Medical Linear Accelerators in Radiation Therapy

Haijun Song, Ph.D.

Dept. of Radiation OncologyDuke University Medical Center

MV Linear Accelerator

Anatomy of a Linac

Anatomy of a Linac

Beam MofiersCone

MLC (Multi-Leaf Collimator)

Radiosurgery with Linac

MLC

Conformal RT (3DCRT) &Intensity Modulation RT (IMRT)

Conformal RT (3DCRT) &Intensity Modulation RT (IMRT)

Dose Map delivered by IMRT

Dose Map delivered by IMRT

Dose Map delivered by IMRT

Image Guided RT (IGRT)

Respiratory Gating RT

Tomotherapy and Robotic

Difference among Radiation Sources

Factors affecting radio-bio response• Gamma vs. Electron

Among Gamma sources• Photon Energy Spectrum• Dose Distribution• Dose Rate

Radioactive IsotopesExample: Cs-137

• single dominant energy, 0.66 MeV• Activity: 6000 Ci and 100 Ci, dual source• Dose rate range: 0.001 - 10 Gy/min

Radioactive Isotopes

Example: Ir-192• Complex energy spectrum: 30 gammas and

40 x rays.• Dose rate, 0.01 - 5 Gy/min being used RT.

Ir-192 photon spectrum

0.0001

0.001

0.01

0.1

1

10

100

1000

0.001 0.01 0.1 1 10

E (MeV)

Perc

ent p

er d

ecay

From kV to MV

Commonly: 50 – 300 kVPBeam Quality: HVL a few mm Al to Cu.Dose Rate: around Gy/min

From kV to MV

Commonly: 6 MV -25 MVBeam quality: PDD10 in water, 67% and up.Dose Rate: around Gy/min

X-Ray tube vs. Linac

Acce. Tube: Travelling Wave

X-Ray ProductionSide emission vs. straight through emmission

Photon energy spectrum from a 6 MV beam

Fl uence/ MeV/ I ndi cent el ect r on

0. 00E+00

2. 00E- 05

4. 00E- 05

6. 00E- 05

8. 00E- 05

1. 00E- 04

1. 20E- 04

1. 40E- 04

0 1 2 3 4 5 6

Fl uence/ MeV/ I ndi centel ect r on

Depth Dose Curves

Dose Distribution: One beam/parallel beams

Dose Distribution: One beam/parallel beams

MV photon and electron beams

Depth Dose of e Beams

0 1 2 3 4 5 6 7 8 90

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

M e V1 81 51 21 086

Per

cent

Dep

th Io

niza

tion

D e p t h ( c m ) i n W a t e r

Dose Measurement

Ion chamber and electrometerIn water for MV photon or electron beamsIn air for KV beamsTracable to NIST Co-60 standard beam

CowDQ

Qw NkMD 60

,*=

Buildup region and skin sparingCompton e1 1 1 1

Compton e2 1 1 1

Compton e3 1 1

Compton e4 1

Depth in water

Total dose 1 2 3 3

Treating Superficial targetsBolus: tissue equivalent, soft to conform to skin

0 1 2 3 40

20

40

60

80

100

S

B dmax

Per

cent

Dep

th D

ose

Depth in Water (cm)

Dose Distribution: One beam/parallel beams

Buildup region and skin sparingcontinued

Skin sparing also means a target at shallow depth will not get adequate dose.

For small animals, “sparing” of a couple of centimeter from skin may “spare” the whole animal.

What do we do when we need to treat superficial target?

Bolus

Treating Superficial targetsBolus: tissue equivalent, soft to conform to skin

0 1 2 3 40

20

40

60

80

100

S

B dmax

Per

cent

Dep

th D

ose

Depth in Water (cm)

bolus

Treating Superficial targetsBolus: tissue equivalent, soft to conform to skin

0 1 2 3 40

20

40

60

80

100

S

B dmax

Per

cent

Dep

th D

ose

Depth in Water (cm)

bolus

Treating Superficial targets

MeV range electron beams /w bolus

kV (suferfical units, orthovoltage units)

MV photon beam with bolus

How useful are MeV electron beams for small animal irradiations?

What can Rad Onc Offer Rad Biology

All offered to human can be offered to other species.

Imaging: CT, MRI

Precise and conformal dose coverage: 3DCRT, IMRT, IGRT, Respiratory Gating, radiosurgery, electron irradiation.

Precision and conformality means sparing healthy tissue, too.

And More: Brachytherapy, hyperthermia, proton, neutron.

Acce. Tube: Standing Wave

Acce. Tube: Short Standing Wave