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Physics
Sunday, March 4, 2018
9:30 a.m. – 10:00 a.m.
Social Q&A
Use your phone, tablet, or laptop to
➢ Submit questions to speakers and moderators
➢ Answer interactive questions / audience response polls
astro.org/RefresherSocialQA
Faculty DisclosuresFaculty and Committee disclosures are also on the 2018 ASTRO Annual Refresher Course website.
Name Employment Funding Sources Ownership or Investments
Leadership
Laurence Court, PhD MD Anderson Cancer Center
None None None
Physics: MRI-guided Radiation Therapy
Laurence Court, PhD
University of Texas MD Anderson Cancer Center
Houston TX
Disclosures
• Employer: UT MD Anderson Cancer Center
• Grants from: NCI, CPRIT, Varian, Elekta, Mobius
Acknowledgements
• Many slides from Geoff Ibbott, Carri Glide-Hurst, Dave Fuller, Ashley Rubinstein, Gabriel Sawakuchi, Daniel O’Brien, Bas Raaymakers, Elekta and ViewRay
Learning Objectives
• To be able to describe the different approaches to MRI-guided radiation therapy
• To be able to discuss some of the dosimetric challenges with MRI-guided radiation therapy (treatment planning, physics QA…..)
Contents
• Why MR-guided RT?
• What does MR-guided RT look like?
• What is the impact of the magnetic field on distributions?
• What ways are there to mitigate this?
• Anything else to worry about (radiation biology experiments)
• Some physics QA challenges
• How many patients have been treated?
• Summary
Why MR for RT?Very brief introduction to what MR can offer us
What clinical benefits could MRbring to radiotherapy?
Slide from Dave Fuller
• Fundamental advantage:– Simultaneous imaging of not only
anatomy, but of functional and spatial/motion of both tumor and normal tissue over time
• Ultimately, we want data that is:– Anticipatory (predictive/early)
– Actionable (changes care)
– Accurate (in time and 3D space)
– Additive (more than 1 feature/function)
Why should the future be MR?
Slide from Dave Fuller
DiagnosisStaging
SimulationTreatment Planning
Tx DeliveryOn-Line Adaption &
Tx Assessment
Off-line Response
Assessment
Role of MRI is growing in Radiation OncologyExpansion to Treatment Time imaging
MR Images courtesy of Philips
MR ScannerSequences and Post-processing S/W
MR ScannerMR Scanner w/ MR-RT Oncology Configuration
Treatment Planning S/W with MR support
MRI Guided Radiation therapy
G. Ibbott, RSNA, Chicago, 2017
MRI-guided RadiotherapyIntroduction to in-room MR-guided RT
Concept of MRI acceleratorAccelerator
MLC
beam
• Simultaneous MRI and irradiation• To do this:
1. Mount the Linac on a rotatable gantry around the MRI magnet ➢ The radiation isocentre is at the centre of the MRI imaging
volume
2. Modify the linac to make it compatible with the MRI
3. Modify the MRI system to➢Minimise material in the beam path and ensure it is
homogeneous ➢Minimise magnetic field at the Linac
• Technical issues• Magnetic interference• Beam absorption• RF interference
Raaymakers et al. PMB 2009 Based on a slide from Elekta
• 15cm central region free from coils
• 8cm Al eq
• Active magnetic shielding (pair of shield coils with opposite polarity)
Raa
ymak
ers
et a
l. P
MB
20
09
Impact of magnetic field on dose distributions
17
Point dose kernels with and without a magnetic field
Raaijmakers et al. PMB 2008
Dose deposition in a magnetic fieldThe Electron Return Effect (ERE)
γ
γ
e-
e-
γ
e-
B = 0
γ
γ
e-
e-
γ
e-
B = 1.5 T
Ra
aijm
ake
rset
al.
PM
B 2
00
8
Dose perturbation effects0 T 1.5 T
0 1 2 3 4 5 6 7 80
20
40
60
80
100
B = 1.5 T
B = 0 T
Depth (cm)
Re
lative
Do
se
(%
)
Raaijmakers et al, Phys Med Biol, 2008
If field covers whole phantom…..R
aa
ijma
kers
et
al, P
hys M
ed
Bio
l, 2
00
8
Impact of surface orientation
Raaijmakers 2007
Varying exit angle
Raaijmakers 2007
lungsoft
tissue
1.5T B-field
6 MV beam
soft
tissue
23
Magnetic-field-induced dose effects in lung
Magnetic-field-induced dose effects
lungsoft
tissue
soft
tissue
Raaijmakers et al, PMB, 2008
24
Rubinstein et al, Med Phys, 2015
8 MV
Beam
25
Ways to mitigate the impact of the magnetic field on the dose distributions
Mitigating dose perturbations
• Magnetic field strength
• System geometry
• Treatment planning
27
28
Princess Margaret Hospital - MR on Rails
G. Ibbott, RSNA, Chicago, 2017
Impact of magnetic field strength
Lower magnetic field strengthViewray MRIdian: Three Co-60 sources and a 0.35 T MRI
30
Dan Low, MRI Guided Radiotherapy, 2017
www.viewray.com
Wooten et al, IJROBP 92, 771-778, 2015
Dan Low, MRI Guided Radiotherapy, 2017
Change system geometry
Parallel orientation
Perpendicular orientation
33
Keall et al, Semin Radiat Oncol, 2014
• The Cross Cancer Institute 6 MV/0.6 T Linac-MRI• The Australian 6 MV/1 T MRI-Linac
Account for perturbations in treatment planning
• Parallel-opposed radiation beams
• IMRT
• Monte-Carlo-based treatment planning
34
Raaijmakers et al, PMB, 2008
Single beam
5 cm beam
Parallel-opposed beams
35
Water
Water
Lung
0 10 20 30 40 50 60 700
20
40
60
80
100
Dose (Gy)
Vo
lum
e (
%)
Parotis LeftParotis Right
Submand Left Submand Right
BrainMyelum
DVH for optimized dose distribution oropharynxComparison between B = 0 T and B = 1.5 T
Raaijmakers et al. Phys. Med. Biol. 52 (2007) p. 7045-54
Mitigating dose perturbations - summary
• Magnetic field strength
• System geometry
• Treatment planning
37
Radiation biology experimentsImpact of magnetic field on dose response
40
Rubinstein et al, Med Phys, 2015
Mouse lung phantom Co-60, 1.5T
2.5 cm beam
Single beamParallel-opposed
beams
Block for Co-60 beam
5 cm diam. poles
Electromagnet coils
PA Irradiation AP Irradiation
The effect of a strong magnetic field on radiation-induced lung damage
• No Magnetic Field
• 9, 10, 10.5, 11, 12, 13 Gy dose groups
• 10 mice per group
• Magnetic Field
• 9, 10, 10.5, 11, 12, 13 Gy dose groups
• 10 mice per group
• Control
• 0 Gy
• 20 mice
42
140 Mice
(C57L)
C57L Mice: • Acute pneumonitis• Chronic fibrosis• No pleural effusions
43
0
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0 2 4 0
D a y s
Pe
rc
en
t s
urv
iva
l
1 1 G y - 0 T
1 1 G y - 1 .5 T
1 2 G y - 0 T
1 2 G y - 1 .5 T
1 3 G y - 0 T
1 3 G y - 1 .5 T
1 0 .5 G y - 0 T
1 0 .5 G y - 1 .5 T
C o n tro l
9 G y
1 0 G y
Post-irradiation survival
0
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0 2 4 0
S u rv iv a l: A ll m ic e
D a y s
Pe
rc
en
t s
urv
iva
l
0 T
1 .5 T
0
2 0
4 0
6 0
8 0
1 0 0
9 1 0 1 1 1 2 1 3
In c re a s e d R e s p . R a te a t 5 M o n th s
D o s e (G y )
%M
ice
Wit
h I
nc
re
as
ed
RR
0 T
1 .5 T
E D 50 (95% C I)
1 0 .7 2 G y (1 0 .4 5 -1 1 .0 0 )
1 0 .5 0 G y (1 0 .4 1 -1 0 .5 9 )
44
> 190 bpm
0
2 0
4 0
6 0
8 0
1 0 0
9 1 0 1 1 1 2 1 3
In c re a s e d L u n g D e n s ity a t 5 M o n th s
D o s e (G y )
%M
ice
Wit
h I
nc
re
as
ed
De
ns
0 T
1 .5 T
E D 50 (95% C I)
1 0 .5 6 G y (1 0 .5 2 -1 0 .6 1 )
1 0 .2 6 G y (1 0 .1 3 -1 0 .3 9 )
> 0.64 g/cm3
45
Pre-irradiation
Pre-irradiation 5 months post-irradiation
0
2 0
4 0
6 0
8 0
1 0 0
9 1 0 1 1 1 2 1 3
R e d u c e d H e a lth y L u n g V o lu m e a t 5 M o n th s
D o s e (G y )
%M
ice
Wit
h R
ed
uc
ed
Vo
l
0 T
1 .5 T
E D 50 (95% C I)
1 0 .5 6 G y (1 0 .4 8 -1 0 .6 4 )
1 0 .3 3 G y (1 0 .1 9 -1 0 .4 8 )
< 0.42 cm3
46
Radiation biology experiments (so far)
• Magnetic field dose not change response (cell experiments)
• Pre-clinical (murine) studies:
• Magnetic field had no impact on survival
• Magnetic field had small (2% or less), but significant impact on respiratory
rate, lung density, and healthy lung volume
Impact on physics QAImpact of the magnetic field on physics QA equipment and measurements
Standard QA measurements
• Ion chamber in solid water or plastic phantom
Effect of magnetic field on dose measurements
Meijsing et al, 2009
Meijsing et al, 2009
-0.5%
0.0%
0.5%
1.0%
1.5%
0 45 90 135 180 225 270 315 0
Re
lati
ve C
ham
be
r R
esp
on
se
Chamber Orientation (deg)
-0.5%
0.0%
0.5%
1.0%
1.5%
0 45 90 135 180 225 270 315 0
Re
lati
ve C
ham
be
r R
esp
on
se
Chamber Orientation (deg)
• IEC1997 requires<= 0.5% variation for reference dosimetry
• Solid Water Phantom• Variation of 1.3%
• Water Phantom• Variation < 0.3%
More measurement effects
PTW 30013 Farmer ChamberPhantom: 30 x 30 x 15 cm3 solid waterChamber: long-axis parallel to magnetic fieldSCD: 143.5 cmDepth: 5 cm
In Water
Slide from O’Brien and Sawakuchi
No magnetic fieldMonte Carlo
(a) 0° orientation
(a) 180° orientation
beam
beam
Slide from O’Brien and Sawakuchi
1.5 T Magnetic Field
Monte Carlo
(a) 0° orientation
(a) 180° orientation
beam
beam
𝑩
𝑩
Slide from O’Brien and Sawakuchi
• Power supply moved away from detector
• Must use MV beam to position at isocenter
• Must calibrate in MR Linac beam
Initial Testing of MR-Compatible ArcCheck QA Device
G. Ibbott, RSNA, Chicago, 2017
MR-guided RT is already here
• January 2014 -June 2016, 316 patients treated
• Online ART MR-IGRT (6 mos)
• Cine gating (9 mos)
MR-Co60 Clinical since Jan, 2014
HFHS MR-Linac Program Summary (ViewRay system)07/19/17 to 01/23/18
• 47 Patients
• 687 tx fractions completed
• Maximum tx/day = 9 26%
19%
17%11%
11%
6%
2%
2%
2%2%
2%Male Pelvis
Abdomen
Lung
Liver
Pancreas
Breast
Chest Wall
Esophagus
Kidney
Bone
H&N
Treatment by Disease Site (%)
46.8%
46.8%
6.4%
Treatment Distribution
SBRT Conventional APBI
Slide from Carri Glide-Hurst
Key Points/Summary
• In-room MRI-guided radiotherapy is here, with more to come
• The permanent magnetic field can impact dose distributions and measurements
• These can be accounted for in several ways
• Radiobiology experiments do not indicate any clinically significant issues (although indicate careful observation of patients)