SXR and DXR

Question 1

which country has the most SXR machines and why

Answer 1

Australia because SXR treat BCC and SCC and UV is higher in Australia

Question 2

Historically EXBT was dominated by kilovoltage what is the problem with this?

Answer 2

Most of the dose is deposited on the skin surface which was is a problem.

Question 3

what is SXR used to treat

Answer 3

BCC and SCC

Question 4

what is DXR used to treat

Answer 4

bony metastasis

Question 5

what is the kV for SXR and DXR

Answer 5

SXR - 50-150kV
DXR - 150-300kV

Question 6

describe how x-rays are produced

Answer 6

• Relies on acceleration of electrons from a cathode to an anode, the kinetic energy of the electrons is transferred to the target atoms – Tungsten.
• X-ray photons are produced by two main processes:
  1. Bremsstrahlung – the release of x-ray photons by bringing the electrons to stop – breaking radiation.
  2. Characteristic – radiation released during the cascade effect of electrons rearranging within the atom.

Question 7

what are the two ways x-ray photons are produced

Answer 7

1.Bremsstrahlung – the release of x-ray photons by bringing the electrons to stop – breaking radiation.
2.Characteristic – radiation released during the cascade effect of electrons rearranging within the atom.

Question 8

what does an x-ray spectrum look like?

Answer 8

a range of photon energies produced via braking and discrete peaks of characteristic photon energy.

Question 9

what are the characteristics of a modern kV SXR machine?

Answer 9

• delivered via dedicated therapy tubes which are designed to withstand higher potentials.
• stationary anode inserted into oil/water cooler
• bipolar tubes
• reflective target
• beryllium window

Question 10

What is the role in the cathode and anode in the generation of x-rays

Answer 10

the filament within the cathode heats up and causes thermionic emission of electrons which are attracted to the Tungsten target in the anode which produces x-ray photons through bremsstrahlung and characteristic radiation.

Question 11

what is a indication of the beryllium window

Answer 11

provides inherent filtration of very low energy photons which are considered unwanted dose.

Question 12

how is heat dissipated in the x-ray tube?

Answer 12

copper block dissipates heat however if it does not dissipate heat enough and therefore a water/oil coiler is needed

Question 13

in terms of SXR and DXR when are water and oil coolers used.

Answer 13

water = SXR as higher energies will boil the water
Oil = DXR

Question 14

what does beam quality refer too

Answer 14

how far the beam penetrates into the body.

Question 15

why are X-rays produced by kV tubes not representative of the maximum beam energy

Answer 15

This is because during SXR and DXR there is a high frequency of low-energy electrons contributing to the dose.

Question 16

what is half-value layer

Answer 16

a parameter which is useful for measuring beam quality produced by kV tubes and it is the amount of (thickness) of a given beam required to attenuate the intensity of the given beam to 50% of the original.

Question 17

what is the purpose of beam filtration

Answer 17

Is mainly needed to filter out unwanted low energy photons that contribute to dose which would be despoited on the skin surfaces.

Question 18

what is beam hardening

Answer 18

this is the process of reducing the beam intensity and unwanted low energy photons by filtering the beam with metals. By hardening the beam the bulk of the electrons are at optimal intensity and aims are removing characteristic peaks.

Question 19

what is absorption edge

Answer 19

The idea of using a metal to filter out unwanted photons and reduce the intensity will produce its own characteristic peaks which can be removed by the addition of another metal and so on…

Question 20

what are the two main types of applicators that help collimate kV imaging

Answer 20

1. open ended
2. closed type

Question 21

how are applicators used for skin apposition

Answer 21

• this requires getting the applicator as parallel to the skin surface as possible which can be a problem if a patient has scabs – these need to be removed.
• The treatment field can also be defined by lead masks and lenses

Question 22

why would you use SXR and DXR for bone cancer in the skull?

Answer 22

As SXR and DXR are low energies, photoelectric and Compton scatter predominant at these energies, these are preferentially absorbed in structures with high z numbers such as bone.

Question 23

what is a limitation of using SXR and DXR for skin cancer

Answer 23

As SXR and DXR are low energies, photoelectric and Compton scatter predominant at these energies, these are preferentially absorbed in structures with high z numbers such as bone which can lead to necrosis.

Question 24

what are the benefits and limitations of using electrons produced from a LINAC for treating skin cancer?

Answer 24

As they are at MV energies, then Compton scatter predominates which is independent of atomic number, so there is no preferential absorption into any particular tissue type with them

Question 25

what needs to be considered when assessing whether to treat a BCC and SCC with a LINAC or SXR/DXR

Answer 25

• palliative patients may be in pain, so would be better to treat on SXR/DXR
• elderly will be more comfortable on SXR than on a LINAC

Question 27

How can you quality assure an SXR or DXR

Answer 27

1. warm the tube
2. check the filters, if the wrong filter is selected the interlock will prevent it from locking in place.
3. check the integrity of matter of the applicators i.e. are there any cracks or damages?

Question 28

describe what an interlock is and it’s purpose in delivering safe treatment

Answer 28

An interlock prevents the wrong filter from being inserted into the machine hence preventing the patient from recieving the wrong dose.