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The Physics of Stereotactic Radiosurgery

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Principles and Practice of Stereotactic Radiosurgery

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Abstract

High-energy ionizing radiation is the tool of radiosurgery. Three major sources for radiosurgery are gamma rays from Cobalt-60 radioactive isotopes, X-ray beams from linear accelerators, and proton beams from charged particle accelerators. Both gamma and X-rays have the same physical characteristics (i.e., both are photons) and their interaction mechanism with material is stochastic. Three major photon interactions are photoelectric absorption, Compton scattering, and pair production. Protons are positively charged and continuously interact with material as they travel. While the dose deposited by photons drops off exponentially with penetration depth, the dose deposited by protons increases very slowly initially then sharply increases, reaching a maximum before rapidly dropping off to 0. Dose deposition characteristic determines fundamental treatment geometry needed. Three major photon beam delivery systems are Gamma Knife, CyberKnife, and conventional linear accelerator (LINAC). Among them, LINAC is the most popular machine and its major components are electron gun, accelerator tube, bending magnet, and treatment head. CyberKnife uses a compact linear accelerator mounted on a robot while Gamma Knife utilizes many Cobalt-60 sources. Beam size has been conventionally determined by a prefabricated circular collimator. However, use of multileaf collimator is increasing in LINAC. In recent Gamma Knife unit, automatic choice among four different collimator sizes (including zero size) is possible. CyberKnife also provides a continuously varying collimator in its latest version. A significantly large dose is delivered in radiosurgery. Therefore, the impact of inaccurate target localization or beam delivery can be disastrous. To minimize such disasters, radiosurgery quality assurance should be stringent.

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Author information

Authors and Affiliations

  1. Department of Radiation Oncology, Virginia Commonwealth University, Richmond, VA, USA

    Siyong Kim Ph.D. & Jatinder Palta Ph.D.

Authors
  1. Siyong Kim Ph.D.
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  2. Jatinder Palta Ph.D.
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Corresponding author

Correspondence to Siyong Kim Ph.D. .

Editor information

Editors and Affiliations

  1. Department of Neurosurgery, SUNY Upstate Medical University, Syracuse, USA

    Lawrence S. Chin

  2. Dept. of Radiation Oncology, Univ. of Maryland School of Medicine, Baltimore, Maryland, USA

    William F. Regine

Glossary

μSv

Microsievert, the SI unit for an equivalent radiation dose that accounts for biological effect.

AAPM

American Association of Physicists in Medicine.

Accelerator

A device that accelerates subatomic charged particles or nuclei to high energies.

Alpha particle

Positively charged particle consisting of two protons and two neutrons.

Beta particle

High-speed electron or positron.

Binding energy

Net energy required to remove an atomic electron from its orbit.

Bq

Becquerel, the Système International (SI) unit of radioactivity equal to one disintegration per second.

Bragg peak

Peak can be seen at the end of the depth dose curve of heavy charged particles such as a proton.

BTS

Beam transport system, a proton accelerator system that carries the protons exiting the accelerator to the patient site.

Cavity

A device through which power is coupled to the accelerating particles in a resonant mode.

Ci

Curie, unit of radioactivity equal to 3.7 × 1010 disintegrations per second.

Collimator

A device capable of collimating radiation.

CT

Computed tomography.

Cyclotron

A particle accelerator in which charged particles are accelerated in a circular path by an alternating electric field in a constant magnetic field.

DD

Depth dose, relative dose level according to the depth of the material when the radiation beam is incident.

Dipole

A pair of electric charges or magnetic poles.

Dose

Energy absorbed per unit mass by radiation.

DVH

Dose–volume histogram, accumulated volume according to dose for a volume of interest.

FF

Flattening filter.

FFF

Flattening filter free.

Filter

A device used to modify the intensity of radiation.

FWHM

Full width at half maximum.

Gamma particle

Electromagnetic radiation particle (photon) emitted by radioactive decay.

Gy

Gray, a unit of dose equivalent to joules per kilogram.

Half-life

Length of time needed for a radioactive substance to lose half of its radioactivity from decay.

Ionization chamber

A chamber for determining the intensity of ionizing radiation by measuring ionization of the enclosed gas.

Ionizing radiation

Electromagnetic or particulate radiation capable of producing ions in its passage through matter.

IMRT

Intensity modulated radiation therapy.

IMSR

Intensity modulated stereotactic radiosurgery, a treatment technique in which radiation intensity is modulated within a field to obtain an optimized dose distribution.

IRIS

A variable aperture collimator in CyberKnife system.

Isocenter

A point in the space that is the center of rotation of the gantry, collimator, and table of an isocentric medical linear accelerator.

Isodose line

A line made by connecting all points of the same dose.

Kernel

A function that describes how the dose is distributed within a medium.

keV

Kiloelectron volts, a unit of energy of particles.

Klystron

A microwave amplifier.

Linear accelerator

A particle accelerator in which the path of the particles is straight.

mA

Milliampere, a unit of electric current.

Magnetron

A diode-type electron tube that generates microwave pulses.

MeV

Million electron volts, a unit of energy of particles.

mMLC

Micro-multileaf collimator, a special collimator consisting of many small collimator leaves that can be individually moved to conform to the shape of the target.

mR

Milliroentgen, a unit of exposure.

MV

Million volts, a unit of beam quality of radiation therapy beams produced in linear accelerators.

OAR

Off-axis ratio, relative dose distribution following a line on a plane normal to the direction of the radiation beam, same as profile.

Output

Magnitude of radiation produced.

PDD

Percent depth dose, relative dose level in percentile according to the depth of the material when the radiation beam is incident.

Photon

Quantum of electromagnetic energy with no mass and charge considered to be both particle and wave.

Positron

Positively charged particle of the same mass and magnitude of charge as an electron.

Profile

Relative dose distribution following a line on a plane normal to the direction of radiation beam, same as the OAR.

Proton

Stable and positively charged subatomic particle.

QA

Quality assurance.

Radioactive decay

Spontaneous disintegration of a radionuclide followed by the emission of ionizing radiation in the form of alpha, beta, or gamma particles.

Radioactivity

Capability of radioactive decay.

Radioisotope

Naturally or artificially produced radioactive isotope of an element.

Range

Distance charged particles can travel.

Resonance

Condition of being resonant in which an increase in the amplitude of a physical quantity can occur.

RF

Radiofrequency, a range of electromagnetic frequencies above sound and below visible light.

SBRT

Stereotactic body radiotherapy.

SRS

Stereotactic radiosurgery.

SRT

Stereotactic radiotherapy.

Synchrotron

A particle accelerator in which charged particles are accelerated around a fixed circular path by an electric field and held to the path by an increasing magnetic field.

TERMA

Total energy released per unit mass, amount of energy released from radiation to the unit mass of the interaction material.

Z

Atomic number; that is, the number of protons in an element.

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Kim, S., Palta, J. (2015). The Physics of Stereotactic Radiosurgery. In: Chin, L., Regine, W. (eds) Principles and Practice of Stereotactic Radiosurgery. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8363-2_4

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  • DOI: https://doi.org/10.1007/978-1-4614-8363-2_4

  • Publisher Name: Springer, New York, NY

  • Print ISBN: 978-1-4614-8362-5

  • Online ISBN: 978-1-4614-8363-2

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