Electrons Flashcards

1
Q

What are some features of electrons?

A
  • have a finite range
  • rapid dose fall odd
  • do not deposite dose at depth and thus reduces normal tissue dose
  • provide high surface dose
  • have wide penumbra which increases at depth
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2
Q

Why do we use electrons?

A
  • provide a uniform dose from the surface to approx 6cm depending on energy
  • useful in treating skin, nose, ears, chestwall, eyelids, scalp, limbs
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3
Q

What is the minimum field size?

A
  • 4x4 is smallest applicator but can be determined by lead cut out
  • if we use smaller there is a loss of lateral electrons and thus the dose is not accurate
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4
Q

What is the electron depth dose?

A
  • the shape of the dpeth dose curve is fairly uniform followed by a rapid drop off
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5
Q

Where is dose prescribed for electrons?

A
  • 80 or 90%
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6
Q

What is the depth in cm at 80%?

A
  • approximately 1/3 of the electron energy
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7
Q

Where is the RP placed?

A
  • on the central axis at the required depth
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8
Q

What happens to the surface dose as the electron energy increases?

A
  • it increases
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9
Q

What happens to the dose as the SSD is extended?

A
  • the lower % lines get wider
  • the higher % lines (80-100%) get narrower and lose depth
  • more MU are required
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10
Q

What is the distance bwteeen the end of applicator to the patient when treating electrons at 100cm SSD?

A
  • 5cm
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11
Q

What occurs with oblique incidence?

A

the skin dose increases dramatically as there are more oblique electrons

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12
Q

What are advantages of electrons compared to superficial?

A
  • sharp dose fall off below the surface
  • less absorption in bone and cartilage
  • good cosmetic results
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13
Q

What are some disadvantages of electrons compared to superficial?

A
  • expensive linac required
  • greater raditaion protection compared to superficial treatments
  • field size limitations
  • %DD less accurate under 4cm
  • dose inhomogentiy on curved surfaces
  • eyes: sheilds can cause scatter, bowing of the isocurves treats a larger area under the surface than at the surface
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14
Q

What are some general disadvantages of electrons?

A
  • dose distribution is signicantly affected by heterogeneities such as air and bone
  • dose within these heterogeneities can be difficult to measure
  • electron beams are difficult to model
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15
Q

What is TSET?

A
  • total skin electron therapy
  • for superifical lesions covering large areas like mycosis fungoides
  • different methods are possible to expose the whole body
  • modified stanford technique
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16
Q

Why is bolus used?

A
  • increase surface dose
  • flatten out irregular surfaces
  • reduce the electron beam penetration in some parts of the treatment field
17
Q

What happens to electrons as they travel to greater depths?

A
  • their paths become more oblique

- higher atomic number material causes a higher directional change

18
Q

What are the main features of a PDD?

A
  • build up region
  • R100
  • R90 (depth of 90% dose in the fall of region)
  • R50
  • Rp
  • bremsstrahlung tail
19
Q

What causes the dose in the build up region?

A
  • knock on electron

- increasingly oblique electron paths (have a longer path then vertical travel and thus more dose)

20
Q

Why is the depth of dmax closer to the dose at surface for higher energy electron beams?

A
  • at higher energy beams, electrons are deflected less as they travel so the depth of maximum dose is deeper
  • since higher energy electrons tend to be deflected less in general their paths tend to be straighter and less oblique at the depth of maximum dose
21
Q

What causes the bremsstrahlung tail?

A
  • due to photon contamination

- varies with energy, 1% in low energy, 5% in higher energy