Photon Therapy: The Basics
The Inverse Square Law:

Undoubtedly one of the most useful properties of radiation emitted from a point source is the inverse square relationship which states that the radiation fluence falls off at a rate proportional to where r is the distance from the point source.

Any radiation diverging from a point in free space exhibits this phenomenon. However, in the real world radiation does not always follow this relationship due to the following:

attenuation,

scattering, and

discrete sources which are not a point.

If you are far enough away from a discrete source it will still approximately act like a point source usually 2X the longest dimension would be appropriate.



The way you utilize this effect in practice is as follows

Suppose you measure some amount of radiation X at a distance of 100 cm.

How much radiation would you measure with the same detector at 110 cm?

This would be equal to: .


Scattering Angle and Energy:

For low energy photons (3050 kV) undergoing Compton scattering, they are almost as likely to be scattered backward as they are to be forward scattered.

However, as the energy of the photons increases, it becomes less and less likely that they will change direction as abruptly (consider it inertia), and in the megavoltage energy range, almost all scattering occurs in a forward direction with very little sidescattering or backscattering.

This is true for electrons as well and is partially responsible for the changes in dmax and surface dose seen below.

Attenuation Coefficients:

There are three different attenuation coefficients that are useful in describing photon interactions in a medium:

Mass attenuation coefficient which is used to describe attenuation: how many photons are removed per unit distance normalized to the medium's density (units of cm2/g).

Mass energy transfer coefficient which is used to describe the amount of energy released per unit path length in a medium normalized to the density of the medium.

This is related to KERMA where is the photon energy fluence.


Mass energy absorption coefficient which is used to describe the amount of energy absorbed in a medium per unit path length normalized to the density of the medium.

This is related to the Dose (D) by: where is the photon energy fluence.


