The Field Size Correction Factor:

  1. The Field Size Correction Factor (FSCF) functions to allow you to take the output of the machine for the reference field size and find the output for another field size.  The dose to a point in a medium can be separated into primary and scattered contributions.
  2. It can be noted that the reference field size for most systems will be the 10 x 10 cm2 field.
  3. Mathematically, the field size correction factor can be split as follows:
  1.  image018
  1. S- is the scatter component from the collimator and LINAC head
  2. S- is the scatter from the patient (or phantom)
  1. In order to measure the individual components, we first simply place an ionization chamber at dmax in a water tank.  With this setup, we can directly measure SC  multiplied by SP (the product).  If we then empty the water tank and add a buildup cap, we can measure SC.  We can then solve for SP  by dividing the product by SC.

 

image019

 

 

The Wedge versus the Compensator versus the Bolus:

  1. Wedge - A wedge (or wedge filter) is a piece of metal that is placed between the radiation source and the patient and is used to modify isodose lines with the patient’s dosimetry.  There are two major uses of wedges: they can be used to compensate for slopes on the patient’s surface or in modifying isodose lines when utilizing non-opposed beam arrangements (this is often referred to as a wedge-pair and is illustrated with the hinge angle description below).
  1. Material: metal.
  2. Position: 20 cm above the patient.
  3. Skin sparing: yes.
  4. Purpose: used to manipulate isodose curves.

 

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  1. Compensator:
  1. Material: metal, tissue equivalent.
  2. Position: 20 cm above the patient.
  3. Skin sparing: yes.
  4. Purpose: compensates for missing tissue.
  1. Bolus:
  1. Material: tissue equivalent.
  2. Position: directly in contact with the skin.
  3. Skin sparing: no.
  4. Purpose: Bypasses skin buildup regions so more dose is deposited at shallower depths (it shifts the %DD curve up toward the skin surface).

Anonymous
2017-09-29, 13:58
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