Electrons

Question 1

What are some features of electrons?

Answer 1

• 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

Question 2

Why do we use electrons?

Answer 2

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

Question 3

What is the minimum field size?

Answer 3

• 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

Question 4

What is the electron depth dose?

Answer 4

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

Question 5

Where is dose prescribed for electrons?

Answer 5

• 80 or 90%

Question 6

What is the depth in cm at 80%?

Answer 6

• approximately 1/3 of the electron energy

Question 7

Where is the RP placed?

Answer 7

• on the central axis at the required depth

Question 8

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

Answer 8

• it increases

Question 9

What happens to the dose as the SSD is extended?

Answer 9

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

Question 10

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

Answer 10

• 5cm

Question 11

What occurs with oblique incidence?

Answer 11

the skin dose increases dramatically as there are more oblique electrons

Question 12

What are advantages of electrons compared to superficial?

Answer 12

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

Question 13

What are some disadvantages of electrons compared to superficial?

Answer 13

• 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

Question 14

What are some general disadvantages of electrons?

Answer 14

• 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

Question 15

What is TSET?

Answer 15

• 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

Question 16

Why is bolus used?

Answer 16

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

Question 17

What happens to electrons as they travel to greater depths?

Answer 17

• their paths become more oblique

- higher atomic number material causes a higher directional change

Question 18

What are the main features of a PDD?

Answer 18

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

Question 19

What causes the dose in the build up region?

Answer 19

• knock on electron

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

Question 20

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

Answer 20

• 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

Question 21

What causes the bremsstrahlung tail?

Answer 21

• due to photon contamination

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