title: Investigation of interfractional shape variations based on statistical point distribution model for prostate cancer radiation therapy.
abstract: OBJECTIVE
  The setup errors and organ motion errors pertaining to clinical target volume (CTV) have been considered as two major causes of uncertainties in the determination of the CTV-to-planning target volume (PTV) margins for prostate cancer radiation treatment planning. We based our study on the assumption that interfractional target shape variations are not negligible as another source of uncertainty for the determination of precise CTV-to-PTV margins. Thus, we investigated the interfractional shape variations of CTVs based on a point distribution model (PDM) for prostate cancer radiation therapy.
  
  
  METHODS
  To quantitate the shape variations of CTVs, the PDM was applied for the contours of 4 types of CTV regions (low-risk, intermediate- risk, high-risk CTVs, and prostate plus entire seminal vesicles), which were delineated by considering prostate cancer risk groups on planning computed tomography (CT) and cone beam CT (CBCT) images of 73 fractions of 10 patients. The standard deviations (SDs) of the interfractional random errors for shape variations were obtained from covariance matrices based on the PDMs, which were generated from vertices of triangulated CTV surfaces. The correspondences between CTV surface vertices were determined based on a thin-plate spline robust point matching algorithm. The systematic error for shape variations was defined as the average deviation between surfaces of an average CTV and planning CTVs, and the random error as the average deviation of CTV surface vertices for fractions from an average CTV surface.
  
  
  RESULTS
  The means of the SDs of the systematic errors for the four types of CTVs ranged from 1.0 to 2.0 mm along the anterior direction, 1.2 to 2.6 mm along the posterior direction, 1.0 to 2.5 mm along the superior direction, 0.9 to 1.9 mm along the inferior direction, 0.9 to 2.6 mm along the right direction, and 1.0 to 3.0 mm along the left direction. Concerning the random errors, the means of the SDs ranged from 0.9 to 1.2 mm along the anterior direction, 1.0 to 1.4 mm along the posterior direction, 0.9 to 1.3 mm along the superior direction, 0.8 to 1.0 mm along the inferior direction, 0.8 to 0.9 mm along the right direction, and 0.8 to 1.0 mm along the left direction.
  
  
  CONCLUSIONS
  Since the shape variations were not negligible for intermediate and high-risk CTVs, they should be taken into account for the determination of the CTV-to-PTV margins in radiation treatment planning of prostate cancer.
authors.affiliation: Department of Medical Technology, Kyushu University Hospital, 3-1-1, Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Shibayama
authors.affiliation: Faculty of Medical Sciences, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Hidetaka
authors.initials: H
authors.lastname: Arimura
authors.affiliation: Graduate School of Medical Sciences, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Taka-Aki
authors.initials: TA
authors.lastname: Hirose
authors.affiliation: Graduate School of Medical Sciences, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.|Japan Society for the Promotion of Science, 8, Ichiban-cho, Chiyoda-ku, Tokyo, 102-8472, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Nakamoto
authors.affiliation: Faculty of Medical Sciences, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Tomonari
authors.initials: T
authors.lastname: Sasaki
authors.affiliation: Faculty of Medical Sciences, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Saiji
authors.initials: S
authors.lastname: Ohga
authors.affiliation: Division of Clinical Radiology Service, Kyoto University Hospital, 54, Kawaharacho, Shogoin, Sakyo-ku, Kyoto, 606-8507, Japan.
authors.forename: Norimasa
authors.initials: N
authors.lastname: Matsushita
authors.affiliation: Department of Medical Technology, Kyushu University Hospital, 3-1-1, Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Umezu
authors.affiliation: Department of Medical Technology, Kyushu University Hospital, 3-1-1, Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Yasuhiko
authors.initials: Y
authors.lastname: Nakamura
authors.affiliation: Faculty of Medical Sciences, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Honda
mesh_terms: D006801:Humans; D008297:Male; D015233:Models, Statistical; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging*; D011880:Radiotherapy Planning, Computer-Assisted; D014057:Tomography, X-Ray Computed
keywords: point distribution model; prostate cancer radiation therapy; shape variation