LII:Radiation Oncology/Physics/Treatment Machines

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Radiation Therapy Treatment Machines

Photons/Electrons

Kilovoltage Units (up to 300 keV)

Contact Therapy Machines
- 40-50 keV
- filtered with 0.5-1.0 mm aluminum to remove very low energy (beam hardened)
- SSD = 2cm
- 50% depth dose at 5 mm

Superficial Therapy Machines
- 50-150 keV
- filtered with 1-4 mm aluminum to remove very low energy (beam hardened)
- SSD = 20 cm
- 50% depth dose at 1-2 cm

Orthovoltage Therapy Machines
- 150-300 keV
- filtered with 1-4 mm copper
- SSD = 50 cm, field 20 x 20 cm
- 50% depth dose at 5-7 cm
- primary treatment machines <1950's

Megavoltage Units (>1 MeV)

Teletherapy Machine (Cobalt)
- Introduced in 1951
- Cobalt-60 decay produces two photons with 1.17 MeV and 1.33 MeV energies
- dose rate of >150 cGy/min
- SSD = 80 cm, field 40 x 40 cm
- 50% depth dose 10 cm
- half-life 5.28 years, requiring replacement of source every ~5 years
- 2cm size of source capsule results in a wide penumbra of the beam (esp compared with 4mm LINAC source size)

Betatron
- Developed in 1940 mainly for physics experiments
- Treatment energies up to 45 MeV (although betatron maximum energies were ~300 MeV)
- Massive size, high cost, and low dose rate prevented wide adoption

Linear Accelerator (linac)
- Introduced in 1953 for clinical practice
- Please see x-ray production section on more details
- Currently the main treatment machine in modern radiation oncology
- SSD = 100 cm, field 40 x 40 cm
- A single machine may produce several different photon beam energies, as well as function in an electron mode

Microtron
- Introduced in 1972 for clinical practice
- Accelerator that linearly accelerates the electrons (like linac), but confines them using a fixed magnetic field (like cyclotron). Electrons move in increasingly large orbits, and desired energy is selected out for extraction and photon production
- Compared to linac, are simple, small, and have easy energy selection. The beam itself is has small energy spread, beam divergence, and size
- Commercial medical energies up to 50 MeV

Heavy particles

Cyclotrons

Accelerate heavy charged particles (protons, heavy ions, deuterons for production of neutron beams)
Developed in 1930's for physics research; introduced in 1960's for clinical practice
Particles are accelerated in circular orbits between two D-shaped half-cyclinders using a fixed magnetic field and variable-radius orbits
Proton energies of >200 MeV are required for sufficient depth penetration
Deuteron energies of ~50 MeV are sufficient for production of neutron beams

Synchrotrons

Accelerate heavy charged particles (protons, heavy ions, deuterons for production of neutron beams)
Similar to cyclotrons, but instead use a variable magnetic field and fixed-radius orbits
Loma Linda has the first dedicated proton facility

Production of X-rays

Electron Volt (eV) - unit of energy used in radiation oncology. Kinetic energy acquired by an electron at rest after being accelerated through 1 volt field. Acceleration distance does not matter, since shorter distance generates stronger field, leading to greater acceleration. Measure of the beam energy

X-rays are produced by two steps:
- 1 - acceleration of electrons
- 2 - collision of these electrons with a metal target, converting their energy into x-rays
  - 2A - Bremsstrahlung (80% at diagnostic energies, 99% at treatment energies)
  - 2B - Characteristic x-ray production via ionization (20% at diagnostic energies, 1% at treatment energies)

Diagnostic and therapeutic x-rays differ in their acceleration of electrons; the collision with target and x-ray production is essentially the same

Diagnostic X-ray electron acceleration
- A metal filament (tungsten) serves as a source of free electrons. The filament is heated to free up more electrons per unit time
- The filament is placed in vacuum to prevent air molecules interacting with the free electrons. Pyrex glass is used to withstand the heat
- High voltage is applied to generate appropriate X-rays

Therapeutic X-ray electron acceleration
- to be continued ...

Electron collision with metal target (and production of x-rays)
- Bremsstrahlung (Radiation)
  - Incoming negative electron is attracted by positively charged nucleus, resulting in directional change
  - Direction change requires the electron to lose energy, radiated as x-ray photon
  - The greater the bending, the greater energy loss, and the higher the generated x-ray energy
  - 180 degree bending results in all of the incident electron energy to be converted to x-ray. This is the maximum beam energy. Diagnostic x-ray are in the 10 - 150 keV range
  - Spectrum of x-rays with different energies is produced, as a function of the different electrons bending differently
  - The lowest energy x-rays are most commonly produced, but many are filtered out in the equipment
  - The process is not very efficient, converting only ~1% into usable EM (x-rays) and remaining 99% into useless EM (heat)
  - The higher the incoming electron energy, the more likely it is converted to x-ray and not heat
- Characteristic X-rays (Ionization)
  - Incoming electron knocks out a K or L shell electron, creating a vacancy in that shell
  - Higher shell electron then "jumps down" to fill the vacancy, and in the process sheds the excess energy as EM radiation (photon)
  - This radiation is characteristic for a given element (since it is a function of the difference in shell energy levels), and is thus called characteristic radiation
  - Characteristic x-rays are produces as a series of peaks, depending on the difference between the source and target shells
  - For tungsten, these are at 59.3 keV (L1->K), 58.0 keV (L2->K), and 67.2 (M->K). The L shells are not visible
  - Characteristic x-rays do not vary in energy as voltage is changed, as their are a function of the different shell energies, caused by "jumping down" of electrons into a vacated orbital spot
- Since x-rays are generated as a complex spectrum, effective energy of the spectrum is defined as the single energy that has the same penetration in water as the actual spectrum. For tungsten at keV energies, it is approximately 1/2.2 (45%). For tungsten at MeV energies, it is approximately 1/3