Physics of Novel Radiation Modalities Particles and Isotopes

Todd Pawlicki, Ph.D.

UC San Diego

Disclosure

• I have no conflicts of interest to disclose.

Learning Objectives

• Understand the physics of proton therapy

• Describe proton dose deposition

• List components of creating a proton beam

• Describe aspects of proton beam planning

• Compare proton and conventional plans

Some Proton History• 1930 Cyclotron invented

– Lawrence EO, Livingston MS. The production of high speed protons without the use of high voltages. Physical Review 1931.

• 1946 Suggested for medical use– Wilson RR. Radiological use of fast protons. Radiology 1946.

• 1958 First patients treated– Tobias CA et al. Pituitary irradiation with high-energy proton beams a preliminary report.

Cancer Research 1958.

– In 1961, the Harvard Cyclotron Laboratory started treating intracranial lesions

• 1991 1st hospital-based system at the LLUMC– Slater JM et al. The proton treatment center at Loma Linda University Medical Center:

rational for and description of its development. IJROBP 1991.

Proton Facilities In Operation

16

9

5

3

22 1

http://www.ptcog.ch (accessed 3/2015)

USA

Japan

Germany

Russia

China

France

Others

Depth Dose

http://commons.wikimedia.org/wiki/Category:Radiation_therapy

PhotonsBragg Peak

Electrons

Photons

Protons

Schulz-Ertner et al. Semin Radiat Oncol, 2006.

SOBP

Koehler and Preston. Radiology(104)191-195, 1972.

Cobalt-60

20 MV X-rays

160 MeV Protons

Particle Properties

Particle Symbol Charge Rest Mass

Electron 1 0.511 MeV

Positron +1 0.511 MeV

Proton +1 1836 0.511 MeV

Neutron 0 1839 0.511 MeV

,e

,e

1

1,p H

1

0,n n

𝐸 = 𝑚𝑐2

Proton (charged particle) Interactions

• Electromagnetic interactions– Excitation

– Ionization

• Bethe-Block formula– S 1/v2

– Bragg peak

p p

e

p p

Proton (charged particle) Interactions

• Nuclear interactionsI. Multiple Coulomb scattering

Small q

II. Elastic nuclear collision

Large q

III. Inelastic nuclear interaction

(i)

(ii)

(iii)

nucleus

pp

p

p

nucleusp

p

, n

e

Ionization Density

0.5 MeV Proton

Hall. Radiology for the Radiologist. 4th ed. 1994.

10.0 MeV Proton

1.0 MeV Electron

0.005 MeV Electron

Linear Energy Transfer (LET)

• Energy transferred per unit track length

• Useful as a simple way to indicate radiation quality and biological effectiveness

LETdE

dl

𝑘𝑒𝑉

𝜇𝑚

Radiation LET (keV/m)

Cobalt-60 -rays 0.2

250 keV x-rays 2.5

10 MeV protons 4.7

150 MeV protons 0.5

Hall. Radiology for the Radiologist. 4th ed. 1994.

Relative Biological Effectiveness

• Equal doses of difference types of radiation do not produce equal biological effects

• RBE depends on – Biological system (cell type)

– Clinical endpoint (early or late effects)

– Energy deposition characteristics

– Dose

Hall. Radiology for the Radiologist. 4th ed. 1994.

RBEx ray

test

D

D

RBE for Protons

• RBE is a function of LET– RBE is not constant with depth

– Careful at distal end of targets and near critical structures

• Clinical RBE for protons 1.1– 1 Gy proton dose 1.1 Gy Cobalt dose

– A single value might not be sufficientCarabe et al. Phys Med Biol. 2012.

1.7

4 6 8 12 14 16 18 200 102

0.6

0.2

0.9

1.1

1.3

1.5

1.0Modulated beam

160 MeV

Depth [cm]

RB

E

low

high

Re

lati

ve d

ose

Clinical RBE

Source: S.M. Seltzer, NISTIIR 5221

Creating Proton Beams

• Energy should be variable starting at 70 MeV

• Maximum energy should be about 250 MeV

𝐹𝑚𝑎𝑔 = 𝑞 ∙ ( 𝑣 × 𝐵)

𝐹𝑒𝑙𝑒 = 𝑞 ∙ 𝐸

𝐹 = 𝑚 ∙ 𝑣2

𝑟

𝑚𝑣 = 𝑞𝐵𝑟

Proton Beams

• Two basic proton accelerator options

– Cyclotron• Protons revolve at the same frequency regardless of energy

or orbit radius

– Synchrotron• The magnetic field strength is increased in synchrony with

the increase in beam energy

𝑚𝑣 = 𝑞𝐵𝑟

Cyclotron

Magnetic Field

Proton Beam

Proton Source

Magnet RF

𝑟 =𝑚𝑣

𝑞𝐵

Clinically Useful Proton Beams

• There are two main approaches

• Passive scattering systems– Fixed depth of penetration

– Fixed modulation

• Active scanning systems– Irradiation the target using a narrow beam

– Beam controlled in three dimensions

Passive Scattering

Goitein et al. Physics Today. 2002.

Active Scanning

Goitein et al. Physics Today. 2002.

Treatment Planning

• Acquisition of imaging data (CT, MRI)

• Delineation of regions of interest

• Selection of plan properties– Beam directions

– Energies

• Conversion of CT values into stopping power

Paganetti. Phys Med Biol. 2012.

Paganetti. Phys Med Biol. 2012.

Range Uncertainty

• Dose calculation– CT Imaging and calibration

– CT conversion to tissue

– CT grid size

– Inhomogeneities

• Other sources– Commissioning measurement uncertainty

– Compensator design

– Beam reproducibility

– Patient setup

Total range uncertainty 2–4% of proton range + 1–2 mm

Dose Distributions

MacDonald et al. Cancer Investigation. 2006.

Photons

Protons

Dose Distributions

Greco & Wolden. Cancer. 2007.

Dose Distributions

Suit et al. Acta Oncologica. 2003.

Proton superior to Photons

Proton superior to Photons

Proton superior to Photons

PTV

Bladder

LT Femoral Head

LT Kidney

Rectum

Protons similar to Photons

Protons similar to Photons

PTV

LT Optic Nerve

Brainstem

LT Cochlea

Summary

• Proton physics differs considerably from photon and electron physics

• Scattering and active scanning are two methods of creating a proton beam

• Proton and conventional plans must be compared carefully – proton plans are not always superior