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Electron Beam Therapy - Reloaded

Qiuwen Wu, Ph.D. Department of Radiation Oncology

Duke University

ET - Reloaded

Electron Photon Proton

• Depth dose and lateral profiles – Suitable for superficial tumors – Requires collimation close to skin

• Clinical e unchanged for decades and underutilized in cancer radiotherapy

• ~ 15% utilization

Introduction: e vs. γ vs. p

EAT – Electron Arc Therapy

• Electron beam delivered in partial arc

• Modifications to linac/cone and TPS – Shortened cone – Different PDDs – Cutout shape

• Patient-specific cast/shield and bolus

• Primarily for chest wall irradiation

• Univ. of Utah

Leavitt 1985 Electron ARC therapy: physical measurement and treatment planning techniques IJROBP 11

McNeely 1988 Electron arc therapy: chest wall irradiation of breast cancer patients IJROBP 14(6)

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ECT – Electron Conformal Therapy

• One or a few electron beams

• Bolus electron conformal therapy

– Tissue equivalent

– Modulate electron energy

– Keep PTV within 90% isodose volume

– Minimize dose to distal/underlying critical structures and normal tissue

Perkins 2001 A custom three-dimensional electron bolus technique

for optimization of postmastectomy irradiation IJROBP51

MERT – Modulated Electron Radiotherapy

• Multiple electron beams

• Energy modulation

– Bolus or energy selection

• Intensity modulation

– Cutout

– eMLC

– pMLC

– Scanning beam

Henzen et al 2014 Beamlet based direct aperture optimization for MERT using a photon MLC. Med Phys, 41

Henzen et al 2014 Monte Carlo based beam model using photon MLC for modulated electron radiotherapy. Med Phys, 41

: 6 MV e− : 6 / 9 / 12 MeV

12 segments

Standard MERT

DEAR – Dynamic Electron Arc Radiotherapy

• Electron radiation is delivered in ARC mode

• Electron applicator and cut-out are kept to provide lateral beam constriction

• Treatment couch is in simultaneous motion with gantry rotation to prevent collision. Beam always normal to skin and SSD = 100 cm

• Couch motion, gantry rotation, and dose rate are modulated to produce desirable dose distributions

Rodrigues, Yin, and Wu. "Dynamic Electron Arc Radiotherapy (DEAR): a feasibility study", Phys Med Biol, 2014. 59(2)

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DEAR design

DEAR design

Gantry

Couch

DEAR Photons

• DEAR: 6, 9, 12 MeV. • Photons: 6X, tangent, no wedge

Plan comparison: chest wall phantom

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DEAR delivery

Room View Patient View

DEAR delivery demo

-50

0

50

Gan

try

Ang

le (d

eg)

Expected

Actual

-4

-3

-2

-1

0

1

Cou

ch V

RT

pos

(cm

)

0 50 100 150 200-20

-15

-10

-5

0

Cou

ch L

AT

pos

(cm

)

Dose (MU)

Gantry Angle(deg)

Couch VRT (cm)

Couch LAT (cm)

0

1

2

3

4

Expected

Actual

-0.6

-0.4

-0.2

0

0.2

0.4

0 5 10 15 20 25 30-0.8

-0.6

-0.4

-0.2

0

Time (s)

Gantry Rot (deg/s)

Couch VRT (cm/s)

Couch LAT (cm/s)

Position Speed

Gantry and couch motion

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0

50

100

150

200

Dose

(MU)

Expected

Actual

120

125

130

135

140

Couc

h VRT

pos (

cm)

0 5 10 15 20 2595

100

105

110

Couc

h LAT

pos (

cm)

Time (s)

Beam Hold

Couch VRT (cm)

Couch LAT (cm)

Beam Hold

Beam hold

Dose (MU)

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120

Do

se R

ate

(MU

/min

)

Time (s)

6e

Actual

Expected

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120

Do

se R

ate

(MU

/min

)

Time (s)

6x

stdev = 1.6 MU/min stdev = 0.2 MU/min

Dose rate modulation

Static (16x10 cm2,G=0⁰)

depth 1.5 cm

DEAR (3x10 cm2,G=315-45⁰)

1.2 1.3

4.2 1.9

Dosimetry: 6e on cylindrical phantom

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• 6 MeV electron • D=1cm cutout • CR film • 148 CP (7MU/CP) • Beam hold (D->e)

Virtual scanning mode

Potential DEAR case

Photons DEAR

Need for fine energy resolution

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Small field electron beam dosimetry

Small field electron beam dosimetry

• Composite dose map

• 6 MeV

• 1x1 cm2 kernel

Small field dosimetry

• 6 MeV

• 10x10 cm2

• PDD

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Small field dosimetry

• 16 MeV

• 10x10 cm2

• PDD

DEAR Summary

Advantages • Uniform dose distributions over

large and curved targets while maintaining narrow penumbra

• Treated area > cone size

• DEAR delivery • Fixed cone/cutout

• DEAR delivery has high accuracy • Expected and actual plans agree

very well

• Trajectory log file can be used as a QC tool

• Dosimetry

Challenges • Small field dosimetry

• Not ready in clinical operation • Couch motion

• Coarse energy selection • Bolus

• Planning tools • Forward and inverse optimization

• Dynamic delivery collimators • Motorized electron collimator (MEC)

• Electron MLC (eMLC)

• QA Tools

Tech

no

log

y C

om

ple

xit

y

Photons Electrons

3DCRT ECT

ARC EAT

IMRT MERT

VMAT DEAR

FFF SFF

Summary: e vs. γ

VMATe

3D Printing

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Prototype eMLC

Eley, J. G., K. R. Hogstrom, et al. (2011). "Potential of discrete Gaussian edge feathering method for improving

abutment dosimetry in eMLC-delivered segmented-field electron conformal therapy." Medical Physics 38(12): 6610-6622. Looking to the Future

Antolak

Commercial eMLC

Gauer, T., D. Albers, et al. (2006). "Design of a computer-controlled multileaf collimator for advanced electron

radiotherapy." Physics in medicine and biology 51(23): 5987-6003. http://euromechanics.com/e_emlc.html https://www.youtube.com/watch?t=66&v=F0BBbHRrjBg

Antolak

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Slide 31 Hogstrom, 2003

Slide 32 Hogstrom, 2003