Analysis of the internal motion of the urethra caused by urethral catheters
PD-0405
Abstract
Analysis of the internal motion of the urethra caused by urethral catheters
Authors: David Sevillano1, Asunción Hervás2, Rafael Rodríguez-Patrón3, Juan David García-Fuentes4, Rafael Colmenares4, Carmen Vallejo2, Fernando López-Campos2, José Antonio Domínguez-Rullán2, Teresa Muñoz2, Miguel Cámara4, Belén Capuz4, Rafael Morís4, Pablo Galiano5, Pedro Retorta4, Sandra Williamson4, María José Béjar4, Daniel Prieto4, Feliciano García-Vicente4
1Hospital Universitario Ramón y Cajal., IRYCIS, Medical Physics, Madrid, Spain; 2Hospital Universitario Ramón y Cajal, IRYCIS, Radiation Oncology, Madrid, Spain; 3Hospital Universitario Ramón y Cajal, IRYCIS, Urology, Madrid, Spain; 4Hospital Universitario Ramón y Cajal, IRYCIS, Medical Physics, Madrid, Spain; 5Hospital Universitario Ramón y Cajal, IRYCIS, Medial Physics, Madid, Spain
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Purpose or Objective
To study differences between the
real position of the urethra and that observed when a Foley catheter is used
for its definition.
Material and Methods
10 prostate SBRT patients were included in this
study. For each patient, two CT scans were performed. The first one with a 4mm-wide Foley urethral catheter and the second with a thin (0.9mm) guide-wire alone.
Both
CTs were registered based on 4 fiducial markers in the prostate. The urethra was
contoured in both situations and data was exported to an in-house software
designed for analysis purposes.
For each patient, the contours of the urethra in
both CTs were divided into 21 sectors according to their relative longitudinal
position along the prostate. Hence, population-based data on urethral position
regardless the size of the prostate was obtained.
For each subsector the mean and SD of the
differences between the urethra contoured with the Foley catheter and that
contoured with the guide-wire were obtained for lateral and Anterior-Posterior
directions.
Margins needed to design a PRV accounting for
90% of the population were also calculated by multiplying the standard
deviation at each subsector by a factor of 2.15, as a 2D geometry was considered.
Results
Differences between urethral
positions in both cases are shown in Figure 1. Larger differences can be
observed in the AP direction compared to those in the lateral direction.
Differences are also larger in areas closer to the base of the prostate. While
no systematic difference is observed in lateral direction, a 5mm difference
occurs in the posterior direction.
Margins calculated in Table 1
increase as the urethra is closer to the base of the prostate. Margins needed
in that area much larger than those usually applied by assuming urethral
diameters of 8-10mm.
According
to these data, the use of a guide-wire to represent the urethra has become the
common practice in our department, as it better represents the real position of
the urethra.
Conclusion
The
use of a Foley catheter in the urethra causes an internal motion of the urethra
relative to the prostate that might have dosimetric consequences. This motion
is of larger importance close to the base of the prostate, where differences in
position caused by the catheter might be larger than 1cm.
The use of a guide-wire to contour the urethra
may avoid the need for larger PRV margins.
Conclusion
The use of a Foley catheter in the urethra causes
an internal motion of the urethra relative to the prostate that might have
dosimetric consequences. This motion is of larger importance close to the base
of the prostate, where differences in position caused by the catheter might be
larger than 1cm.
The use of a guide-wire to contour the urethra
may avoid the need for larger PRV margins.