Prostate Implants
As part of the prostate brachytherapy trial we are examining
the effectiveness of implants by comparing information about
patients' post implantation condition against reconstructions of
their implant dosimetry. There are several ways of doing this and
we are currently looking at the different ways of representing
the implant reconstruction, either from orthogonal simulator
films, CT, or more recently, ultrasound images. We are also
looking at ways of comparing the actual dosimetry with the
intended pre-planned dosimetry and with any post operative
complications experienced by the patients. We can do this by
simply comparing reconstructed isodoses to digitized images of
the patients anatomy or more quantitatively by looking at the
homogeneity of the dose given to the prostate by producing dose
volume histograms and comparing these to the pre-planned
dosimetry and the dosimetry of other implants.
Orthogonal simulator films produce the most accurate isodose
reconstructions but they do not allow the prostate, its self to
be imaged. The volume occupied by the radioactive sources is
calculated and it is assumed that this volume, being equal to the
known volume of the prostate, occupies the same space as the
prostate. CT films enable both sources and prostate to be imaged
but the volume of the prostate is exaggerated to an uncertain
degree. Thus sources can be seen within the prostate but a
comparison of volumes is not directly possible.
Critical areas about the prostate gland include the urethra as it
traverses the prostate, the wall of the rectum, the bladder and
neurovascular bundles which converge upon the gland. All these
regions can be located on ultrasound images but only the rectum
and bladder are visible from CT imaging. It is possible to
calculate the doses received by these regions from CT
reconstructions and from the preliminary planning of ultrasound
studies. The ultrasound pre-planning allows doses to these
regions to be minimised before the actual implant takes place.
Dose volume histograms allow a subjective intercomparison to be
made between preliminary and actual implant dosimetry, between
the various forms of reconstructed dosimetry and between
different patients, it is not possible however to obtain a
definitive description of the success or failure of any
particular implant. The histograms calculate dose received by
each unit volume of an implant. Natural dose volume histograms
can be used to represent the degree of homogeneity of dose within
an implant and this can be used as a guide to the probable
success or failure of an implant. By removing the effects of the
inverse square law rate of attenuation of dose received from a
single point source, the degree of homogeneity of dose actually
received can be clearly seen as a deviation of the implant
volume, at the point of the treatment dose, from the behaviour of
volume that would have been treated by a single point source.