Dose 2016

Derivation of formalism to convert ionisation in plastic to dose to water Scaling photon energy fluence

In water at a reference depth

In solid plastic at an equivalent depth

Phantoms irradiated with the same MV photon beam

z

ref

z

eq

Ψ

Ψ

s

w

The energy fluences at corresponding depths in the two media can be related using O’Connor’s density scaling theorem

According to this theorem energy fluences can be related through the inverse square law, once all distances in irradiation geometry ( SSD, field size and depth ) are scaled with the inverse ratio of relative electron densities of the two media

2   

+ +

  

Ψ

(

)

SSD

z

r z

=

eq

eq

size field r :

ref

ref

,

w

(

)

Ψ

eq eq r z

SSD

z

(ref : in water)

ref

ref

,

s

O'Connor, J. E, PMB 1, 352-369, 1957