L 13:Electron beam Therapy Flashcards
(33 cards)
Range of Electrons
Range in cm = 1/2 of energy in MeV
What can be used to measure the surface electron dose
- TLD
- OSLD - in ViVo
- Farmers Chamber
- Extrapolation Chamber
- Parallel Plane Chamber
Gold std for surface electron dose measurement
Extrapolation chamber
How much electrons are deposited per depth
2MeV/cm
Eg: so for 6MeV beam (6/2=3cm). By 3 cm all the dose is deposited
Clinically useful electrons
6-20 MeV
How do electrons work in the body
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.
Low energy Electrons
- The lower the energy, the sharper is the dose dropoff beyond the therapeutic range.
*
DEpth dose can be measured by
Ion chambers
Diodes
FIlm
* PDD increase with increase in energy
How to pick which beam for which tumor
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
Beam flatness is achieved by
Dual scattering foil
Optimally +/- 3%
X-ray contamination in electron beams
6-12 MeV = 0.5-1%
12-15 MeV = 1-2%
15-20 MeV = 2-5 %
Uses of Bolus
- Flatten an irregular surface
- Reduce the penetration of electrons
- Increase surface dose
- Conform the dose distribution to the PTV
How to find the thickness of Pb required for blocking?
Minimum thickness lead used for 5% of transmission of dose is
- energy of the electron beam / 2
- Add a safety margin of +1 to this.
How to find the thickness of Cerrobend required for blocking?
thickness of Pb x 1.2
How to know the diameter of the field size
Energy (MeV)/ 2.5 = diameter of the field size in cm
Electron backscatter factor
EBS
increases with
* low energy
* high Z
To prevent backscatter what do u do?
Place a bolus
Total skin irradiation
Used of mycosis fungoides (cutaneous lymphoma)
2-9 MeV is used
Field shaping is done using
lead or Cerrobend blocks
What is used for electron treatment planning
Pencil beam algorithm
monte Carlo
why no bragg peak for electrons
As they back scatter. They have higly variable track lengths from scatter
TO know the lectron depth
Dmax = Eo/5
Practical range for differenet densities
Rp = E0 (p water/pair) / 2
Linac for electrons and photons
