title: Localization of the prostatic apex for radiation treatment planning.
abstract: OBJECTIVE
  To determine whether retrograde urethrogram, or the combination of computed tomography (CT) scan/retrograde urethrogram is more accurate for locating the magnetic resonance imaging (MRI) designated prostatic apex, and to determine whether patients treated in our department with CT/urethrogram are receiving the prescribed minimal dose to the MRI identified prostatic apex.
  
  
  METHODS
  Seventeen patients with early stage prostate cancer were enrolled in a prospective study to determine the location of the prostatic apex. All of the patients agreed to undergo MRI in addition to retrograde urethrogram, and CT of the pelvis for three dimensional (3D) treatment planning. The prostatic apex was identified on each of the studies and measured from a reference point (the most superior portion of the pubic symphysis). The location of the prostatic apex as measured by retrograde urethrogram alone and by CT/urethrogram was compared to the location of the prostatic apex as measured by MRI. Because of MRI's ability for multiplanar capabilities, and high soft tissue contrast in the region of the prostate, it was assumed to be more accurate for identifying the location of the prostatic apex, and was used as the gold standard.
  
  
  RESULTS
  The location of the prostatic apex as determined by the urethrogram alone was on average 5.8 mm caudad to the location on MRI (p = 0.012), while the location of the prostatic apex as determined by CT/urethrogram was 3.1 mm caudad to the location on MRI (p = 0.150). If the prostatic apex is defined at 12 mm instead of 10 mm above the urethrogram tip, the statistically significant difference between the urethrogram and the MRI is no longer present. Based on these results, all 17 patients received the minimum prescribed dose to the prostatic apex.
  
  
  CONCLUSIONS
  CT/urethrogram correlates better with the location of the MRI determined prostatic apex, than does the urethrogram alone. Locating the prostatic apex 12 mm above the urethrogram tip better localizes the prostatic apex, while also avoiding the error that can potentially lead to a geographic miss. This in fact assures that all of our patients receive the minimum prescribed dose to this critical site of extraprostatic extension, while also decreasing the amount of normal tissue that is included in the treatment volume.
authors.affiliation: Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA 19111, USA.
authors.forename: O
authors.initials: O
authors.lastname: Algan
authors.forename: G E
authors.initials: GE
authors.lastname: Hanks
authors.forename: A H
authors.initials: AH
authors.lastname: Shaer
mesh_terms: D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D011446:Prospective Studies; D011467:Prostate / Q000000981:diagnostic imaging / Q000473:pathology*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology* / Q000532:radiotherapy; D011880:Radiotherapy Planning, Computer-Assisted; D012669:Seminal Vesicles / Q000000981:diagnostic imaging / Q000473:pathology; D014057:Tomography, X-Ray Computed; D014521:Urethra / Q000000981:diagnostic imaging