The Utility of Penile Bulb Contouring to Localise the Prostate Apex as Compared to Urethrography

Purpose: High-precision radiotherapy relies on accurate anatomic localisation. Urethrography is often used to localise the prostatic apex. However, urethrography is an invasive localisation procedure and may introduce a systemic error. The penile bulb (PB) is contoured to minimise the risk of erectile dysfunction. The purpose of this study is to assess the value of using the PB, as an alternative to urethrography, to localise the prostate. Methods and Materials: The PB was localised on 10 patients treated with simplified intensity-modulated arc radiotherapy at computed tomography simulation during treatment weeks 1 and 7. All patients underwent placement of fiducial markers. Urethrography was used only at simulation. Distances from the superior PB contour to the inferior prostate contour, the apex fiducial marker, and to the inferior prostate contour were obtained as well. The PB was contoured by two observers independently. Agreement coefficients and analysis of variance were used to assess reliability between rates and consistency of measurements over time. Results: The PB-apex distance was greater than or equal to the urethrogram-apex distance in 24/30 (80%) measurements, and the median difference was 3 mm and was consistent between raters. The greatest variation in PB-IM distance between weeks was 6 mm, the median was 3 mm, and the agreements of measurements between weeks for raters 1 and 2 were 0.79 and 0.69, respectively. These differences were not statistically different and were consistent with the computed tomography slice thickness. Conclusions: The PB can be used to identify the prostate apex and can be reliably contoured between observers. Measurements are consistent between patients and through the duration of treatment. The PB distance measurements support studies indicating that urethrography causes a shift of the prostate superiorly. The distance from the PB to prostate apex remains stable during treatment for individual patients but varies between patients.