1

Very High Energy Electr ons(VHEE) and 4D IMRT

Lech Papiez, Indiana University

VHEE and 4D IMRT- Issues and perceptions

1. VHEE as a legitimate alternative to photonbeam therapy.

2. Definite 4D IMRT in radiation therapy.3. Obstacles in delivering true 4D IMRT with

existing technology.4. Fast scanning of pencil beam VHEE as a

vehicle for 4D IMRT.

Dose for basic arrangement of VHEEbeams – single broad and narrow beam

150 MeV

Comparison of dose from singlebeamlets - VHEE vs. photons

2

Comparison of ratio of target to integraldose from parallel opposed beamlets

T T

Comparison of penumbra for singleand parallel opposed beams

Penumbra (measured as the dis tance between 90% and 20% of dose maximu m ata given dep th) for single and parallel opposed electr on and pho to n beams.

Message I• Parameters (depth of beam penetration,

penumbra, integral dose) of basic VHEE beamarrangements are adequate for dosedistribution shaping for 3D conformal therapyand IMRT therapy.

• VHEE beam properties suggest advantage intheir clinical utilization due to their better ratio ofdose to target vs. integral dose.

• Potential shortcomings of VHEE (surface dose,photoneutrons).

Exploring potential advantage –case 1VHEE multi-energy (50,100,150, 200,

250 MeV) optimal planning

10

20

30

40

50

60

70

80

90

100

Prostate model case

Optima l VHEE planfrom nine coplanar beams

3

-14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4-2

-1

0

1

2

3

4

Tissue TissueBone

cm

cm

0.90.8

200 MeVTissue Air

0.5 0.7

-14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4-2

-1

0

1

2

3

4

cm

cm

0.9

0.80.7

0.5

15 MVTissue Air

Exploring potential advantage –case 2Air-water interface : VHEE vs. x-ray

Radiusof air cavity = 2 cm

Isodoseline step5% ofmaximum dose

Advantages of VHEE therapy vs.photon therapy

Message II• VHEE provide potential advantages over photon

therapy by avoidance of electronic disequilibriumand dose variation at tissue interfaces

• VHEE provide potential advantages over photontherapy due to their more favorable ratio of dosedelivered to the target vs. integral dose delivered tothe body

• VHEE can be particularly effective if wholespectrum of electron beam energies will beavailable for treatment and if VHEE therapy can beeffectively integrated with photon beam therapy

Message III• VHEE based treatments avoid high dose at

surface for multiple beam arrangements

• VHEE based treatments provide negligibledose equivalent enhancement fromphotoneutrons and induced radioactivity

• VHEE based treatments require shieldingcomparable to 50 MeV electron or photontreatments

4

VHEE advantage in 4D IMRTdue to the feasibili ty of fas t,electromagnetic scann ing of

VHEE pencil beams

3D IMRT – prin ciple (join t optimization forintensity maps over all beams)

T

Beam AP

Beam LAT

OAR 1

OAR 2

Beam AP

Beam LAT

1

10

0

4D IMRT - mov ing organs (phase I)

T

Beam LAT

Beam AP

OAR 1

OAR 2

OAR 3

OAR 4

1

1

00

00

OAR 2

OAR 3

4D IMRT – movi ng organ s (phas e 2)

T

OAR 1

OAR 4

Beam AP

Beam LAT0

0

0

0

0.5

0.5

5

4D IMRT – principle (join t optimi zation forintensi ty maps over all beams and phases)

Phase I

Phase II

Beam AP Beam LAT

11

0

0 0 00

0 0 0 0.50.5

4D IMRT vs. 3D IMRT - I

• Best possible 3D plan requires optimizationthat involves search for beamlet intensities thatcover jointly all beam dire ctions

• Best possible 4D plan requires optimizationthat involves search for beamlet intensities thatcover jointly all beam dire ctions and all bodygeometries

• No 3D plan for any stationa ry geometry can,under general cond ition s, achieve asfavora ble dose crit eria as 4D plan optimizedsimu ltaneou sly for all body geometries

4D IMRT vs. 3D IMRT – II• Notice the similarity

– The optimal plan for stationary body, as well as for movingbody, requires evaluation of criteria for dose distributionaccumulated over all time of body exposure

• Notice the difference– The delivery of optimal 4D plan requires proper redistribution

of beamlet intensities over all geometries of the treated body– this is equivalent to realization of black and white“intensity” movie for each beam (TV therapy).

– In contrast, the optimal 3D plan requires only properaccumulation of beamlet intensities for each beam over theentire interval of irradiation – this is equivalent to realizationof black and white “intensity” (still) picture for each beam.

Geom

etry4

Geom

etry2

Beam angle 4Beam angle 2 Beam angle 3

Geom

etry3

Optim

izationcriteria

forG

eometry

1

Beam angle 1

One phase planning (single 3D) –cases (i) and (ii)

Gatin

g

Mo

tion

do

seartifacts

Mo

tion

do

seartifacts

Mo

tion

do

seartifacts

Mo

tion

do

seartifacts

6

Optim

izationcriteria

forG

eometry

4

Optim

izationcriteria

forG

eometry

2

Beam angle 4Beam angle 2 Beam angle 3

Optim

izationcriteria

forG

eometry

3

Optim

izationcriteria

forG

eometry

1

Beam angle 1

Separate phase planning (multi 3D) –case (iii)

Gatin

gG

ating

Gatin

gG

ating

Geom

etry4

Geom

etry2

Beam angle 4Beam angle 2 Beam angle 3

Geom

etry3

Geom

etry1

Beam angle 1

Separate phase planning on 4Dgeometry (3.5 D) – case (iv)

Optim

izationcriteria

forreference

geometry

Gatin

gG

ating

Gatin

gG

ating

Geom

etry4

Geom

etry2

Beam angle 4Beam angle 2 Beam angle 3G

eometry

3

Beam angle 1

True 4D planning and delivery –case (v)

Geom

etry1

Geom

etry2

Geom

etry3G

eometry

4

Optim

izationcriteria

forreference

geometry

Fast

scann

ing

Towards 4D IMRT with DMLC delivery– what can be gained?

• Plan for single phase (3D)• Target moving and OAR immobile• DMLC delivery basic goal is to impose the planned

intensity map over the target• Non-uniqness of the solution for DMLC to moving

target allows to reduce the dose to OAR• Passive solutions through DMLC motion correlation

with phase of target shift• Active solution with the modification of leaf velocities

7

IMRT to moving target andstationary organ at risk (OAR)

Increase of dose to OAR for adversecorrelation of OAR and leaf motion

Leading Leaf

Following Leaf

Sensitive Organ

TargetPoints

Time (s)

Integral Intensity Intended = 6.0 MUIntegral Intensity Delivered = 21.0 MU� 250% increase in integral monitor units

Intensi tydeliver ed

to the target

Intensity delive redto OAR

Reduction of dose to OAR for “good”correlation of OAR-leaf motions

Leading Leaf

Following Leaf

Sensitive Organ

TargetPoints

Integral intensity from plan = 6.0 MUIntegral intensity delivered = 3.67 MU40% reduction of intensity delivered

Time (s)

Inten sit y delive redto the target

Intensity deliveredto OAR

DMLC IMRT for 4D therapy –statistical analysis

Integ

ralMU

toO

AR

Phase Shi ft (radian s)

Average

Intended

OAR

TargetAdverse phase correlation

“good” phase correlation

Intensity planned and deliveredto target

8

LeadingLeaf

Following Leaf

Sensitive Organ

TargetPointsUnmodifiedTrajectories

Pos

itio

n(L

abra

me

ofre

fere

nce)

Ti me (s)

Static Geometry

Modified (Case2)

Unmodified(Case1)

82% reduction in integral MU from static case74% reduction in integral MU relative case 1

Partial 4D IMRT can provide generousreduction of dose to OAR

Conclusions

• VHEE therapy is a feasible alternative tophoton and proton therapy (for sometreatments VHEE is dosimetrically superior tophoton therapy)

• True 4D IMRT treatment delivery isequivalent to creation of black and whitemovie displayed on the screen of each beamfield

• VHEE electromagnetic scanning of electronpencil beam provides the physically viabletool for delivery of the true 4D IMRT

Acknowle dgme nts

• Vadim Moskvin, Colleen DesRosiers (IU-simulation, evaluation and planning for VHEE)

• George Sandison, Colin Yeboach (TBakerCC –optimized VHEE IMRT vs. photons andprotons)

• Malka et al (CNRS, Ecole Polytechnique,Palaiseau, France–laser based VHEE, emshaped VHEE beams)

• Thomas Bortfeld – critique and discussion• Dharani Rngaraj, Ryan McMahon (IU, WashU –

DMLC IMRT for moving targets and OAR)