title: Variation in position and volume of organs at risk in the small pelvis.
abstract: In preparation for studies of dose volume of ionizing radiation and long-term side effects, we assessed both variation in position and volume of organs at risk in the small pelvis.
  
  
  METHODS
  On 10 men and seven women we delineated the sigmoid, rectum, anal sphincter, bladder, penile bulb, and cavernous bodies in two CT scans taken between five to 69 days apart.
  
  
  RESULTS
  The measured overlap of the two delineated volumes divided by the maximum possible overlap, was below 50% for the sigmoid in six of 17 patients, for the distal 4 cm of the sigmoid in five of 17 patients, for the rectum in none of 17 patients, for the anal sphincter in three of 17 patients and for the urinary bladder in none of 17 patients. The smaller volume divided by the larger volume was below 50% in three of 17 patients for the sigmoid, in six of 17 patients for the 4 distal cm of the sigmoid, in two of 17 patients for the rectum, in two of 17 patients for the anal sphincter and in seven of 17 patients for the urinary bladder. For the urinary bladder the largest deviation was found cranially, 4.0 cm (SD 2.0 cm), the caudal part being relatively fixed. For the rectum the largest deviation was found in the anterior wall, 1.8 cm (SD 0.7 cm), with maximum documented variation in cranial direction of 3.2 cm (SD 1.8 cm).
  
  
  CONCLUSIONS
  The sigmoid varies considerably in documented position with the largest deviation anteriorly, the urinary bladder change in volume with the extension mainly located cranially and for the rectum the anterior wall is the most mobile with the distension becoming more pronounced cranially. In modeling dose-volume effects one may consider our results.
authors.affiliation: Department of Oncology-Clinical Cancer Epidemiology, The Sahlgrenska Academy, Göteborg University, Göteborg, Sweden. ann-charlotte.waldenstrom@oncology.gu.se
authors.forename: Ann-Charlotte
authors.initials: AC
authors.lastname: Waldenström
authors.forename: David
authors.initials: D
authors.lastname: Alsadius
authors.forename: Niclas
authors.initials: N
authors.lastname: Pettersson
authors.forename: Karl-Axel
authors.initials: KA
authors.lastname: Johansson
authors.forename: Erik
authors.initials: E
authors.lastname: Holmberg
authors.forename: Gunnar
authors.initials: G
authors.lastname: Steineck
authors.forename: Markus
authors.initials: M
authors.lastname: Müller
mesh_terms: D000328:Adult; D000368:Aged; D012809:Colon, Sigmoid / Q000528:radiation effects; D005260:Female; D005833:Genital Neoplasms, Female / Q000532:radiotherapy*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009929:Organ Size; D010388:Pelvis / Q000528:radiation effects*; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D012007:Rectum / Q000528:radiation effects; D001743:Urinary Bladder / Q000528:radiation effects; D014781:Viscera / Q000528:radiation effects*