Motion Management Strategies
Gig S Mageras, PhD, FAAPM
Peng Peng Zhang, PhD
Memorial Sloan‐Kettering, New York
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Disclosure
Support from:
• NIH/NCI award R01 126993
• Varian Medical Systems
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Outline
Topics in this talk:
• Focus on intra‐fx motion (respiratory & prostate)
• Effects of motion on RT
• Motion mitigation in TP process
• Studies of comparative motion management strategies
Additional topics in chapter:
• Types of motion management
• Real‐time imaging: achievable accuracy
• Residual uncertainties w current motion management
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EFFECTS OF MOTION ON RT aapm ss/mageras
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Motion affects CT
• Standard CT appears distorted: scan times long wrt respiratory motion time scales
Chen SRO 2004:
• Max object lengthening or shortening ~ motion extent
NSCLC example
“Free breathing” CT
• Max displacement of object center ~ ½ motion extent
Static
Φ = π/4
Φ = 5π/4
Chen 2004
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Motion affects CBCT
• Motion during ~1 min acquisition  image blurring,
streaking, reduced tissue boundary visibility
Respiration
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Peristalsis
(figure courtesy Michael Lovelock)
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Motion during radiation delivery
• “Blurs” dose, broadens dose gradients
• Interplay effect with moving MLC: – Significant over single field/fx
– Similar to physical compensator over multiple fields/fx
planned profile **** delivered profile, peak-trough motion 10mm
Breast IMRT
(Chui 2003)
Single 2Gy fx
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Average over 30 fx
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Motion affects on particle beams
Proton treatment of liver:
Equivalent path length changes
(Lu 2007)
Interplay with scanned
particle beams
(Bert 2008)
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Motion in prostate:
Intra‐fx motion trajectory from EM tracking (Calypso)
LAT
Case 1
Case 3
SI
AP
Case 2
Case 4
(Data courtesy Dennis Mah)
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MOTION MITIGATION IN THE TREATMENT PLANNING PROCESS:
RESPIRATORY SITES
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Respiration‐correlated CT: • Cine acquisition (Low 2003, Pan 2004)
• Acquisition > 1 resp. cycle each couch position
• Retrospective image sorting wrt resp. phase
Abdominal
displacement
Pan et al 2004
• Helical acquisition (Ford 2003, Vedam 2003, Keall 2004,
Fitzpatrick 2006)
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Internal target volume
• ICRU 62:
– Internal Margin (IM): Variations in size, shape, and position of CTV relative to anatomic reference points; e.g., movements of respiration
– Internal Target Volume (ITV): Volume encompassing the CTV and IM (ITV = CTV + IM)
• Common approach: ITV encompasses GTVs in RCCT, then margin added for CTV
• Commonly available s/w tools:
• Loop through RCCT images while
delineating ITV
• Max intensity projection: each voxel set
to max CT# for that voxel in RCCT set
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(Underberg, 2005 )
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Example clinical process: Gated treatment of pancreas at MSKCC (1/3)
• Fiducials: implanted markers or stent
• Plan CT: BH helical CT + IV contrast
• RCCT: cine scan + record respiration, 20 repeat img/position, phase sorted GE Discovery 8-row PET/CT, Varian RPM
Slide courtesy Karyn Goodman
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Gated treatment of pancreas at MSKCC (2/3)
•
•
•
•
Select gate from GTV motion, usually 30‐70%
GTV/CTV delineation on plan CT
ITV delineation: loop over in‐gate RCCT images
Lung HFx: ITV includes all RCCT images
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70
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Gated treatment of pancreas at MSKCC (3/3)
• Fiducials delineated in RCCT: – At gate onset  DRR, compare with radiographs
– In‐gate ITVfid  DRR, compare with fluoro
DRR
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Radiograph
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Uncertainties related to RCCT (1/3)
• RCCT phase sorting: amplitude variations cause discontinuity artifacts (Rietzel 2005, 0%
phase
Lu 2006, Fitzpatrick 2006, Abdelnour 2007)
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Artifacts in phase-sorted RCCT
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Uncertainties related to RCCT (2/3)
• Deviations in GTV volume/shape caused by RCCT artifacts (Persson 2010)
– Variation in lung GTV larger than delineation error
– Variation largest in bins adjacent to end inspiration
8 patients, 2 delineations/pt
EE
Bin 4
(EE)
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Uncertainties related to RCCT (3/3)
• RCCT is a single sample:
– Breathing pattern may vary (Purdie 2006, Britton 2007)
– Tumor baseline variation (Sonke 2008)
– Tumor size & shape changes (Kupelian 2005)  Motion estimates require verification at treatment
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Uncertainties in ITV definition (1/2)
• Max intensity projection from RCCT may underestimate lung tumor extent (left)
• Alternatives:
– Verification of GTV on each RCCT image (middle)
– DIR + propagation of GTV contours (right)
MIP
MIP + GTV verification in RCCT
Union of GTVs
(Ezhil 2009)
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Uncertainties in ITV definition (2/2)
• Gating window ITV can be underestimated in RCCT with irregular breathing
– 2D dynamic MRI, lung ca, n=8
– “RedCAM” = simulated RCCT from dMRI
– “dSGP” = moving phantom tumor using dMRI trajectories
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(Cai 2010)
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Motion‐encompassing ITV may overestimate internal margin Dosimetric analysis: Internal margin < implied by motion extent – Wolthaus IJROBP 70:1229, 2008
– Mutaf IJROBP 70:1561, 2008
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Incorporating respiratory motion into dose calculation
• Enables evaluation of dose to CTV & normal tissues • Makes use of RCCT + deformable image registration:
– Dose calculation on each 3D image in RCCT set
– DIR between each image & selected ref image
– Dose grids warped to ref image & summed aapm ss/mageras
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Studies of cumulative dose (1/3)
(Admiraal, RO 86:55,2008) • Stage I NSCLC n=10, ITV from MIP, PTV=ITV, dose calc on avg CT
• Small differences between PTV static dose & CTV cumulative dose
Static dose
Cum. dose
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PTV
CTV
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Studies of cumulative dose (2/3)
(Wu, MP 35:1440, 2008) • SBRT liver n=5, ITV=union of DIR‐prop. GTV, PTV=ITV
• CTV D95/D99 <static PTV in 2/5 cases
• Small motion effects on normal liver, kidneys
8 mm motion
Static
Cum.
Dose difference, Pt 5
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Studies of cumulative dose (2/3)
(Starkschall, IJROBP 5:1560, 2009) • Stage III NSCLC n=15, 0‐2cm, ITV = union of rigid‐propagated CTVs, PTV=ITV+(5‐to‐10 mm), model‐based DIR, 3DCRT/IMRT
• 6/15 cases CTV D99 differs >3%, no cases >5%
• Comparison cum. dose to ITV not assessed
• Negligible effect on normal organs
Small motion
Large motion
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Uncertainties related to cumulative dose
• DIR inaccuracies  dose mapping errors
– Self‐consistency errors
– Absence of landmarks, RCCT artifacts
– Tumor growth/regression
TP dose distribution
SD inconsistency in DIR vectors
Dose SD error
(Salguero MP 38:343, 2011)
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MOTION MITIGATION IN THE TREATMENT PLANNING PROCESS:
PROSTATE
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Intra-fraction prostate motion modeling and
dosimetric analysis (Hossain et al, 2008)
• Motion tracking
• Single fraction prostate SBRT, 50‐70 min
• Record intrafraction motion via Cyberknife dual x‐ray system
• Track fiducial markers
• Motion modeling
• Displace isocenter using marker trajectory
• Recalculate dose
• Investigate the dosimetric effects of
intrafraction motion • Finding:
• DVH shows clinically important changes when motion > 5mm
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Paradigm for dosimetric action levels in hypofractionated prostate VMAT
(Zhang 2011)

Hypofractionated prostate RT benefits from
VMAT due to shortened Tx time

Intra-fraction motion management: how to
intervene?
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Motion in prostate:
Intra‐fx motion trajectory from EM tracking (Calypso)
LAT
Case 1
Case 3
SI
AP
Case 2
Case 4
(Data courtesy Dennis Mah)
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Characteristics of motion trajectories
Trajectories of EM transponder centroid in prostate
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(Zhang 2011)
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Cumulative dose from trajectories
180
D j   Dij ( MLCi , D i , vo  vi )
i 1
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(Zhang 2011)
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Effect of intra‐fraction motion on Tx plan
Static plan:
CTVD95% = 95%
Static plan:
UDmax = 99.3%
Static plan:
RWD1cc = 86.5%
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Static plan:
BWD1cc = 98.8%
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Setting Action Thresholds
Patient specific
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(Zhang 2011)
Generic 2mm
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Sensitivity and specificity analysis
Plan
TP = dose violation
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Plan
TN = no violation
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STUDIES OF COMPARATIVE MOTION MANAGEMENT STRATEGIES aapm ss/mageras
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Cumulative lung dose for different motion management strategies • NSCLC, T1N0‐T2N3, n=18 (Hugo 2009)
• Compare – Breath hold at end inspiration,(BH) – target tracking with aperture,(TT) – mid‐ventilation aperture (MVA)
ΔMLCTT 4%/5mm
• RCCT, DIR & dose accumulation
• Obs. differences in MLD ≤1Gy for excursions ≤13mm
13mm excursion
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ΔMLCBH 5%/5mm
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Target tracking vs RC inverse planning
(Zhang 2008)
• Lung ca, n=4, 10‐30mm excursion
• Compare
– Target tracking: static IMRT plan each RCCT
– RC inverse plan: optimized using motion PDFs/cum. dose
29mm excursion, cumulative dose
70 Gy
68
66
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Target tracking 4D inverse planning
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Summary
Respiratory targets:
• Motion affects imaging, radiation delivery
• RCCT + motion encompassing ITV widely used
• Uncertainties:
– Irregular breathing introduces artifacts in RCCT, affects tumor size/shape, hence ITV
– Max intensity projection may underestimate lung tumor motion extent
– RCCT represents single sample but breathing is variable  motion requires verification at tx
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Summary
Respiratory targets, cont’d:
• Cumulative dose:
– CTV cumulative dose similar to static ITV dose for early stage lung, less clear for advanced stage
– Small effect on normal tissue dose, relative to static case
– Cum. dose calc cumbersome at present
• When motion extent< ~15mm, motion mitigation in TP is effective, provided mean target position is controlled
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Summary
Prostate:
• Motion also affects imaging, dose delivery
• Real‐time motion tracking (EM, radiographic) means of assessing effect on TP, motion mitigation strategies
– Studies have used population motion data
– Pt‐specific motion data feasible at simulation aapm ss/mageras
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Analyze dose end points vs. motion CTV D95%
SI mean
SI max
AP mean
AP max
LAT mean
LAT max
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CTV Dmin
6.5Gyx5
Rectum wall Dmax
7Gyx5
Rectum wall D1cc
7.5Gyx5
Bladder wall Dmax
8Gyx5
Bladder wall D1cc
8.5Gyx5
Urethra Dmax
(Zhang 2011)
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Comparisons of two intrafraction motion
management strategies (Su et al, 2010)
• Study design
• 17 patients
• Setup and intrafraction monitoring using Calypso
• Intervention strategy I
• Interrupt Tx whenever motion > action threshold=3mm
• Margin required: LR=1.1mm, SI=1.8mm, and AP=2.3mm
• Intervention strategy II
• Automatically reposition pt at time intervals of 2 min
• Margin required: LR=0.5mm, SI=1mm, and AP=1.5mm
•Strategy II substantially increases #interruptions, and treatment time (factor of 8)
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