Now, we've already seen that a linear accelerator is a complex piece
of equipment,
from a mechanical point of view and from an electrical point of view.
In order to safely deliver a radiation dose to the patient,
we need to monitor the performance characteristics of a machine
throughout its lifetime.
So, both mechanically and electronically,
these things will be looked at on a regular basis.
The most fundamental thing that we have to do, however,
is to examine the beam parameters.
And if you remember that a patient is being treated on a daily basis
for maybe up to five weeks,
it's important that the beam parameters do not change during that period.
In order to successfully treat a patient,
we need to understand how radiation affects human tissue.
And, obviously, we can't use real people to find this out,
so we have to use an alternative, and the alternative is water.
The interaction properties of water and radiation closely mimics
the interaction properties of radiation and human tissue.
So, whenever a new machine is installed,
one of the most fundamental measurements that we can make
is to measure the interaction between the radiation and the water,
and we do this by using a water tank.
We use ionisation chambers placed inside the water tank,
which we've filled with water to a depth approximately two thirds full,
and for certain field sizes,
we scan across the water phantom and down to different depths.
So, the measurements used, at different field situations,
at different depths in the water,
are the measurements which are fed into the treatment planning computer
and can be used to plan individual patient's treatments.
The water tank is the most fundamental instrument
to examine the beam parameters,
but it's large, awkward and not easily used.
So, we have to use other materials and other instruments to do this.
Now, here we have a phantom.
Now, this can be water or Perspex.
Now, this phantom is used in conjunction with an ionisation chamber.
The ionisation chamber is inserted into the phantom,
and the end of the cable fits onto the electrometer,
which is situated outside in the console area of the unit.
Now, this ionisation chamber is used for photon measurements.
However, a multi-modality machine such as this
can also produce therapeutic electrons, and in order to measure the electrons,
we use a different type of chamber, a parallel plate chamber.
And this chamber is used in connection with a solid water phantom.
And the ionisation chamber fits into the phantom and,
according to the energy that we're measuring,
various slices of different thicknesses can overlay it.
This ionisation chamber is also connected to an electrometer,
which sits outside in the console area.
The readings that we obtain on the electrometer
have various calculations performed on them,
so that we can determine the output of the unit at any particular time.
A machine should be able to last about 14 to 15 years,
if it's well taken care of.
So, these broad aspects,
then, of simple mechanical and complex mechanical checks,
simple and complex electronic checks and also dosimetric,
the measurement of dose, checking the beam parameters.
Quality assurance applies to all machines,
from the simplest machine, such as a kilovoltage machine,
which would be used for supersum, super-official work,
through the microselectron, which we've seen,
right through to the multi-modality megavoltage machines.
So, quality assurance as a concept applies to every piece of equipment
in the radiotherapy department.