EPID dosimetry

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Using measured radiological opacity (thickness) to monitor radiotherapy dose

It is the contention of this work that comparison between EPID images and Monte Carlo predictions of EPID images provides a means to verify radiotherapy treatment delivery, without reliance on (possibly unreliable) TPS calculations.

It is the further contention of this work that a much simpler treatment verification procedure (involving much simpler calculations than than are generally used for “EPID dosimetry”) would be of value to local radiotherapy clinics.

The method proposed here is not better than conventional EPID dosimetry techniques, nor is it likely to be preferable in many situations; it is simply an alternative technique. Here, in Queensland, where a number of clinics are beginning to treat with IMRT and are looking for a simple and inexpensive way to use the EPID data that they are already producing to verify the delivery of complex radiotherapy treatments, the method proposed herein has a place.

Continuing to develop this technique, rather than following a more conventional path, presents risks and challenges. QUT is unlikely to be recognised as a contributor to EPID dosimetry research so long as we are not actually evaluating a parameter called “dose”.  But we might produce something useful…

The calibration method is very simple:

1.Calculate the transmissions from EPID images of a large field passing through a series of homogeneous, planar, plastic phantoms, with different physical thicknesses. 2.Identify the calibration relationship between transmission and physical thickness of plastic phantom.
3.When obtaining further images of different objects using different fields, use the calibration relationship obtained above to calculate the physical thickness of plastic which would produce the same transmission as seen in the image of each object. This is the (plastic-equivalent) radiological thickness of the object.This process is advantageous as a means of verifying radiotherapy treatments because the measured radiological thickness of a given object will depend on:

(a)The physical thickness of the object;
(b)The density and internal composition of the object;
(c)The energy of the radiotherapy beam;
(d)The profile of the radiotherapy beam;
(e)The field size of the radiotherapy beam;
(f)The field shape of the radiotherapy beam;
(g)The number of monitor units of exposure of the object; and
(h)The position of the object in relation to the radiotherapy beam.

Consequently, changes to any of these important treatment parameters will show up as changes to the measured radiological thickness.

And, this process is advantageous as a means of verifying radiotherapy treatments because:

(i)Normalisation, estimation, prediction and the use of correction factors (including correction for field size and phantom scatter) are not required;
(j)Modification of the EPID panel or acquisition software is not required;
(k)It produces results which are absolute (not relative); and
(l)It produces results which are easily compared with Monte Carlo calculations.This process is disadvantageous as a means of verifying radiotherapy treatments because the measured radiological thickness of a given object will also depend on:

(a)The day-to-day performance of a given EPID panel; and
(b)Minor, inadvertent changes to the lateral position of the EPID panel.

One remaining disadvantage of the radiological thickness measurement process is that:
(c)The results produced are not  values of “dose”.
(The first two of these problems also affect EPID dosimetry calculations made using more-conventional means, and I’d suggest that the third is easily mitigated by the fact that the measurement is absolute. An absolute measure of “centimetres of plastic” is at least as good as a relative measure of “percentage dose”.?)

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