Chapter 5 Treatment Machines in External Beam Radiotherapy

Question 1

When were X-Rays discovered and by whom?

Answer 1

X-Rays were discovered in 1895 by Wilhelm Rontgen .

Question 2

When did radiotherapy first start?

Answer 2

Radiotherapy started soon after the discovery of X-rays.

Question 3

Name three main instruments that were used to perform radiotherapy in its early years.

Answer 3

X-Ray Tubes, Van de Graaff generators and Betatrons.

Question 4

When were Cobalt-60 machines introduced to radiotherapy?

Answer 4

The Cobalt-60 teletherapy machine was invented in the early 50s by H.E. Johns.

Question 5

What are the two main machines used in modern Radiotherapy?

Answer 5

Linacs and Cobalt-60 teletherapy units.

Question 6

Other than the linac, name two other machines used for electron and X-ray radiotherapy?

Answer 6

Microtrons and betatrons.

Question 7

Apart from the electron, what are four particles sometimes used for radiotherapy?

Answer 7

Protons, neutrons, heavy ions, negative pi mesons.

Question 8

What range of X-ray energy is typically found in clinical X-ray beams/units?

Answer 8

10 kVp - 50 MV X-ray photons are produced using electrons with energies of 10 keV- 50MeV.

Question 9

What are the two groups of X-rays in X-ray production? State whether they are discrete or continuous.

Answer 9

Bremsstrahlung X-rays (continuous) & Characteristic X-rays (discrete).

Question 10

Explain the origin of Bremsstrahlung X-rays in X-ray production.

Answer 10

An electron passing close to the nucleus of an atom interacts with the nuclear field through the Coulomb interaction, losing some of its kinetic energy and producing a Bremsstrahlung photon.
The production of a bremsstrahlung photon in the deceleration of a charged particle in the midst of another charged particle is also called radiative loss.

In the production of the bremsstrahlung photon, the electron is also deflected/scattered through a small angle. Due to the loss of energy in the process, the electron is said to be inelastically scattered.

Since a range of energies of bremsstrahlung photons may be produced from the electrons (from 0 to the max kinetic energy of the electron), the bremsstrahlung spectrum is a continuous spectrum.

Bremsstrahlung or “braking radiation” is also produced in synchrotron and cyclotrons when magnetic fields are used to bend particles moving at high speeds. In those cases, it is called synchrotron radiation (radiation produced by a relativistic particle) and cyclotron radiation (radiation produced by a non-relativistic particle).

Question 11

Explain the origin of characteristic X-rays in X-ray production.

Answer 11

An electron may collide with orbital electrons in target atoms, resulting in ejection of the electron and an orbital vacancy. An electron in an orbit of higher energy may then fill the vacancy, thereby emitting energy in the form of a photon. Characteristic X-rays are thus discrete “line” spectra corresponding to energy transitions in the orbital electrons of a given element. Different elements are thus associated with
corresponding characteristic X-ray spectra.

Auger electrons arise when the energy is instead transferred to another electron, instead of being released in the form of a photon. The auger electron then has enough energy to escape the atom.

Question 12

What are Auger electrons?

Answer 12

Auger electrons are electrons that are released from atoms following an energy transition of a higher shell electron to a lower shell orbital vacancy. Usually the energy difference between the higher shell and lower shell is released in the form of a photon. In this case, the energy is transferred to another electron, supplying it with enough energy to escape the atom.

Question 13

What is the fluorescent yield of an element?

Answer 13

The fluorescent yield, ω, is the number of characteristic photons released per vacancy in a shell. 0<= ω <=1. For low Z elements, ω is closer 0 and for high elements, ω is closer to 1. Eg. for copper (Z=29) ω=0.5 .

Question 14

For a thick target, give the formula for the intensity of an emitted photon in bremsstrahlung production.
[Hint: Unfiltered]

Answer 14

The thick target spectrum follows the following formula:

I(hv)= CZ(E_k - hv)

where C is a proportionality constant, Z is the atomic number of the target material, E_k is the kinetic energy of the incident electron and hv is the energy of the photon.

Question 15

How is target thickness classified or analysed in X-ray production.

Answer 15

“According to the range R of electrons of a given kinetic energy E_k in the target material, targets are divided into two main groups: thin and thick.
A thin target has a thickness much smaller than R, while the thickness of a thick target is of the order of R.”

Question 16

What are superficial X-rays?

Answer 16

These are X-rays produced from electrons with kinetic energy in the range 10 - 100 keV.

Question 17

What are orthovoltage X-rays?

Answer 17

These are X-rays produced from electrons with kinetic energy in the range 100 - 500 keV.

Question 18

What are megavoltage X-rays?

Answer 18

These are X-rays produced from electrons with kinetic energies above 1 MeV.

Question 19

Which machines/devices are used to produce X-rays in the superficial, orthovoltage and megavoltage range?

Answer 19

“Superficial and orthovoltage X rays are produced with X ray tubes (machines), while megavoltage X rays are most commonly produced with linacs and sometimes with betatrons and microtrons.”

Question 20

Describe shape of the spectrum for the following:

(1) Unfiltered thin target
(2) Unfiltered thick target
(3) Filtered thick target

Answer 20

(1) Rectangular spectrum showing equal probabilities/intensities for each photon energy up till E-k, which is the max photon value. After this energy, intensity falls to 0.
(2) Pyramidal spectrum consisting of stacked thin target spectra. The final spectrum is simply a triangle with the max intensity being at 0 energy and minimum being at the E-k value.
(3) This is the classical X-ray spectrum detected upon exiting the X-ray tube window. The intensity is 0 for low energy photons, rises to a maximum intensity then falls back down to 0 at the E_k value.

Question 21

How is X-ray beam quality described quantitatively?

Answer 21

“Various parameters, such as photon spectrum, half-value layer (HVL), nominal accelerating potential (NAP) and beam penetration into tissue equivalent media, are used as X ray beam quality indices
● A complete X ray spectrum is very difficult to measure; however, it gives the most rigorous description of beam quality.
● The HVL is practical for beam quality description in the superficial (HVL in aluminium) and orthovoltage (HVL in copper) X ray energy range, but not practical in the megavoltage energy range because in this energy
range the attenuation coefficient is only a slowly varying function of beam energy.
● The effective energy of a heterogeneous X ray beam is defined as that energy of a monoenergetic photon beam that yields the same HVL as does the heterogeneous beam.
● The NAP is sometimes used for describing the megavoltage beam quality. The NAP is determined by measuring the ionizationn ratio in a water phantom at depths of 10 and 20 cm for a 10 × 10 cm2 field at the nominal source to axis distance (SAD) of 100 cm.
● Recent dosimetry protocols recommend the use of tissue–phantom ratios or percentage depth doses (PDDs) at a depth of 10 cm in a water phantom as an indicator of megavoltage beam effective energy (beam
quality index).”

Question 22

How are gamma rays produced?

Answer 22

Gamma rays are produced when an unstable (radioactive) nucleus undergoes gamma decay.
For gamma production, there is need for a radioactive source.

Question 23

What are important characteristics of a gamma ray source used for the purpose of external beam radiation therapy?

Answer 23

The important characteristics of radioisotopes in external beam radiotherapy
are:
— High gamma ray energy;
— High specific activity;
— Relatively long half-life;
— Large specific air kerma rate constant , Gamma_AKR

Question 24

Define ‘specific activity’ of a radioisotope.

Answer 24

Specific activity is the activity per unit mass of the radioactive nuclide.

a= A/m
a=( lambda * N)/m

a= ln(2)*N_a/(t_half * m_A)

where N_a= avogadro’s number
t_half= half-life time
m_A= atomic mass number

Question 25

Define air kerma rate in air as a function of the specific air kerma rate constant of a radioactive nuclide.

Answer 25

A= source activity
d= distance between the source and point of interest
Gamma_AKR= specific air kerma rate constanct 

Air kerma rate= (A * Gamma_AKR)/ (d^2)

Question 26

Based on ideal characteristics for a gamma ray source, name three possible sources for EXBRT ?

Answer 26

(1) Cobalt-60 : most suitable
(2) Cesium-137
(3) Europium- 152

Question 27

What is a teletherapy machine?

Answer 27

“Treatment machines incorporating gamma ray sources for use in external beam radiotherapy are called teletherapy machines. They are most often mounted
isocentrically, allowing the beam to rotate about the patient at a fixed SAD. Modern teletherapy machines have SADs of 80 or 100 cm.
The main components of a teletherapy machine are: a radioactive source; a source housing, including beam collimator and source movement mechanism; a gantry and stand in isocentric machines or a housing support assembly in stand-alone machines; a patient support assembly; and a machine console.”

Question 28

What is the typical activity of a cobalt-60 teletherapy source?

Answer 28

” Typical source activities are of the order of 5000–10000 Ci (185–370 TBq) and provide a typical dose rate at 80 cm from the teletherapy source of the order of 100–200 cGy/min. “

Question 29

How long are cobalt-60 teletherapy sources used after first installation?

Answer 29

Teletherapy sources are usually replaced within one half-life after they are installed; however, financial considerations often result in longer source usage.

Question 30

What are the international regulations on head leakage from a teletherapy machine?

Answer 30

” Some radiation will escape from the unit even when the source is in the
beam off position. The head leakage typically amounts to less than
1 mR/h (0.01 mSv/h) at 1 m from the source. International regulations
require that the average leakage of a teletherapy machine head be less
than 2 mR/h (0.02 mSv/h) at 1 m from the source. “

Question 31

Give examples of electrostatic accelerators and cyclic accelerators in medicine.

Answer 31

“Examples of electrostatic accelerators used in medicine are superficial
and orthovoltage X ray tubes and neutron generators. The best known example
of a cyclic accelerator is the linac; other examples are microtrons, betatrons and
cyclotrons. “

Question 32

What are the different sections of a linac machine?

Answer 32

Linacs are usually mounted isocentrically and the operational systems are
distributed over five major and distinct sections of the machine, the:
● Gantry;
● Gantry stand or support;
● Modulator cabinet;
● Patient support assembly (i.e. treatment table);
● Control console.

Question 33

List the main beam-forming components of a medical linac.

Answer 33

—The main beam forming components of a modern medical linac are
usually grouped into six classes:
(i) Injection system;
(ii) RF power generation system;
(iii) Accelerating waveguide;
(iv) Auxiliary system;
(v) Beam transport system;
(vi) Beam collimation and beam monitoring system.

Question 34

Explain what is an injection system for a linac and how it works.

Answer 34

Pg. 140- 142 ROP

Question 35

Explain what is a radiofrequency power generator system for a linac and how it works.

Answer 35

pg. 143 ROP