Electron Output:

 

 

  1. On a LINAC when delivering electron fields, the jaw size is fixed at a constant value.  This is due to the huge variation in output with jaw size especially at lower energies (see left figure above) which can vary as much as 200%.
  2. Output is much more stable when the cone size is varied, however (see right figure above) and varies only about 15%.
  3. This large difference is due to the huge amount of scattering surfaces in the LINAC head as compared to the cone.

 

image027

 

Isodose Curves:

 

image028

 

  1. Electron isodose curves show the following trends:
  1. At low energies, the low dose isodose lines (20,30%) bulge out, but the high dose isodose lines (90,100%) are fairly straight.
  2. At high energies, the low dose isodose line tends to remain straighter but the high dose isodose lines pinch in.
  3. When extended SSDs are used to treat a patient there is more penumbra.
  1. Uniformity Index:
  1. The uniformity index is a way to quantify the amount of penumbra in an electron beam.
  2. It is defined as the ratio of the area of 90% isodose curve to that of the 50% isodose curve.
  3. Values greater than 0.7 are considered acceptable for fields larger than 10x10 cm2.

 

 

  1. Inhomogeneities have a strong effect on electron fields.  Consider the above image (this can be generalized to any low density/ high density environment such as bone and water):
  1. The water increases lateral scatter to the surrounding air and leads to hot spots adjacent to it.
  2. The air attenuates the field much less and the isodose curves extend deeper.
  3. A loss of scatter from the air causes a significant pinching in of the isodose curves traversing the water.
  4. The most important thing to remember is that Monte Carlo must be used to calculate around inhomogeneities as the pencil beam algorithms are inadequate.

 

Anonymous
2014-06-25, 16:17
Good material and great organization. The practice oral examinations were a great tool to prepare you for the setting of the actual exam.