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Flashcards in Superficial Deck (53):
1

What are the room design requirements? (8)

- Adequate room
- console area
- interlocked door
- warning lights/sign
- shielding
- machine mountings
- water supply
- storage space

2

What is the shielding requirements for <500kV?

- lead lined walls
- shielding doors

3

What is the shielding requirements for >500kV?

- concrete, high density concrete walls
- maze

4

What is a kilovolt?

- a unit of force equal to 1000 volts

5

What is a megavolt?

- a unit of force equal to 1,000,000 volts
- beams with the energy range of 4-25MV are used (linacs)

6

What is kilovoltage?

- the amount of electrical energy applied so that the electrons accelerate from the cathode towards the anode

7

What is electron voltage?

- a unit of energy equal to the energy acquired by an electron accelerated through a potential difference of 1 volt

8

What is 1 KeV?

- 1000 electron volts

9

What is 1 MeV?

- 1,000,000 electron volts

10

What is the conversion of electrons into x-rays in kilovoltage?

1. the electron producing X ray beams in the x ray tube originate in the heated filament (cathode)
2. these are accelerated in a vaccum towards the target (anode)

11

What is the effeicieny for x-ray production in the superficial and orthovoltage energy range?

- 1% or less

12

What happens to most of the electron kinetic energy deposited in the x-ray target?

- is transformed into heat and must be dissipated through an efficient target cooling system

13

What should the target material in a superificial and orthovoltage machine be?

- should have a high atomic number and high melting point
e.g. tungsten Z value = 74

14

What is an atomic number?

- the number of protons found in the nucleus of an atom

15

Why do you need to warm-up the kilovoltage machine slowly?

- heat dissipation in the target is very high and if this occurs in a cold target it will cause damage
- the tungsten will expand faster and larger than it copper surround and will crack
- morning warm-up of 20-30 minutes is needed

16

What information is required in the control panel?

- machine warmed up
- interlocks set
- patient ID checked (name, D.O.B., db no.)
- filter correct
- applicator correct
- exposure time set

17

What does a filter do?

- removes the softer photon beam components which would simply irradiate the most superficial layers and produce an undesirable rapid attenuation through those layers (HARDENS THE BEAM)
- prevents excessive photons with unwanted energies contributing to the dose (SMOOTHS THE BEAM)
- makes the beam more HOMOGENEOUS

18

What is the effect of filtration on the spectrum of a heterogeneous beam?

- though intensity of the beam is reduced when a filter is added, its quality (penetration) is improved as the low energy component is virtually removed from the beam

19

How is beam quality/penetration expressed?

- in terms of kV and Half-Value Layer (HVL)

20

What is the Half-Value Layer (HVL)

- is a measure of the penetration of a kv XRT beam
- the thickness of the material at which the intensity of radiation entering it is reduced by one half
- expressed in units of distance (mm)

21

What is a HVL 1mm Al suitable for?

- lesions 1-3mm in depth

22

What is a HVL 2mm Al suitable for?

- lesions 3-5mm in depth

23

What is a HVL 5mm Al suitable for?

- lesions 5-7mm in depth

24

What is a HVL 9mm Al suitable for?

- lesions 7-8mm in depth

25

What is a HVL 13mm Al suitable for?

- lesions 9-10mm in depth

26

What does increasing filtration cause?

- decrease in the number of x-ray photons, intensity

27

What is beam monitoring?

- tells us how much radiation is being produced

28

What are the 2 methods of controlling radiation output?

1. timer and backup timer
2. dose monitor

29

What beam monitoring is needed for <150kV (superficial)?

- a timer is sufficient

30

What beam monitoring is needed for >150kV (orthovoltalge)?

- need a timer and dose monitor

31

What are lead cutout (shields) castellated?

- ensures a reduction in the definition of the field edge on the skin

32

What is skin apposition?

- the applicator must be parallel to the skin
- ideally all the applicator edges should be the same distance away from the skin

33

What are difficulties with skin apposition?

- convex surfaces - "stand in"
- concave surfaces - "stand off"

34

How can stand-off/stand-in be corrected?

- if measurements are taken of actual SSD's across the area

35

What is the effect of SSD on depth dose?

- increased SSD causes increased effect on % DD in general due to the inverse square law
- at small SSD this effect is increased

36

What is the ISL?

- the intensity of radiation from a point source is inversely proportional to the distance from the source.

37

What is the dominant interaction process at superficial energies?

- photoelectric absorption
- photons have shallow penetration and backscatter accounts for a significant amount of dose delivery

38

What is the dominant interaction process at larger energies?

- comption scatter
- photons penetrate further, do not interact at shallow depths and back scatter reduces
- scatter that does occur goes in a forward direction

39

What is the beam characteristics at kilovoltage and why?

- As a result of the photoelectric effect
- less penetration
- more interactions at or just below skin due to back scatter (can be ask much as 50%)
- photoelectric absorption occurs, denser tissue will absorb more energy

40

What is backscatter factor?

- ratio of a quantity of radiation at the surface (of a patient) to the quantity of radiation at the same point but without a patient
- back scatter increases as field size increases and thus less MU needed
- more backscatter occurs from dense structures such as bone

41

What is photoelectric absorption?

- the higher atomic number of bone means that it absorbs more energy
- below 100kV, 3mm Al HVL, bone absorbs 4.5 times more energy than muscle

42

What directional movement does roll refer too?

- translatinal

43

What directional movement does yaw refer too?

- coronal

44

What directional movement does pitch refer too?

- sagittal

45

What is grenz?

- grenz rays are produced at low kilovoltages (10-20kV) giving them a very low penetration power
- half their energy is absorbed within the first half millimeter of tissue (treat to 1mm)
- now use topical chemo or surgery to treat

46

What is contact/papillon technique?

- 40-50kV
- treat 1-2mm
- recommended for patient with rectal/anal tumours who are not fit enough for general anaesthesia or surgery
- cancer has to be small and superficial with no evidence of LN involvement

47

What is orthovoltage?

- treatment with 150-500kV
- more penetrative than superficial (2-3cm below skin)
- multuple beams used to decrease skin reaction
- not used when MV machines invented

48

What depth is superfical for?

- lesions within first 5mm of skin

49

What are indications for RT?

- patient medically unfit or refuse surgery
- cosmetic reason
- functional reason (to avoid nerve or function damage)
- patient at risk of microscopic disease or where small volume recurrence has occured
- older patients
- mutliple lesions
- patient prone to keloid scar formation

50

What are some features of basal cell carcinoma?

- starts with suble change to skin (small bump or flat red path)
- develop slowly
- may appear as non-healing sore

51

What are the three tyoes of BCC?

- nodular
- infiltrating
- superficial

52

What are some features of squamous cell carcinoma?

- highly varibale clinical appearance
- raised scaly lump
- ulcer or reddish skin plaque
- crusted sore
- can appear as persistent small ulcer on lip
- slow growing
- can metastasise

53

What are the treatment options for BCC/SCC?

- Mohs surgery
- standard surgical removal
- cutterage and cautery
- photodynamic therapy
- topical immunotherapy
- cryotherapy