Chapter 5 Treatment Machines in External Beam Radiotherapy Flashcards
(35 cards)
When were X-Rays discovered and by whom?
X-Rays were discovered in 1895 by Wilhelm Rontgen .
When did radiotherapy first start?
Radiotherapy started soon after the discovery of X-rays.
Name three main instruments that were used to perform radiotherapy in its early years.
X-Ray Tubes, Van de Graaff generators and Betatrons.
When were Cobalt-60 machines introduced to radiotherapy?
The Cobalt-60 teletherapy machine was invented in the early 50s by H.E. Johns.
What are the two main machines used in modern Radiotherapy?
Linacs and Cobalt-60 teletherapy units.
Other than the linac, name two other machines used for electron and X-ray radiotherapy?
Microtrons and betatrons.
Apart from the electron, what are four particles sometimes used for radiotherapy?
Protons, neutrons, heavy ions, negative pi mesons.
What range of X-ray energy is typically found in clinical X-ray beams/units?
10 kVp - 50 MV X-ray photons are produced using electrons with energies of 10 keV- 50MeV.
What are the two groups of X-rays in X-ray production? State whether they are discrete or continuous.
Bremsstrahlung X-rays (continuous) & Characteristic X-rays (discrete).
Explain the origin of Bremsstrahlung X-rays in X-ray production.
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).
Explain the origin of characteristic X-rays in X-ray production.
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.
What are Auger electrons?
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.
What is the fluorescent yield of an element?
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 .
For a thick target, give the formula for the intensity of an emitted photon in bremsstrahlung production.
[Hint: Unfiltered]
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.
How is target thickness classified or analysed in X-ray production.
“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.”
What are superficial X-rays?
These are X-rays produced from electrons with kinetic energy in the range 10 - 100 keV.
What are orthovoltage X-rays?
These are X-rays produced from electrons with kinetic energy in the range 100 - 500 keV.
What are megavoltage X-rays?
These are X-rays produced from electrons with kinetic energies above 1 MeV.
Which machines/devices are used to produce X-rays in the superficial, orthovoltage and megavoltage range?
“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.”
Describe shape of the spectrum for the following:
(1) Unfiltered thin target
(2) Unfiltered thick target
(3) Filtered thick target
(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.
How is X-ray beam quality described quantitatively?
“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).”
How are gamma rays produced?
Gamma rays are produced when an unstable (radioactive) nucleus undergoes gamma decay.
For gamma production, there is need for a radioactive source.
What are important characteristics of a gamma ray source used for the purpose of external beam radiation therapy?
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
Define ‘specific activity’ of a radioisotope.
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
