SEMINAL VESICLE MOTION RELATIVE TO THE PROSTATE

The clinical target volume (CTV) is a tissue volume that contains a
demonstrable extent and location of the malignant growth and/or subclinical
microscopic malignant disease, which have to be eliminated. To account for
uncertainties regarding movement of the CTV, variation in shape and size of the
CTV and variations in beam geometry characteristics, a margin is added to the
CTV. The planning target volume (PTV) is the CTV plus this margin. Use of
fudicial markers, especially with on-line position verification, will reduce
geometrical uncertainties. This will make it possible to use a smaller margin,
resulting in a lower radiation dose delivered to normal tissue surrounding the
CTV. For a prostate cancer patient with a risk of involvement of the seminal
vesicles the CTV contains the prostate and seminal vesicles.
The fiducial markers are only implanted in the prostate and corrections for
translation and in future also rotation will be based on the position of the
fiducial markers in the prostate. Although there are studies reporting about
shape variation of the prostate and seminal vesicles, only a few studies used
repeat CT scans in combination with fiducial markers. Because the prostate
shape has rounded edges, rotations of the prostate relative to the seminal
vesicles cannot be assessed in a precise manner by means of repeat CT scans
without fiducial markers. Using fiducial markers in combination with repeat CT
scans will allow a much more precise assessment of the motion of the seminal
vesicles relative to the prostate. For the use of an off-line or on-line
treatment verification protocol based on intraprostatic fiducial markers, this
data is required to determine the minimal margin that can be used for the
seminal vesicles. Until now there is no study providing sufficient data to
determine this minimal margin.

To quantify the motion of the seminal vesicles.

Twenty patients will be included in this study. A week after implantation of 4
fiducial markers the planning CT scan will be made. During the period of
radiotherapy 3 additional CT scans will be made for each patient, in week 2, 4
and 6 of treatment. The prostate, fiducial markes and seminal vesicles will be
outlined on each CT scan. The difference in position of the seminal vesicles
relative to the fiducial markers and prostate will be determined.

Three additional CT scans will be made. This will take about 10 minute per CT
scan. The aim is to plan de CT scans directly before or after the treatment of
that day.
The additional CT scans will increase the effective radiation dose, but this an
insignifcant increase compared to the radiation dose received by the small
pelvis.