Ch 4. Clinical Radiation Generators The physics of Radiation Therapy, pp. 45 - 70 Ch 4. Clinical Radiation Generators

Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Kilovoltage Units Up to above 1950 X-rays generated at voltages up to 300 kVps Still some use in the present era, esp. treatment of superficial skin lesions Kilovoltage Therapy Grenz-Ray Therapy Contact Therapy Superficial Therapy Orthovoltage Therapy or Deep Therapy Supervoltage Therapy

Kilovoltage Units Grenz-Ray Therapy Energy : < 20 kV Very low depth of penetration No longer used in R/T Contact Therapy Energy: 40 – 50 kV Short SSD (< 2 cm) Produces a very rapidly decreasing depth dose Max irradiated tissue : skin surface Application: Tumor not deeper than 1 – 2 mm

Kilovoltage Units Superficial Therapy Energy: 50 – 150 kV HVLs: 1.0- – 8.0-mm Al Applicator or cone attached to the diaphragm SSD: 15 – 20 cm Tube current: 5 – 8 mA Application: tumors confined to about 5-mm depth

Kilovoltage Units Orthovoltage Therapy or Deep Therapy Energy: 200 – 300 kV Tube current: 10 – 20 mA HVLs: 1 – 4 mm Cu Cones or movable diaphragm (continuous adjustable field size) SSD: 50 cm Application: tumor located < 2 –3 cm in depth Limitation of the treatment: skin dose Depth dose distribution Increase absorbed dose in bone Increase scattering

Kilovoltage Units Supervoltage Therapy Energy: 500 – 1000 kV Technical problem Insulating the high-voltage transformer Conventional transformer systems were not suitable for producing potential > 300 kVp The problem solved by invention of resonant transformer

Kilovoltage Units Resonant transformer units Used to generate x-rays from 300 to 2000 kV At resonant frequency Oscillating potential attains very high amplitude Peak voltage across the x-ray tube becomes very large

Megavoltage Therapy X-ray beams of energy > 1 MV Accelerators or γray produced by radionuclides Examples of clinical megavoltage machines Van de Graaff generator Linear accelerator Betatron Microtron Teletherapy γray units (e.g. cobalt-60)

Van de Graaff Generator Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Van de Graaff Generator Electrostatic accelerator Energy of x-rays: 2 MV (typical), up to 10 MV Limiation: size high-voltage insulation No longer produced commercially Technically better machine (e.g. Co-60 units & linear accelerators)

Linear Accelerator Kilovoltage Units Van de Graaff Generator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Linear Accelerator Use high frequency electromagnetic waves to acelerate charged particles (e.g. electrons) to high energies through a linear tube High-energy electron beam – treating superficial tumors X-rays – treating deep-seated tumors

Linear Accelerator Fig 4.5. A block diagram of typical medical linear accelerator

Linear Accelerator Types of EM wave 1. Traveling EM wave Required a terminating (“dummy”) load to absorb the residual power at the end of the structure Prevent backward reflection wave 2. Standing EM wave Combination of forward and reverse traveling waves More efficiency Axial beam transport cavities and the side cavities can be independently optimized More expensive Requires installation of a circulator (or insulator) between the power source the structure prevent reflections from reaching the power source

The Magnetron A device that produces microwaves Functions as a high-power oscillator Generating microwave pulses of several microseconds with repetition rate of several hundred pulses per second Frequency of microwave within each pulse is about 3000 MHz Peak power output: 2 MW (for low-energy linacs, 6MV or less) 5 MW (for higher-energy linacs, mostly use klystrons)

The Magnetron The cathode is heated by an inner filament Electrons are generated by thermionic emission Pulse E-field between cathode & anode Electron accelerated toward the anode Static B-field perpendicular to the plane of cavities Electron move in complex spirals toward the resonant cavities Radiating energy in form of microwave

The Klystron Not a generator of microwaves Microwave amplifier Needs to be driven by a low-power microwave oscillator

The Klystron Electrons produced by the cathode Passed in the drift tube (field-free space) Electrons are accelerated by –ve pulse into buncher cavity Lower level microwave set up an alternating E field across the buncher cavity Electrons arrive catcher cavity Generate a retarding E-field Electrons suffer deceleration KE of electrons converted into high-power microwaves Velocity of e- is altered by the action of E-field (velocity modulation) Some e- are speed up Other are slowed down

Auxiliary system The linac auxiliary system consists of several services that are not directly involved with electron acceleration, yet make the acceleration possible and the linac viable for clinical operation. ● A vacuum pumping system producing a vacuum pressure of ~10–6 torr in the accelerating guide and the RF generator; ● A water cooling system used for cooling the accelerating guide, target, circulator and RF generator; ● An optional air pressure system for pneumatic movement of the target and other beam shaping components; ● Shielding against leakage radiation.

The Linac X-Ray Beam Production of x-rays Electrons are incident on a target of a high-Z material (e.g. tungsten) Target – need water cooled & thick enough to absorb most of the incident electrons Bremsstrahlung interactions Electrons energy is converted into a spectrum of x-rays energies Max energy of x-rays = energy of incident energy of electrons Average photon energy = 1/3 of max energy of x-rays Designation of energy of electron beam and x-rays Electron beam - MeV (million electron volts, monoenergetic) X-ray beam – MV (megavolts, voltage across an x-ray tube, hetergeneous in energy)

linac treatment head 31

Lead or tungsten Opening from 0 x 0 to 40 x 40 cm at SSD 100 cm

Narrow pencil about 3 mm in diameter Uniform electron fluence across the treatment field e.g. lead Electron scatter readily in air Beam collimator must be achieved close to the skin surface

The Effect of Flattening Filter 34

Vertical and Horizental Chambers

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Linac generations ● Low energy photons (4–8 MV): straight-through beam; fixed flattening filter; external wedges; symmetric jaws; single transmission ionization chamber; isocentric mounting. ● Medium energy photons (10–15 MV) and electrons: bent beam; movable target and flattening filter; scattering foils; dual transmission ionization chamber; electron cones. ● High energy photons (18–25 MV) and electrons: dual photon energy and multiple electron energies; achromatic bending magnet; dual scattering foils or scanned electron pencil beam; motorized wedge; asymmetric or independent collimator jaws. ● High energy photons and electrons: computer controlled operation; dynamic wedge; electronic portal imaging device (EPID); multileaf collimator (MLC). ● High energy photons and electrons: photon beam intensity modulation with MLC; full dynamic conformal dose delivery with intensity modulated beams produced with an MLC.

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Sections of a Linac The linacs are usually mounted isocentrically and the operational systems are distributed over five major and distinct sections of the machine: (I) gantry (2) gantry stand or support (3) modulator cabinet (4) patient support assembly, i.e., treatment couch (5) control console 39

Betatron Kilovoltage Units Van de Graaff Generator Linear Accelerator Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Betatron Electron in a changing magnetic field experiences acceleration in a circular orbit Energy of x-rays: 6 – 40 MV Disadvantage: low dose rate Small field size

Microtron Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Microtron Electron accelerator which combines the principles of both linear accelerator and the cyclotron Advantage: Easy energy selection, small beam energy spread and small size

Cyclotron Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Cyclotron Charged particle accelerator Mainly used for nuclear physics research As a source of high-energy protons for proton beam therapy Have been adopted for generating neutron beams recently

Cyclotron Structures Short metallic cylinder divided into two section (Ds) Highly evacuated Placed between the poles of a direct current magnet Alternating potential is applied between two Ds

Cyclotron Positive charged particles (e.g. protons or deuterons) are injected at the center of the two Ds Under B-field, the particles travel in a circular orbit Accelerated by E-field while passing from one D to the other Received an increment of energy Radius of its orbit increases

Machine Using Radionuclides Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Machines Using Radionuclides Radionuclides have been used as source of γrays for teletherapy Radium-226, Cesium-137, Cobalt-60 60Co has proved to be most suitable for external beam R/T Higher possible specific activity Greater radiation output Higher average photon energy Radionuclide Half-Life (Years) γRay Energy MeV I- Value ( Rm2_) Ci – h Specific Activity Achieved in Practice (Ci/g) Radium-226 (filtered by 0.5 mm Pt) 1622 0.83 (avg.) 0.825 ~ 0.98 Cesium-137 30.0 0.66 0.326 ~ 50 Cobalt-60 5.26 1.17, 1.33 1/30 ~ 200

Cobalt-60 Unit Source From 59Co(n, γ) nuclear reactor Stable 59Co → radioactive 60Co In form of solid cylinder, discs, or pallets Treatment beam 60Co →60Ni + 0β(0.32 MeV) + γ(1.17 & 1.33 MeV) Heterogeneity of the beam Secondary interactions βabsorbed by capsule → bremsstrahlung x-rays (0.1MeV) scattering from the surrounding capsule, the source housing and the collimation system (eletron contamination)

Cobalt-60 teletherapy machine Theratron-780, AECL (now MDS Nordion), Ottawa, Canada

Methods for moving the teletherapy source from the BEAM-OFF into the BEAM-ON position and back: Source on a sliding drawer Source on a rotating cylinder

Penumbra The region, at the edge of a radiation beam, over which the dose rate changes rapidly as function of distance from the beam axis 1. Transmission penumbra 2. Geometric penumbra

Transmission penumbra Source Transmission penumbra The region irradiated by photons which are transmitted through the edge of the collimator block The inner surface of the blocks is made parallel to the central axis of the beam The extent of this penumbra will be more pronounced for larger collimator opening Minimizing the effect The inner surface of the blocks remains always parallel to the edge of the beam Collimator SDD SSD

Radiation source: not a point source Geometric penumbra Radiation source: not a point source e.g. 60 Co teletherapy → cylinder of diameter ranging from 1.0 to 2.0 cm From considering similar triangles ABC and DEC DE = CE = CD = MN = OF + FN – OM AB CA CB OM OM AB = s (source diameter) OF = SSD DE = Pd ( penumbra) Pd = s (SSD + d – SDD) SDD Parameters determine the width of penumbra

Geometric penumbra (con’t) Solutions Extendable penumbra trimmer Heavy metal bars to attenuate the beam in the penumbra region Secondary blocks Placed closed to the patient for redifining the field Should not be placed < 15 – 20 cm, excessive electron contaminants Definition of physical penumbra in dosimetry Lateral distance between two specified isodose curves at a specified depth At a depth in the patient, dose variation at the field border Geometric, transmission penumbras + scattered radiation produced in the patient

Heavy Particle Beams Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Heavy Particle Beams Advantage Dose localization Therapeutic gain (greater effect on tumor than on normal tissue) Including neutrons, protons, deuterons, αparticles, negative pions, and heavy ions Still experimental Few institutions because of the enormous cost

Neutrons Sources of high energy neutron beams D-T generator, cyclotrons, or linear accelerators D-T generators 2H + 3H → 4He + 1n + 17.6 MeV 1 1 2 0 Monoenergetic (14 MeV) Isotropic (same yield in all directions) Major problem Lack of sufficient dose rate at the treatment distance 15 cGy/min at 1 m Advantage Its size is small enough to allow isocentric mounting on gantry

Mostly in forward direction Cyclotron Stripping reaction 2H + 9Be → 10Be + 1n 1 4 5 0 Mostly in forward direction Spectrum of energies (40% - 50% of deuteron energy) Fig 4.15. Neutron spectra produced by deuterons on beryllium target

Protons and Heavy Ions Energy of therapeutic proton beams 150 – 250 MeV Sources: produced by cyclotron or linear accelerator Major advantage Characteristic distribution of dose with depth Bragg peak

Range energy relationship for protons Use to calculate the range for other particles with the same initial velocity R1/R2 = (M1/M2) · (Z2/Z1)2 R1, R2 — particle range M1,M2 — Masses Z2, Z1 — the charges of the two particle e.g. Protons (150 MeV), Deuterons (300 MeV), Helium ions (600 MeV) have same range of about 16 cm water Range  A/Z2

Negative Pions Pi meson (pion, π) Protons and neutrons are held together by a mutual exchange of pi mesons Mass : 237x of electron Charge : π+, π-, π0 Decay: π+ → μ+ + ν (mean life: 2.54 x 10-18) π- → μ- + ν (mean life: 2.54 x 10-18) π0 → hν1 + hν2(mean life: 2.54 x 10-18) μ— mesons; ν — neutrinos

Negative Pions Pi meson (pion, π) (con’t) Sources: nuclear reactor Cyclotron or linear accelerator with protons (400 – 800 MeV) and beryllium as target material Energy range of pion interest in R/T — 100 MeV Range in water about — 24 cm Problems Low dose rates Beam contamination High cost