Particle Accelerators in Medicine: Medical Physics

CHAPTER 2
MEDDICAL PHYSICS
Specific Objectives:
At the end of this chapter students will be able to:
 Familiarize with the glossary of different particle accelerators used in medicine.
 To name the types of particle accelerator built for nuclear physics and particle physics
research in English
 Understand their use in medicine, mainly for treatment of illness like cancer.
 Deal with categories of particle accelerator known as: electrostatic and cyclic.
Activity 1: READING
Text: Particle Accelerators in Medicine
The best-known examples of electrostatic accelerator are the x-ray tube and the neutron
generator. Three types of x-ray tube (Crookes tube, Coolidge tube, field emission tube) are
discussed in this chapter; neutron generator is discussed briefly in Sect. 9.6.3. Cyclic
accelerators fall into two categories: linear and circular. Many different types of circular
accelerator have been designed for research purpose and most are also used in medicine,
such as the betatron, microtron, cyclotron, and synchrotron.
Of all cyclic accelerators, the linear accelerator is by far the most important and most widely
used in medicine because of its versatility and compact design. Actually, one can say that
modern radiotherapy achieved its successes as a result of the advances that were introduced
during the past few years in the linear accelerator technology. In this chapter circular
accelerators are discussed briefly, before the chapter undertakes a detailed discussion of the
practical aspects of linear accelerators used clinically for cancer therapy.
1-1 Basic Characteristics of Particle Accelerators
Numerous types of accelerators have been built for basic research in nuclear physics and
high-energy physics, and most of them have been modified for at least some limited use in
radiotherapy. X-ray machine is the simplest accelerator and is widely used in medicine
both for diagnosis of disease in diagnostic radiology and for treatment of disease in
radiotherapy. In addition to megavoltage linear accelerators which are the most widely used
machines in radiotherapy, other accelerators used in medicine are cyclotrons for proton and
neutron radiotherapy as well as for production of positron emitting radionuclides for PET
studies; betatrons and microtrons for x-ray and electron beam radiotherapy; and
synchrotrons for hadron radiotherapy.

Irrespective of the accelerator type two basic conditions must be met for particle acceleration:
1. Particle to be accelerated must be charged.
2. Electric field must be provided in the direction of particle acceleration.
The various types of accelerators differ in the way they produce the accelerating electric field
and in how the field acts on the particles to be accelerated.
As far as the accelerating electric field is concerned there are two main classes of accelerators:
electrostatic and cyclic.
In electrostatic accelerators the particles are accelerated by applying an electrostatic electric
field through a voltage difference, constant in time, whose value fixes the value of the final
kinetic energy of the accelerated particle. Since the electrostatic fields are conservative, the
kinetic energy that the particle can gain depends only on the point of departure and point of
arrival and, hence, cannot be larger than the potential energy corresponding to the maximum
voltage drop existing in the machine. The kinetic energy that an electrostatic accelerator can
reach is limited by the discharges that occur between the high voltage terminal and the walls
of the accelerator chamber when the voltage drop exceeds a certain critical value (typically 1
MV).
The electric fields used in cyclic accelerators are variable and non-conservative, associated
with a variable magnetic field and resulting in some closed paths along which the kinetic
energy gained by the particle differs from zero.
If the particle is made to follow such a closed path many times over, one obtains a process of
gradual acceleration that is not limited to the maximum voltage drop existing in the
accelerator. Thus, the final kinetic energy of the particle is obtained by submitting the charged
particle to the same, relatively small, potential difference a large number of times, each cycle
adding a small amount of energy to the total kinetic energy of the particle.
Cyclic accelerators fall into two main categories: linear accelerators and circular accelerators,
depending on particle’s trajectory during the acceleration.
In a linear accelerator the particle undergoes rectilinear motion, while in a circular accelerator
the particle’s trajectory is circular. All cyclic accelerators except for the linear accelerator fall
into the category of circular accelerator.
Examples of electrostatic accelerators used in medicine are: superficial and orthovoltage x-
ray machines and neutron generators. In the past, Van de Graaff accelerators have been used
for megavoltage radiotherapy; however, their use was discontinued with the advent of first the
betatron and then the linear accelerator. For medical use, the best-known example of a cyclic
accelerator is the linear accelerator (linac); all other examples fall into the circular accelerator
category and are the microtron, betatron, cyclotron, and synchrotron.

1.2 Practical Use of X Rays
Roentgen’s discovery of x rays in 1895 is one of several important discoveries that occurred
in physics at the end of the nineteenth century and had a tremendous impact on science,
technology, and medicine in particular and modern society in general. Two other discoveries
of similar significance are Becquerel’s discovery of natural radioactivity in 1896 and a by
Marie Curie and Pierre Curie in 1898.
Studies in x-ray physics stimulated developments of modern quantum and relativistic
mechanics and triggered the practical use of x rays in medicine and industry. The use of x
rays in diagnosis of disease developed into modern diagnostic radiology, while the use of x
rays in treatment of cancer developed into modern radiotherapy. Concurrently with these two
medical specialties, medical physics has evolved as a specialty of physics dealing with the
physics aspects of diagnosis and treatment of disease, mainly but not exclusively with x rays.
On a smaller scale x-ray research generated new research modalities, such as x-ray
crystallography, x-ray spectroscopy, and x-ray astronomy. Scientific x-ray research to
date resulted in 14 Nobel Prizes: eight of these in Physics, four in Chemistry and two in
Medicine.
1.2.1 Medical Physics
Medical physics is a specialty of physics dealing with the application of physics to medicine,
most generally in three areas: (1) Diagnostic imaging physics (∼25 % of total effort); (2)
Nuclear medicine physics (∼5 % of total effort); and (3) Radiotherapy physics (∼70 % of
total effort). While nuclear medicine concentrates mainly on application of unsealed
radionuclides for diagnosis and treatment of disease, the use of x rays forms an important
component of diagnostic radiology as well as radiotherapy. The former uses x rays in the
photon energy range from 50 kVp to 150 kVp produced by x-ray tubes; the later uses x rays in
a much wider energy range extending from 50 kVp to 25 MV, produced by x-ray machines in
the kilovolt range and linear accelerators in the megavolt range.
1.2.2 Industrial Use of X Rays
X rays for industrial use are produced by x-ray machines or linear accelerators and cover a
wide variety of purposes dealing with safety and quality assurance issues, such as:
1. Inspection of luggage, shoes, mail, cargo containers, etc…
2. Nondestructive testing and inspection of welds, cast metals, parts of automobiles and
airplanes, iron reinforcement bars, cracks and pipes inside concrete structures.
3. Food irradiators for sterilization and pest control.
4. Radiation based sterilizators of surgical equipment and blood irradiators.
5. Small animal irradiators for radiobiological experiments.

1.2.3 X-Ray Crystallography
X-ray crystallography is a study of crystal structures through the use of x-ray diffraction
techniques. X rays are very suitable for this purpose because their wavelength in the 0.1 ˚A
(∼100 keV) to 1 ˚A (∼10 keV) range is of the order of typical crystalline lattice separations.
An x-ray beam striking a crystalline lattice is scattered by the spatial distribution of atomic
electrons and the imaged diffraction pattern provides information on the atomic or molecular
structure of the crystalline sample. In 1912 Max von Laue established the wave nature of x
rays and predicted that crystals exhibit diffraction phenomena.
Soon thereafter, William H. Bragg and William L. Bragg analyzed the crystalline structure
of sodium chloride, derived the Bragg relationship 2d sin φ = mλ (Fig. 1.8) linking the lattice
spacing d with x-ray wavelength λ, and laid the foundation for x-ray crystallography. The
crystal lattice of a sample acts as a diffraction grating and the interaction of x rays with the
atomic electrons creates a diffraction pattern which is related, through a Fourier transform, to
the electron spectral distribution in the sample under investigation.
Instrumentation for x-ray diffraction studies consists of a monoenergetic x-ray source, a
device to hold and rotate the crystal, and a detector suitable for measuring the positions and
intensities of the diffraction pattern.
Monoenergetic x rays are obtained by special filtration of x rays produced either by an x-ray
tube or from an electron synchrotron storage ring. The basic principles of modern x-ray
crystallography are essentially the same as those enunciated almost 100 years ago by von
Laue and the Braggs; however, the technique received a tremendous boost by incorporation of
computer technology after the 1970s, increasing significantly the accuracy and speed of the
technique.
1.2.4 X-Ray Spectroscopy
X-ray spectroscopy is an analytical technique for determination of elemental composition of
solid or liquid samples in many fields, such as material science, environmental science,
geology, biology, forensic science, and archaeometry.
The technique is divided into three related categories: the most common of them is the x-ray
absorption spectrometry (also called x-ray fluorescence spectrometry), and the other two are
x-ray photoelectron spectrometry and Auger spectrometry. All three techniques rely on
creation of vacancies in atomic shells of the various elements in the sample under study as
well as on an analysis of the effects that accompany the creation of vacancies (e.g., emission
of photoelectron, emission of characteristic line spectrum, and emission of Auger electron).
Like other practical emission spectroscopic methods, x-ray spectroscopy consists of three
steps:
1. Excitation of atoms in the sample to produce fluorescence emission lines (or
photoelectrons or Auger electrons) characteristic of the elements in the sample. The most
common means for exciting characteristic x-ray photons for the spectroscopic analysis is by

use of x rays produced by x-ray machines; however, energetic electrons and heavy charged
particles such as protons are also used for this purpose. Excitation by electrons is called the
primary or impulse excitation; excitation by photons is called secondary or fluorescence
excitation; excitation by heavy charged particles is called particle-induced x-ray emission
(PIXE).
2. Measurement of intensity and energy of the emitted characteristic lines (or electrons). All
methods for determining x-ray wavelengths λ use crystals as gratings and are based on the
Bragg law mλ = 2d sin φ where d is the lattice spacing and m is an integer. The dynamic
range of these methods extends from 20 ˚A to 0.1 ˚A corresponding to photon energies of 6
keV to 130 keV, and the range of detectable elements in an unknown sample extends from
beryllium (Z = 4) to uranium (Z = 92).
3. Conversion of measured data to concentration or mass with the nanogram range reached
with standard spectrometers. The main disadvantage of the technique is that only a thin
surface layer of the order of a few tenths of a millimeter can be analyzed because of
absorption effects of the low energy fluorescence radiation. This requires a perfectly
homogeneous sample for accurate results.
While x-ray spectroscopy was initially used to further the understanding of x-ray absorption
and emission spectra from various elements, its role now is reversed and it is used as a non-
destructive analytical tool for the purpose of chemical analysis of samples of unknown
composition.
Source: Biological and Medical Physics, Biomedical Engineering by Ervin B. Podgorsak
Radiation Physics for Medical Physicists
Second Edition
Activity2: COMPREHENSION
Exercise: 1
A/ Read the text and find the best answer for the following questions
1- What are the outstanding examples of electrostatic accelerator?
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2- How many x-ray tubes are provided in the text? List them.
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3- How many categories are hooked on cyclic accelerators? List them.
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4- What are the required conditions for particle acceleration?