• No interplay effect between moving tumor
and moving leaves
• More time for appropriate and efficient
IGRT
Summary
• VMAT is a new technology that will require
new vigilance in QA
– Synchronization of dose rate controls, gantry
and MLC speed
– Thing we don’t know yet that may go wrong
• Hypo-fraction requires increased vigilance
in QA
– Tight margins
– Limited (or no) averaging of random setup
errors
– Limited opportunities to recover if an error is
caught mid-treatment
Final thoughts
• Hypo-fractionated radiation therapy
– Has the potential to do real
immediate harm to the patient
– May result in more severe morbidity
than traditional XRT
– Has little room for error
– Is more akin to surgery than XRT
and the same degree of care is
required
• VMAT and CBCT are
enabling technologies that
make hypo-fractionation
easier to implement
Acknowledgments
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Charles Mayo, Ph.D.
Lei Dong, Ph.D.
Weilang Du, Ph.D.
Milos Vicic, Ph.D.
Rebecca Howell, Ph.D.
Song Gao, Ph.D.
Ramaswamy Sadagopon, MS
Oleg Vassiliev, Ph.D
Karl Prado, Ph.D.
Issac Rosen, Ph.D.
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Kara Bucci, M.D.
Joe Chang, M.D., Ph.D.
James Cox, M.D.
Thomas Guerrero, M.D., Ph.D.
Melinda Jeter, M.D., M.P.H.
Ritsuko Komaki, M.D.
Zhongxing Liao, M.D.
Mary Frances McAleer, M.D,
Ph.D.
• James Welsh, M.D.