title: A planning target volume margin formula for hypofractionated intracranial stereotactic radiotherapy under cone beam CT image guidance with a six-degrees-of-freedom robotic couch and a mouthpiece-assisted mask system: a preliminary study.
abstract: OBJECTIVE
  A planning target volume (PTV) margin formula for hypofractionated intracranial stereotactic radiotherapy (SRT) has been proposed under cone beam CT (CBCT) image guidance with a six-degrees-of-freedom (6-DOF) robotic couch.
  
  
  METHODS
  CBCT-based registration using a 6-DOF couch reportedly led to negligibly small systematic positioning errors, suggesting that each in-treatment positioning error during the treatment courses for the patients employing this combination was predominantly caused by a random gaussian process. Under this assumption, an anisotropic PTV margin for each axis was formulated based on a gaussian distribution model. 19 patients with intracranial lesions who underwent additional post-treatment CBCT were consecutively selected, to whom stereotactic hypofractionated radiotherapy was delivered by a linear accelerator equipped with a CBCT imager, a 6-DOF couch and a mouthpiece-assisted mask system. Time-averaged patient-positioning errors during treatment were estimated by comparing the post-treatment CBCT with the reference planning CT images.
  
  
  RESULTS
  It was suggested that each histogram of the in-treatment positioning error in each axis would approach each single gaussian distribution with a mean of zero. The calculated PTV margins in the x, y and z directions were 0.97, 1.30 and 0.88 mm, respectively.
  
  
  CONCLUSIONS
  The empirical isotropic PTV margin of 2 mm used in our facility for intracranial SRT was consistent with the margin calculated by the proposed gaussian model.
  
  
  CONCLUSIONS
  We have proposed a PTV margin formula for hypofractionated intracranial SRT under CBCT image guidance with a 6-DOF robotic couch.
authors.affiliation: 1 Department of Radiology, Self Defense Forces Central Hospital, Tokyo, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Naoi
authors.forename: N
authors.initials: N
authors.lastname: Kunishima
authors.forename: K
authors.initials: K
authors.lastname: Yamamoto
authors.forename: K
authors.initials: K
authors.lastname: Yoda
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D054893:Cone-Beam Computed Tomography; D019583:Dose Fractionation, Radiation; D004867:Equipment Design; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D008397:Masks; D008875:Middle Aged; D056888:Patient Positioning; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D065287:Robotic Surgical Procedures; D025321:Surgery, Computer-Assisted