title: Assessment of interfractional prostate motion in patients immobilized in the prone position using a thermoplastic shell.
abstract: The aim of this study was to evaluate the interfractional prostate motion of patients immobilized in the prone position using a thermoplastic shell. A total of 24 patients with prostate calcifications detectable using a kilo-voltage X-ray image-guidance system (ExacTrac X-ray system) were examined. Daily displacements of the calcification within the prostate relative to pelvic bony structures were calculated by the ExacTrac X-ray system. The average displacement and standard deviation (SD) in each of the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions were calculated for each patient. Based on the results of interfractional prostate motion, we also calculated planning target volume (PTV) margins using the van Herk formula and examined the validity of the PTV margin of our institute (a 9-mm margin everywhere except posteriorly, where a 6-mm margin was applied). In total, 899 data measurements from 24 patients were obtained. The average prostate displacements ± SD relative to bony structures were 2.8 ± 3.3, -2.0 ± 2.0 and 0.2 ± 0.4 mm, in the SI, AP and LR directions, respectively. The required PTV margins were 9.7, 6.1 and 1.4 mm in the SI, AP and LR directions, respectively. The clinical target volumes of 21 patients (87.5%) were located within the PTV for 90% or more of all treatment sessions. Interfractional prostate motion in the prone position with a thermoplastic shell was equivalent to that reported for the supine position. The PTV margin of our institute is considered appropriate for alignment, based on bony structures.
authors.affiliation: Department of Radiation Oncology and Image-applied Therapy, Kyoto University Graduate School of Medicine, 54 Shogoin Kawahara-cho, Sakyo-ku, Kyoto 606-8507, Japan.
authors.forename: Itaru
authors.initials: I
authors.lastname: Ikeda
authors.forename: Takashi
authors.initials: T
authors.lastname: Mizowaki
authors.forename: Yohei
authors.initials: Y
authors.lastname: Sawada
authors.forename: Manabu
authors.initials: M
authors.lastname: Nakata
authors.forename: Yoshiki
authors.initials: Y
authors.lastname: Norihisa
authors.forename: Masakazu
authors.initials: M
authors.lastname: Ogura
authors.forename: Masahiro
authors.initials: M
authors.lastname: Hiraoka
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002114:Calcinosis / Q000000981:diagnostic imaging*; D006801:Humans; D007103:Immobilization / Q000295:instrumentation*; D008297:Male; D008875:Middle Aged; D056888:Patient Positioning / Q000295:instrumentation*; D016684:Prone Position; D011467:Prostate / Q000000981:diagnostic imaging*; D011469:Prostatic Diseases / Q000000981:diagnostic imaging*; D011835:Radiation Protection / Q000295:instrumentation; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000295:instrumentation*
keywords: intensity-modulated radiotherapy; interfractional prostate motion; prone position; prostate cancer; thermoplastic shell