title: Dosimetric effects of prone and supine positions on post-implant assessments for prostate brachytherapy.
abstract: OBJECTIVE
  Post-implant dosimetric assessment is essential for optimal care of patients receiving prostate brachytherapy. In most institutions, post-implant computed tomography (CT) is performed in the supine position. This study aimed to assess variability in dosimetric parameters with postural changes during acquisition of post-implant CT scans.
  
  
  METHODS
  In total, 85 consecutive patients were enrolled in this study. Fifty-three patients underwent seed implantation alone, and the remaining 32 received a combination of seed implantation and external beam radiotherapy. For post-implant analyses, CT scans were obtained in two patient positions, supine and prone. To evaluate differences in dosimetric parameters associated with postural change, the dosimetric data obtained in the supine position were defined as the standard.
  
  
  RESULTS
  The median prostate volume was 22.4 ml in the supine and 22.5 ml in the prone position (p = 0.51). The median prostate D90 was 120.1% in the supine and 120.3% in the prone position, not significantly different. The mean prostate V100 was 97.1% in the supine and 97.0% in the prone position, again not significantly different. Median rectal V100 in supine and prone positions were 0.42 ml and 0.33 ml, respectively (p < 0.01). Rectal D2cc was also significantly decreased in the prone as compared with the supine position (median, 59.1% vs. 63.6%; p < 0.01). A larger post-implant prostate volume was associated with decreased rectal doses in the prone position.
  
  
  CONCLUSIONS
  Though there were no significant differences among prostate D90 assessments according to postural changes, our results suggest that post-implant rectal doses decreased in the prone position.
authors.affiliation: Department of Radiology, Keio University School of Medicine, Tokyo ; Department of Radiology, National Hospital Organization Saitama Hospital, Saitama.
authors.forename: Toshio
authors.initials: T
authors.lastname: Ohashi
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Momma
authors.forename: Shoji
authors.initials: S
authors.lastname: Yamashita
authors.forename: Kunimitsu
authors.initials: K
authors.lastname: Kanai
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Watanabe
authors.forename: Takashi
authors.initials: T
authors.lastname: Hanada
authors.forename: Naoyuki
authors.initials: N
authors.lastname: Shigematsu
keywords: brachytherapy; position; post-implant; prone; prostate cancer