ESTRO 2022 - Abstract Book

S348

Abstract book

ESTRO 2022

1 Hospital Universitario Ramón y Cajal., IRYCIS, Medical Physics, Madrid, Spain; 2 Hospital Universitario Ramón y Cajal, IRYCIS, Radiation Oncology, Madrid, Spain; 3 Hospital Universitario Ramón y Cajal, IRYCIS, Urology, Madrid, Spain; 4 Hospital Universitario Ramón y Cajal, IRYCIS, Medical Physics, Madrid, Spain; 5 Hospital Universitario Ramón y Cajal, IRYCIS, Medial Physics, Madid, Spain 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. Materials 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.