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CSB092 > Superficial > Flashcards

Superficial Flashcards

1
Q

What are the room design requirements? (8)

A
  • Adequate room
  • console area
  • interlocked door
  • warning lights/sign
  • shielding
  • machine mountings
  • water supply
  • storage space
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2
Q

What is the shielding requirements for <500kV?

A
  • lead lined walls

- shielding doors

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3
Q

What is the shielding requirements for >500kV?

A
  • concrete, high density concrete walls

- maze

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4
Q

What is a kilovolt?

A
  • a unit of force equal to 1000 volts
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5
Q

What is a megavolt?

A
  • a unit of force equal to 1,000,000 volts

- beams with the energy range of 4-25MV are used (linacs)

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6
Q

What is kilovoltage?

A
  • the amount of electrical energy applied so that the electrons accelerate from the cathode towards the anode
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7
Q

What is electron voltage?

A
  • a unit of energy equal to the energy acquired by an electron accelerated through a potential difference of 1 volt
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8
Q

What is 1 KeV?

A
  • 1000 electron volts
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9
Q

What is 1 MeV?

A
  • 1,000,000 electron volts
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10
Q

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

A
  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)
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11
Q

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

A
  • 1% or less
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12
Q

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

A
  • is transformed into heat and must be dissipated through an efficient target cooling system
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13
Q

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

A
  • should have a high atomic number and high melting point

e. g. tungsten Z value = 74

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14
Q

What is an atomic number?

A
  • the number of protons found in the nucleus of an atom
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15
Q

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

A
  • 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
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16
Q

What information is required in the control panel?

A
  • machine warmed up
  • interlocks set
  • patient ID checked (name, D.O.B., db no.)
  • filter correct
  • applicator correct
  • exposure time set
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17
Q

What does a filter do?

A
  • 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
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18
Q

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

A
  • 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
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19
Q

How is beam quality/penetration expressed?

A
  • in terms of kV and Half-Value Layer (HVL)
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20
Q

What is the Half-Value Layer (HVL)

A
  • 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)
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21
Q

What is a HVL 1mm Al suitable for?

A
  • lesions 1-3mm in depth
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22
Q

What is a HVL 2mm Al suitable for?

A
  • lesions 3-5mm in depth
23
Q

What is a HVL 5mm Al suitable for?

A
  • lesions 5-7mm in depth
24
Q

What is a HVL 9mm Al suitable for?

A
  • lesions 7-8mm in depth
25
Q

What is a HVL 13mm Al suitable for?

A
  • lesions 9-10mm in depth
26
Q

What does increasing filtration cause?

A
  • decrease in the number of x-ray photons, intensity
27
Q

What is beam monitoring?

A
  • tells us how much radiation is being produced
28
Q

What are the 2 methods of controlling radiation output?

A
  1. timer and backup timer

2. dose monitor

29
Q

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

A
  • a timer is sufficient
30
Q

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

A
  • need a timer and dose monitor
31
Q

What are lead cutout (shields) castellated?

A
  • ensures a reduction in the definition of the field edge on the skin
32
Q

What is skin apposition?

A
  • the applicator must be parallel to the skin

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

33
Q

What are difficulties with skin apposition?

A
  • convex surfaces - “stand in”

- concave surfaces - “stand off”

34
Q

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

A
  • if measurements are taken of actual SSD’s across the area
35
Q

What is the effect of SSD on depth dose?

A
  • increased SSD causes increased effect on % DD in general due to the inverse square law
  • at small SSD this effect is increased
36
Q

What is the ISL?

A
  • the intensity of radiation from a point source is inversely proportional to the distance from the source.
37
Q

What is the dominant interaction process at superficial energies?

A
  • photoelectric absorption

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

38
Q

What is the dominant interaction process at larger energies?

A
  • 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
Q

What is the beam characteristics at kilovoltage and why?

A
  • 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
Q

What is backscatter factor?

A
  • 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
Q

What is photoelectric absorption?

A
  • 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
Q

What directional movement does roll refer too?

A
  • translatinal
43
Q

What directional movement does yaw refer too?

A
  • coronal
44
Q

What directional movement does pitch refer too?

A
  • sagittal
45
Q

What is grenz?

A
  • 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
Q

What is contact/papillon technique?

A
  • 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
Q

What is orthovoltage?

A
  • 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
Q

What depth is superfical for?

A
  • lesions within first 5mm of skin
49
Q

What are indications for RT?

A
  • 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
Q

What are some features of basal cell carcinoma?

A
  • starts with suble change to skin (small bump or flat red path)
  • develop slowly
  • may appear as non-healing sore
51
Q

What are the three tyoes of BCC?

A
  • nodular
  • infiltrating
  • superficial
52
Q

What are some features of squamous cell carcinoma?

A
  • 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
Q

What are the treatment options for BCC/SCC?

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

CSB092 (9 decks)

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