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Virtual Source Distance (Effective SSD):
- Electrons do not follow an inverse square relationship with distance from the source. The reason being is that there are significant amounts of scatter from many materials in the LINAC head that behave like multiple sources for the electrons.
- One can, however, develop an empirical relationship that uses the virtual source distance (VSD). The virtual source distance is found by taking a series of measurements at various SSD’s.
- Suppose you have a standard SSD (usually 100 cm), and you wish to know the output for another SSD then the inverse square relationship using the VSD would be:
- Q0 - is the chamber reading at the standard SSD.
- Qg - is the chamber reading at an extended SSD with gap g.
- VSD - is the virtual source distance.
- dmax - is the depth of maximum dose for the electron energy of interest.
- g - is the gap and is defined as the difference between the standard SSD and the extended SSD (for instance SSD’s of 100 cm and 110 cm would have a gap of 10 cm).
- We can rewrite the above equation in the following form:
- If we then produce a plot of versus the gap g (see above) we get a curve whose slope (f) is equal to .
- We can then solve for the VSD with: .
- VSDs should be tabulated with energy and field size as both have an effect especially at low energies and small field sizes.
- In general, the VSD increases with field size and energy, but this is the norm, not the rule.