Post on 14-Feb-2019
(Un-)Certainties in SABR
Johan Cuijpers PhD, VU University medical centre, Amsterdam.
Disclosures
• No personal disclosures • VUmc has a master research agreement with Varian
Medical Systems
Content
• (Un-)Certainties in SABR for lung – Pre-Treatment Imaging
– 4DCT
– Treatment Planning & Dosimetry – Dealing with motion – Dose calculation accuracy – Plan summation – Multiple lesions
– Image Guidance
– Setup accuracy – 6D corrections
Pre-treatment Imaging
• 4DCT – One breathing cycle per table
position is acquired! – Artefacts due to finite time
resolution CT-scanner – Variability of breathing pattern
Imaging Artefacts 4DCT
0% bin
50% bin
20% bin
70% bin
Amplitude variability during 4DCT
-0,5
0
0,5
1
1,5
2
0 20 40 60 80 100 120 140
Ampl
itude
RPM
(cm
)
Time (s)
smallseries
largeseries
VUmc 4DCBCT protocol
Acquire full length 4DCT
Acquire 2nd short 4DCT
Acquire 4D-CBCT
Is Quality of 4DCT OK? • phase errors • artefacts 4DCT
Send data to TPS
Treat Adapt plan
Quality of short 4DCT OK?
Tumor motion as in 4DCT?
yes no
yes no
yes no
Plan
•8x2.5mm •10 bins •At least 10 images per breathing cycle •Time resolution 1/10 of breathing cycle
4D CBCT
8
Content
• (Un-)Certainties in SABR for lung – Pre-Treatment Imaging
– 4DCT – Treatment Planning & Dosimetry
– Motion management – Dose calculation accuracy – Plan summation – Multiple lesions
– Image Guidance – Setup accuracy – 6D corrections
Dosimetric margins for breathing motion
• Respiratory motion leads to penumbra blurring • Penumbra blurring can be compensated by increasing field size
– asymmetric margins around mid-position – symmetric margins around mean tumor position
• Dosimetric margins are smaller than ‘ITV’ margin (=½ App)
Uncertainties – Blurring depends on
– motion pattern – the steepness of the penumbra – PTV size
Dosimetric Margin versus amplitude
0
2
4
6
8
10
12
14
16
18
0 5 10 15
SD
Mar
ge
80 long95 long80 vherk95 vherk
Multi Institution assessment of accuracy of ITV
Hurkmans et al, Int J Radiat Oncol Biol Phys. 2010 Oct 13
ROSEL Trial
Multi Institution assessment of accuracy of Mid-V
Hurkmans et al, Int J Radiat Oncol Biol Phys. 2010 Oct 13
ROSEL Trial
Negative Margin Relative to ITV
Cuijpers et al: Radiother Oncol. 2010 Dec;97(3):443
Reduction ITV Expiration (cranial) side: -0.2 App + 1.3 (mm)
Inspiration (caudal) side: -0.3 App + 2.2 (mm)
-12,0
-10,0
-8,0
-6,0
-4,0
-2,0
0,0
0 5 10 15 20 25 30 35 40
mIT
V80
(mm
)
App (mm)
Expiration
Inspiration
Amplitude Monitored Treatment Delivery
• Verification on amplitude using Varian RPM
• Gated therapy with: – Gating window is set to full amplitude range – RPM system guards amplitude during treatment – If breathing amplitude during irradiation is larger than during
CT, beam holds
CT and CBCT with similar breathing
CBCTs with different breathing
Treatment planning @ VUmc
• Dose prescription SABR @ Vumc – Prescription dose
– 3x18 Gy – 5x11 Gy – 8x7.5 Gy
– Normalized on 80% – Coverage PTV: V80% >99% – Dmax PTV > 100% (>130% of prescription dose)
• Use Average Intensity Projection for dose calculation • RapidArc with 2 arcs (CW, CCW) • 10 MV FFF beam • Avoidance sector to spare the contralateral lung
Ave-IP good approximation
(Un-)Certainties due to small field dosimetry
1. Static Field • 1 x 1 cm2 – 3 x 3 cm2
a) Dose Deviation in center
With courtesy to Wilko Verbakel
Absolute output Eclipse
Small lesion in low density lung tissue
4400 5400 6400 7400
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
4400 5400 6400 74000
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
4400 5400 6400 7400Dose (cGy)
4400 5400 6400 7400
Dose (cGy)
4400 5400 6400 7400
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
4400 5400 6400 7400Dose (cGy)
4400 5400 6400 7400Dose (cGy)
4400 5400 6400 74000
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
9.3 cm3
7.1 cm3
1.6 cm3
11.4 cm3
5.5 cm3
2.6 cm3
3.9 cm3
9.3 cm3
PTVs 3x18
AAA versus Acuros
With courtesy to Miguel Palacios
4400 5400 6400 7400
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
4400 5400 6400 74000
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
4400 5400 6400 7400Dose (cGy)
4400 5400 6400 7400
Dose (cGy)
4400 5400 6400 7400
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
4400 5400 6400 7400Dose (cGy)
4400 5400 6400 7400Dose (cGy)
4400 5400 6400 74000
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
9.3 cm3
7.1 cm3
1.6 cm3
11.4 cm3
5.5 cm3
2.6 cm3
3.9 cm3
9.3 cm3
PTVs 3x18
-865
2.76 cm
With courtesy to Miguel Palacios
AAA versus Acuros
4400 5400 6400 7400
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
4400 5400 6400 74000
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
4400 5400 6400 7400Dose (cGy)
4400 5400 6400 7400
Dose (cGy)
4400 5400 6400 7400
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
4400 5400 6400 7400Dose (cGy)
4400 5400 6400 7400Dose (cGy)
4400 5400 6400 74000
20
40
60
80
100
Dose (cGy)
Vol
ume
(%)
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
AAAAcuros
9.3 cm3
7.1 cm3
1.6 cm3
11.4 cm3
5.5 cm3
2.6 cm3
3.9 cm3
9.3 cm3
PTVs 3x18
-790
0.6 cm
With courtesy to Miguel Palacios
AAA versus Acuros
Acuros vs AAA
How to deal with Multiple lesions?
• Preferably in one optimization using single isocentre • However: the human body is not a rigid body
– Multiple isocentres necessary for independent setup on each lesion
– Dose distributions per lesion have mutual influence
• VUmc/London Ontario flow chart
Flowchart: H. Tekatli et al (submitted)
With courtesy to Hilal Tekatli
Single isocentre
With courtesy to Hilal Tekatli
Multiple iso-centres
With courtesy to Hilal Tekatli
(Un-)Certainty in previously delivered dose
• Accounting for previously delivered dose @VUmc
– Rigid 3D registration of old and new pCT – Reconstruct plan on new pCT and recalculate dose – Convert isodose lines to volumes for optimization
– With pitch/roll – Use 6D rigid registration (Velocity) – Convert isodose lines to Volume – Use this volume in optimization in Eclipse
Uncertainties
- Deformations - Changes in Anatomy
Including previous treatments
Including previous treatments
Including previous treatments
Content
• (Un-)Certainties in SABR for lung – Pre-Treatment Imaging
– 4DCT – Treatment Planning & Dosimetry
– Dealing with motion – Small field dosimetry – Inhomogeneities – Plan summation – Multiple lesions
– Image Guidance – Setup accuracy – PTV margins
Setup Uncertainties
State-of art practice in SABR is on-line soft tissue matching using CBCT
Treatment Margins determined by: On-line setup accuracy CBCT Roll / Pitch / Yaw errors Deformations Intrafraction variability
Intra-fraction motion: VUmc data •Intra-fraction motion measured by repeat CBCT
–Supine position
–Two (or one) arms above the head
–Mattress for patient comfort
–Knee cushion
–No further immobilization
–Fast RapidArc™ treatment
AP (mm) SI (mm) LR (mm)
Novalis Tx
(6D)
39 pats,193 fractions
Spine 0.8 0.8 1.0
ITV 1.7 1.4 1.1
True Beam (FFF)
32 pats,140 fractions
ITV 1.4 1.3 1.2
1 SD of intra-fraction motion
Total variance
Components of random variation in ITV positioning (1 SD).
AP (mm) SI (mm) LR (mm)
Intra-fraction variability 1.7 1.4 1.1
CBCT+couch accuracy 0.5 0.5 0.5
Inter-observer variability 0.5 0.5 0.5
Total variance 1.8 1.6 1.3
Correction of Pitch & Roll positioning error
Pitch Roll ExacTrac
X-ray correction
mean 0.97 -0.07
SD 1.12 0.88
Residual error on CBCT
mean -0.13 0.07
SD 0.51 0.55
Before and after Exactrac robotics couch corrections 39 patients, 193 Treatment fractions
y = -0.36 xR2 = 0.28
-5
-4
-3
-2
-1
0
1
2
3
4
5
-5 -4 -3 -2 -1 0 1 2 3 4 5
roll ET
roll
CB
CT
y = -0.074 xR2 = -0.022
-5
-4
-3
-2
-1
0
1
2
3
4
5
-5 -4 -3 -2 -1 0 1 2 3 4 5
pitch ET
pitc
h C
BC
T
Residual Pitch error Residual Roll error
Correction for roll&pitch
PTV margin in case of hypo fractionation
• For hypofractionated treatments setup errors are to be treated as systematic errors
– ‘Random part’ vHerk margin recipe 2.5Σ+0.7σ not applicable
• Statistical probability of getting prescription dose in GTV
PTV margin
Σ=1.5mm 1 3 5 8
2.5*Σ 3.8 mm 0.9 (0.9)3 (0.9)5 (0.9)8
2.8*Σ 4.2 mm 0.95 (0.95)3 (0.95)5 (0.95)8
3.4*Σ 5.1 mm 0.99 (0.99)3 (0.99)5 (0.99)8
X
PTV margin
Σ=1.5mm 1 3 5 8
2.5*Σ 3.8 mm 0.9 0.73 0.59 0.43
2.8*Σ 4.2 mm 0.95 0.86 0.77 0.66
3.4*Σ 5.1 mm 0.99 0.97 0.95 0.92
Radiobiological margin
However: -Dose outside PTV is not equal to zero -Small underdosage PTV is not very detrimental for TCP Selvaraj et al, Med. Phys. 40 (2013) - MC simulation of setup errors - less than 1% TCP loss calculation
Σ=1 mm, σ=3 mm
In Conclusion
• Uncertainties exist in all parts of the SABR process – 4DCT – Motion management & Target Volume definition – Treatment planning – Treatment delivery
• An appropriate level of certainty can be obtained by
– Appropriate 4DCT protocols – Appropriate motion management margins & verification – Modern calculation algorithms (with special attention to small
field dosimetry) – Properly chosen PTV-margins
Thank you for your attention
Q4 2017: VUmc SABR/SMART symposium www.sbrt.eu