title: Cone beam CT (CBCT) evaluation of inter- and intra-fraction motion for patients undergoing brain radiotherapy immobilized using a commercial thermoplastic mask on a robotic couch.
abstract: Patients receiving fractionated intensity-modulated radiation therapy (IMRT) for brain tumors are often immobilized with a thermoplastic mask; however, masks do not perfectly re-orient the patient due to factors including the maximum pressure which can be applied to the face, deformations of the mask assembly, patient compliance, etc. Consequently, ~3-5mm PTV margins (beyond the CTV) are often recommended. We aimed to determine if smaller PTV margins are feasible using mask immobilization coupled with 1) a gantry mounted CBCT image guidance system and 2) position corrections provided by a full six-degree of freedom (6-DOF) robotic couch. A cohort of 34 brain tumor patients was treated with fractionated IMRT. After the mask set-up, an initial CBCT was obtained and registered to the planning CT. The robotic couch corrected the misalignments in all 6-DOF and a pre-treatment verification CBCT was then obtained. The results indicated a repositioning alignment within our threshold of 1.5 mm (3D). Treatment was subsequently delivered. A post-treatment CBCT was obtained to quantify intra-fraction motion. Initial, pre-treatment and post-treatment CBCT image data was analyzed. A total of 505 radiation fractions were delivered to the 34 patients resulting in ~1800 CBCT scans. The initial median 3D (magnitude) set-up positioning error was 2.60 mm. Robotic couch corrections reduced the 3D median error to 0.53 mm prior to treatment. Intra-fraction movement was responsible for increasing the median 3D positioning error to 0.86 mm, with 8% of fractions having a 3D positioning error greater than 2 mm. Clearly CBCT image guidance coupled with a robotic 6-DOF couch dramatically improved the positioning accuracy for patients immobilized in a thermoplastic mask system; however, such intra-fraction motion would be too large for single fraction radiosurgery.
authors.affiliation: Department of Radiation Oncology, Odette Cancer Centre, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, ON, Canada M4N 3M5. Alex.Lightstone@sunnybrook.ca
authors.forename: A W
authors.initials: AW
authors.lastname: Lightstone
authors.forename: May
authors.initials: M
authors.lastname: Tsao
authors.forename: Parminder S
authors.initials: PS
authors.lastname: Baran
authors.forename: Gordon
authors.initials: G
authors.lastname: Chan
authors.forename: Geordi
authors.initials: G
authors.lastname: Pang
authors.forename: Lijun
authors.initials: L
authors.lastname: Ma
authors.forename: Fiona
authors.initials: F
authors.lastname: Lochray
authors.forename: Arjun
authors.initials: A
authors.lastname: Sahgal
mesh_terms: D001932:Brain Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D015331:Cohort Studies; D054893:Cone-Beam Computed Tomography / Q000379:methods; D006801:Humans; D007103:Immobilization / Q000379:methods*; D008397:Masks; D009068:Movement; D058958:Organs at Risk; D056888:Patient Positioning / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061106:Radiotherapy Setup Errors / Q000517:prevention & control*; D011881:Radiotherapy, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012371:Robotics / Q000379:methods*