PHYS C12

Alternative Treatment Options

neutron generator, cyclotron, or neutron recator producing neutrons

synchroton or cyclotron generating heavy ions (protons, He, C, N)

Proton Therapy

Precise, non-invasive, non-surgical

Unlike x-rays, protons stop at tumour site, minimising damage to surrounding healthy tissue, allowing for higher doses

Fewer side effects

When is proton therapy especially useful

Treating childhood cancers, as minimising healthy tissue damage is crucial in a growing body

When/why might proton therapy not be preferred

fast-growing tumours - requires longer planning, set-up, and delivery

widespread cancer - radiation must be delivered broadly

higher cost, limited availability

Bragg Peak

proton enters tissue with high velocity

it interacts mainly with orbital electrons, losing energy through dose deposition during ionisation and excitation

Energy loss per unit distance (thus dose) inversely proportional to proton velocity

Near the end of its range, proton slows significantly, electron interaction probability increases sharply, just before stopping proton deposits lots of energy in a very short distance (Bragg peak)

After peak, proton stopped, dose drops to ~0

Clinical use of bragg peak

peak is positioned at tumour depth to minimize exit dose (beyond tumour)

Spread-Out Bragg Peak (SOBP)

Bragg peak of monoenergetic proton targets narrow depth ranges but to treat entire tumour, bragg peak is laterally spread

Incident photon beam energy is modulated, giving multiple peaks that are super-imposed into a wide, flat plateau, increasing size of treatment field

Gives flexibility in distrubuting dose

Penumbra depends on system design and setup parameters, is it possible to shield parts of

the beams to remove low-dose 'tails'

Heavy Ion Beam Therapy

Also have Bragg peak

Higher LET, thus biological effectiveness, than protons - Beam directly cleaves DNA in multiple sites, even with low oxygen, resulting in cells being unable to repair

Higher mass limits lateral scattering

Sharper longitudinal dose fall off than protons

Nuclear interactions activate nuclei of irradiated atoms, inducing localised radioactivity that can be externally imaged for tumour and normal tissue dose verification

Heavy Ions beyond Bragg peak

near the end of their range, heavy ions come to a stop in the tumour

their biological effectiveness increases by 3-4 times due to denser ionisation

This may allow destruction of radioresistant tumours and overcome therapeutic resistance

But it also may damage healthy tissue beyond peak

Synchrotron

Microwave energy is fed into RF cavities which creates an electric field that accelerates electrons to almost speed of light

When electrons deflected by magnetic field, they emit broad spectrum of electromagnetic radiation

Particle Acceleration

Electron gun: thermionic emission from filament

Klystron: microwave RF power

Linear accelerator: RF cavities generate electric field that rapidly oscillates at exactly the correct frequency so that e- passing through cavities gain kinetic energy from electric field in forward direction (accelerated)

Booster Ring

e- gain more energy as they repeatedly circulate, passing through RF cavities in the ring

Dipole magnets bend e- into circular path

Quadrupole magnets focus beam

Magnet’s field strengths increase as electron energy increase to keep them on same circular path

operates at 3GeV in Australia

Storage Ring

where electrons circulate at constant high ebergy to be stored and used

Insertion devices produce intense x-rays

Dipole magnets bend e- into circular path

RF cavities replace energy lost by electrons

Imaging and Medical Beamline (IMBL) at Australian synchrotron

Provides dynamic, 3D, high-resolution X-ray images that reveal minute differences at air, tissue, and bone interfaces

Can visualise blood vascularisation, lung air movement, and tissue/organ structure in far greater detail than MRI

Developing new methods that could destroy whole tumours with minimal disruption to healthy tissue:

Microbeam Radiotherapy (MRT)

Stereotactic Synchrotron Radiotherapy (SSRT)

Ultra-High Dose Rate (FLASH) Radiotherapy

Proton Synchrotron

Most famous one is in CERN's accelerator complex

Accelerates protons delivered by either the Proton Synchrotron Booster or heavy ions from the Low Energy Ion Ring

Cyclotron

10-590 MeV

charged particles injected into centre of cyclotron chamber and are accelerated within electrodes called dees in a spiral path, which allows for a longer acceleration path than a straight line accelerator

Cyclotron - Ion Acceleration

Alternating RF (10-30 MHz) voltage applied to two hollow D-shaped electrodes, creating an electric field in narrow gap between them

Uniform magnetic field applied perpendicular to the dees, creating a force that bends ions into circular path

When ions reach gap, electric field is timed to accelerate them toward the opposite dee

As ions gain velocity each time they pass through gap, their orbital radius (thus spiral path) increases

Cyclotron

size of dees determines spiral path length, hence maximum attainable energy

Electric fields stays synchronised as the longer distance ions must cover counteracts their faster velocity, allowing them to always arrive at gap at same frequency

Once ions reach outer edge, they are deflected as a high-speed beam towards a target

Cyclotron - Proton beam production

If source is hydrogen gas, electric fields ionises it

Metal foil strips e- from H- ions, leaving protons

Proton beam used for clinical treatment or to bombard a target, causing nuclear reactions (p,n) that produce radionuclides, some of which are used for PET

(p,n) nuclear reactions

Incident proton ejects one or more neutrons from target nucleus to stabilise it

Target atom becomes radioisotope and its atomic number number increases by one

This is becasue neutrons are not bound by electrostatic forces, so require less energy to escape than protons

Proton Treatment - Source of beam and Room design

Cyclotron (only protons, fixed energy, smaller, lower cost) or synchrotron (protons or carbon ions, 70-250 MeV, larger, higher cost, less radiation activation in room)

1-5 treatment rooms

360° rotating gantries

2m thick concrete walls

Cyclotron: Neutron Production

used for neutron therapy when tumours resistant to x-rays and neutron radiography, which images water or lubricants inside metal parts

Cyclotron beam directed onto beryllium target mounted in rotatable isocentric gantry structure

Fast neutrons produced by either(p+Be where fast proton knocks out a neutron from Be target nucleus or d+Be where a deuteron (1 proton, 1 neutron) strikes a Be target, stripping proton and producing recoil neutron that retains some of the deuteron’s inital kinetic energy

In both processes, each neutron produced converts one Be atom to a B atom

Neutron Generator

contain compact linear particle accelerators

Deuterium or tritium gas is ionised

deuterium ions (deuteron), tritium ions (triton), or a mixture are accelerated by a high voltage into a negatively charged Ti or Sc target loaded with deuterium, tritium or a mixture

Fusion of two deuterons (D + D) or a deuteron and a triton (D + T) forms a Helium ion and a neutron

Nuclear Reactor

Each fission (one nucleus absorbs a neutron and becomes two smaller nuclei) event liberates ~200 MeV energy that is distributed among reaction products

Each time Uranium-235 nucelus undergoes fission, it releases 2-3 neutrons that create a self-sustained fission chain reaction

Nuclear Reactor Core Components

reactor fuel (e.g. U-235) - undergos fission when struck by a free neutron

moderator - slows fission-produced neutrons down to be reflected back into fuel, sustaining chain reaction

control rods (boron or hafnium) - absorb neutrons, their position affects the number of neutrons available to induce fission to control the fission rate

Open Pool Australian Lightwater (OPAL) Reactor Functions/Purposes

Irradiation for radioisotope production

Dopes Si for semiconductors

Delayed and prompt neutron activation analysis

Nuclear research, education, and training

Open Pool Australian Lightwater (OPAL) Reactor

operates at 20 MW power

Compact core of 4 x 4 fuel assembly array with 5 control rods, cooled with demineralised water and surrounded by a zirconium alloy vessel containing heavy water that reflects neutrons back towards core

low enriched uranium fuel

Water Phantoms

large, tissue-equivalent container filled with water

used to measure and verify radiation dose distributions

What is Dose Deposition in Water Dependant On

beam type (electron, x-ray, etc)

beam energy

field size

source-to-patient distance

Skin Sparing

Dose deposition concentrated at large depths, which allows pateint’s sensitive skin to recieve low dose

X-ray Dose Distribution

x-rays release secondary electrons, indirectly proportional to x-ray energy, which deposit dose (indirect ionisation)

dose rapidly rises until a maximum at depth of tumour, then almost exponentially decreases with depth because photon energy fluence (number of photons at given energy) decreases, causing energy and range of electrons to decrease

This delivers lower dose to patient’s highly sensitive skin (before maximum) and healthy tissue beyond tumour (after maximum)

depth of maximum dose directly proportional to beam energy

Dose Distribution for Neutron Beams

Indircet ionisation - neutrons interact with nucleus, producing protons or heavier nuclei, that deposit dose

More skin sparing than electrons, less than x-rays because recoil protons are not strongly forward-peaked and have shorter ranges, so deposit dose quicker and shallower

Dose Distribution for Electron Beams

High surface dose (very little skin sparing)

Maximum dose at short depth due to fast dose build-up, so best for superficial tumours

After dmax, dose drops rapidly due to their short range which limits dose to deep healthy tissue, and levels off at low dose Bremsstrahlung tail (x-ray contamination of beam)

Higher incident electron energy = more Bremsstrahlung contamination

Electron Beam Dose Distribution Link to Denisty

Higher density (bone) decreases dose transmission, creating under-dosage behind the bone

and potential backscatter hotspots

Lower density (lung) increases electron penetration, thus dose to underlying tissue

Dose Distributions for Heavy Charged Particle Beams

Penetrate tissue, lose energy but do not diverge from direction of motion

Range in tissue depends on incident kinetic energy and mass

Just before particle expended all of its kinetic energy, its energy loss per unit distance traveled increases drastically, resulting in high dose deposition at that depth (bragg peak)

Because of their large mass, heavy charged particles lose their kinetic energy only by interacting with target atom’s orbital electrons (not through Bremsstrahlung interactions with target nuclei)