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
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
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
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