L 13:Electron beam Therapy

Question 1

Range of Electrons

Answer 1

Range in cm = 1/2 of energy in MeV

Question 2

What can be used to measure the surface electron dose

Answer 2

• TLD
• OSLD - in ViVo
• Farmers Chamber
• Extrapolation Chamber
• Parallel Plane Chamber

Question 3

Gold std for surface electron dose measurement

Answer 3

Extrapolation chamber

Question 4

How much electrons are deposited per depth

Answer 4

2MeV/cm
Eg: so for 6MeV beam (6/2=3cm). By 3 cm all the dose is deposited

Question 5

Clinically useful electrons

Answer 5

6-20 MeV

Question 6

How do electrons work in the body

Answer 6

Electrons interact with matter by
* ionization and excitation—the most predominant interaction in soft tissues,
* bremsstrahlung—more significant at higher energies and in higher-atomic-number materials,
* elastic scattering by atomic nuclei, and
* elastic scattering by orbital electrons. Collisions resulting in secondary electron production (d rays) are possible but rare.

Question 7

Low energy Electrons

Answer 7

• The lower the energy, the sharper is the dose dropoff beyond the therapeutic range.
  *

Question 8

DEpth dose can be measured by

Answer 8

Ion chambers
Diodes
FIlm

Question 9

How to pick which beam for which tumor

Answer 9

Ex: tumor is 3 cm deep them do 3 x 3 = 9MeV
* 3.3 x 90% depth
* As the energy increases the % surface dose increases

Question 10

Beam flatness is achieved by

Answer 10

Dual scattering foil
Optimally +/- 3%

Question 11

X-ray contamination in electron beams

Answer 11

6-12 MeV = 0.5-1%
12-15 MeV = 1-2%
15-20 MeV = 2-5 %