Outlines
Establishing SBRT Program:
Physics & Dosimetry
‰ Illustrate the difference between SBRT vs. CRT
‰ Introduce the major procedures in SBRT:
Lu Wang, Ph.D.
¾
Simulation while assessing target motion;
Radiation Oncology Department
¾
Target definition: how to account for motion;
Fox Chase Cancer Center
¾
Treatment planning consideration;
¾
Treatment delivery with imaging guidance.
‰ Present physics and dosimetry issues involved.
SBRT
CRT
Sim
Sim
Target
Definition
Target
Definition
Treatment
Planning
Treatment
Planning
Treatment
Delivery
Treatment
Delivery
SBRT - Simulation
Sim
Stereotaxy?
Stereotaxy
Patient
Immobilization?
Patient Motion
Assessment?
Body
Body Localizer
Localizer
Localizer
rods
Body frame
Tumor
Origin
(0,0,0)
Body pillow
(0,0)
(0,0)
Indexing laths
Isocenter
Patient
Stereotaxy refers to a 3D superposition of a fixed coordinate
system upon a given organ.
clamp
Carbon fiber base board
SBL provides a rigid reference for patient setup and re-alignment, as well as assists image fusion.
1
Stereotaxy
3D Images Provide Stereotaxy
in Extracranial Sites
Y
Localizer
rods
Tumor
(0,0)
Origin
(0,0,0)
Bony landmarks (x, y, z)
(0,0)
Isocenter
X
Z
Conventional vs SBRT
* Slide courtesy of Brian Kavanagh / University of Colorado
Motion Assessment
SBRT must be based on 3D images !
Patient Immobilization
¾SBRT simulation begins with patientpatient-specific mold making
and immobilization of patient.
4DCT Simulation
SBRT requires a determination of patient-specific internal
target volume (ITV)
What imaging procedures to use in order to determine the ITV?
2
Multiphase CT Scans & Image Fusion
Frame
Frame registration
registration
helps
helps to
to contrast
contrast the
the
tumor
tumor motion
motion
Scans taken at:
•Inspiration breath hold
• Expiration breath hold
• Normal breathing
Establish stereotaxy
for target localization
Composite Target or ITV
Motion Study with MRI Cine
fiducial
tumor
P
Three targets from normal breathing, inspiration and expiration breath hold
scans. Due to motion artifact, the target on the normal breathing appears to have
two separate targets (green).
Motion Comparison
A
tumor
SBRT – Target Definition
3 CTs vs. MRI
AP direction
PTV
MRI cine
3 CT scans
30
Motion (mm)
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10
Patient #
SI direction
MRI cine
3 CT scans
3 CT scans
30
Patient-specific ITV
is defined?
Other uncertainties
have been determined?
25
20
Motion (mm)
Motion (mm)
25
MRI cine
LAT direction
15
10
5
20
15
10
5
0
0
0
1
2
3
4
5
Patient #
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
Patient #
3
ITV Definition
4D MIP Approach:
Other Uncertainties
3 Multiphase CT Approach:
Inspiration
Normal breathing
¾ Whether using 3D imaging guidance?
If yes, how much uncertainty is associated with the
imaging system?
¾ Can setup error completely be corrected by imaging
guidance?
If not, what is the residual error (image resolutions,
..etc)?
expiration
Patient specific ITV
Volume Comparison
Comparison
Comparison of
of Target
Target Definitions
Definitions
Parameter
3D standard
3 Multiphase CT
4D Union
Planning CT
scan
FreeFree-breathing sliceslicebased
Max. inspiration and
expiration breath holding
+free+free-breathing CTs
4D CT
(or MIP)
GTV or ITV
Standard GTV
GTV_free U GTV_insp U
GTV_exp
GTV1 U GTV2
U … U GTVn
GTV+0.0 cm
ITV+0.0 cm
ITV+0.0 (or
0.3) cm
CTV+0.5 (cm) setup +
CTV+0.3CTV+0.3-0.5cm (setup)
CTV+0.3CTV+0.3-0.5
cm (setup)
CTV
PTV
ITV
I T V V o lume C o mp ar i so n
35
ITV_4D
population based tumor motion
30
ITV_3CT
GTV_conv
25
20
15
10
GTV_insp
GTV1
5
.
Range of
tumor
motion
.
GTV_norm
GTV_free
0
…
1
2
3
4
5
6
7
8
9
10
GTV_exp
Pat ient
Volume Comparison
SBRT – Treatment Planning
PTV
Treatment
Planning
PT V C o mp ariso n
80
70
PTV_4D
60
PTV_3CT
PTV_conv
50
40
What beam
margin?
30
Beam arrangement
and energy?
Target
conformality?
20
10
0
1
2
3
4
5
Pat ient
6
7
8
9
10
What dose fall-off?
How much dose
inhomogeneity?
Co-planar or
non-coplanar
PITV index
and 50% IDL
More underline issues maybe exist: such as commissioning for small fields
4
Dose Fall-Off
Typical Margins Used
SBRT
™Various margins were reported: from 0 – 1.0 cm
CRT
™It depends on the dose inhomogeneity desired.
™ It also depends on the machine to be used for treatment
planning – such as Cyberknige - may use multiple
isocenters – thus, the dose inhomogeneity is high.
90%
60%
MC Investigation of Block Margin
Non-coplanar Beam Arrangement
25
3
Density_lung = 0.10 g/cm
No suggestion
for block margin, …
V20 of two lungs (%)
Density_lung = 0.25 g/cm
20
Density_lung = 0.50 g/cm
3
c
3
15
10
b
a
5
0
0
2
4
6
8
10
12
14
16
18
20
Beam margin (mm)
Coplanar Beam Arrangement
Energy Effect in Patient
15 MV- MC
6 MV - MC
90
70
60
50
40
30
10
Wang et al J. Appl. Clinc Med. Phys. (2002)
5
Energy Effect in Patient
Target Conformality
Different protocol has different specifications. E.g. RTOG
0915 Requires:
¾ PITV is close to unit as much as possible (1.2 – 1.5)
¾ 50% line also should follow the shape of the target – the
distance from the prescribed IDL to the 50% IDL < 2.0 cm
¾ The prescription isodose surface must be ≥ 60% and <
90% of the maximum dose (normalized to the maximum
dose point).
¾ High dose spillage requirements: - the cumulative volume of
all tissue outside the PTV receiving a dose >105% of the prescription
dose should be no more than 15% of the PTV volume.
Wang et al J. Appl. Clinc Med. Phys. (2002)
……
SBRT – Treatment Delivery
Linear Accelerators with features
especially suitable for SBRT
Treatment
Delivery
Delivery
System?
Adopted system?
Specialized SRT:
Imaging
Guidance?
Gating or
Tracking?
Bony alignment or
soft tissue alignment?
Is surrogate moving
in phase of the target?
Novalis, Cyberknife, Trilogy?
On Board Imaging – Cone Beam
Cone Beam CT Guidance
after bony alignment
6
Cone Beam CT Guidance
Image
Image Guidance
Guidance using
using CT-on-Rails
CT-on-Rails
after soft tissue alignment
PRIMART
Siemens PRIMARTOM in a treatment room.
Treatment Setup
Image
Image Guidance
Guidance using
using CT-on-Rails
CT-on-Rails
Treatment setup based on a
stereotactic box to establish the
rigid relationship with the
machine reference.
3-D Target Localization with
a SOMATOM CT
Gantry Moves During
Slice Acquisition
CT Gantry Retracts
Prior to Treatment
Zero the table coordinate
Carbon Fiber
Tabletop
Rails
PRIMART
PRIMART
ZXT
Daily Target Localization: Bony
Landmarks Shifts
Localizer
rods
Tumor
(0,0 (0,0)
)
Isocenter
CT Gantry Retracts
Prior to Treatment
Localizer
rods
Tumor
Bony landmarks (x, y, z)
SOMATOM Sliding Gantry
Patient Relocalization for Treatment
New coordinates
from Treatment CT
Coordinates from SIM CT
Origin
(0,0,0)
ZXT Table
SOMATOM Sliding Gantry
Origin
(0,0,z)
(0,0
)
Bony landmark (x’, y, z’)
PRIMART
∆X = X’ – X, ∆Y=Y’ – Y, ∆Z = Z’ -Z
ZXT
SOMATOM Sliding Gantry
Shift the table if necessary
7
Physics & Dosimetry Issues
™Small field dosimetry - beam commissioning for small
fields (output factor, profile…etc.)
Summary: SBRT Requirements
¾ Higher confidence in tumor targeting and
appropriate accounting of internal organ
movement.
™ Quality assurance of imaging guidance system – known
the accuracy and limitations
¾ Reliable mechanisms for generating focused,
sharply delineated dose distributions
™ Using IMRT vs. conformal approach – is there any
benefit in using IMRT and when to use it?
¾ Reliable accurate patient positioning accounting
for target motion related to time dependent organ
movement
SUMMARY:
SBRT Physics and Technology
™
™
™
™
™
CT simulation:
simulation: Need assess tumor motion
Immobilization:
Immobilization: Minimize motion, breathing effects
Planning:
Planning: Small field dosimetry considerations
Repositioning:
Repositioning: High precision patient setset-up.
Relocalization:
Relocalization: Identify tumor location in the treatment
field:
Thank you!
* MV/ kV xx-ray, implanted markers and/or setset-up fiducials
* Motion tracking and gating systems
* RealReal-time tumor tracking systems with implanted markers
™
Treatment delivery techniques
* Adapted conventional systems
* Specialized SRT: Novalis,
Novalis, CyberKnife,
CyberKnife, Trilogy…
Trilogy…
8