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

lecourt@mdanderson.org

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)