title: Inter- and Intrafraction Target Motion in Highly Focused Single Vocal Cord Irradiation of T1a Larynx Cancer Patients.
abstract: OBJECTIVE
  The purpose of this study was to verify clinical target volume--planning target volume (CTV-PTV) margins in single vocal cord irradiation (SVCI) of T1a larynx tumors and characterize inter- and intrafraction target motion.
  
  
  METHODS
  For 42 patients, a single vocal cord was irradiated using intensity modulated radiation therapy at a total dose of 58.1 Gy (16 fractions × 3.63 Gy). A daily cone beam computed tomography (CBCT) scan was performed to online correct the setup of the thyroid cartilage after patient positioning with in-room lasers (interfraction motion correction). To monitor intrafraction motion, CBCT scans were also acquired just after patient repositioning and after dose delivery. A mixed online-offline setup correction protocol ("O2 protocol") was designed to compensate for both inter- and intrafraction motion.
  
  
  RESULTS
  Observed interfraction, systematic (Σ), and random (σ) setup errors in left-right (LR), craniocaudal (CC), and anteroposterior (AP) directions were 0.9, 2.0, and 1.1 mm and 1.0, 1.6, and 1.0 mm, respectively. After correction of these errors, the following intrafraction movements derived from the CBCT acquired after dose delivery were: Σ = 0.4, 1.3, and 0.7 mm, and σ = 0.8, 1.4, and 0.8 mm. More than half of the patients showed a systematic non-zero intrafraction shift in target position, (ie, the mean intrafraction displacement over the treatment fractions was statistically significantly different from zero; P<.05). With the applied CTV-PTV margins (for most patients 3, 5, and 3 mm in LR, CC, and AP directions, respectively), the minimum CTV dose, estimated from the target displacements observed in the last CBCT, was at least 94% of the prescribed dose for all patients and more than 98% for most patients (37 of 42). The proposed O2 protocol could effectively reduce the systematic intrafraction errors observed after dose delivery to almost zero (Σ = 0.1, 0.2, 0.2 mm).
  
  
  CONCLUSIONS
  With adequate image guidance and CTV-PTV margins in LR, CC, and AP directions of 3, 5, and 3 mm, respectively, excellent target coverage in SVCI could be ensured.
authors.affiliation: Department of Radiation Oncology, Erasmus MC Cancer Institute, University Medical Center Rotterdam, Rotterdam, The Netherlands. Electronic address: s.kwa@erasmusmc.nl.
authors.forename: Stefan L S
authors.initials: SL
authors.lastname: Kwa
authors.affiliation: Department of Radiation Oncology, Erasmus MC Cancer Institute, University Medical Center Rotterdam, Rotterdam, The Netherlands.
authors.forename: Abrahim
authors.initials: A
authors.lastname: Al-Mamgani
authors.affiliation: Department of Radiation Oncology, Erasmus MC Cancer Institute, University Medical Center Rotterdam, Rotterdam, The Netherlands.
authors.forename: Sarah O S
authors.initials: SO
authors.lastname: Osman
authors.affiliation: Department of Radiation Oncology, Erasmus MC Cancer Institute, University Medical Center Rotterdam, Rotterdam, The Netherlands.
authors.forename: Anne
authors.initials: A
authors.lastname: Gangsaas
authors.affiliation: Department of Radiation Oncology, Erasmus MC Cancer Institute, University Medical Center Rotterdam, Rotterdam, The Netherlands.
authors.forename: Peter C
authors.initials: PC
authors.lastname: Levendag
authors.affiliation: Department of Radiation Oncology, Erasmus MC Cancer Institute, University Medical Center Rotterdam, Rotterdam, The Netherlands.
authors.forename: Ben J M
authors.initials: BJ
authors.lastname: Heijmen
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods; D019583:Dose Fractionation, Radiation; D005240:Feasibility Studies; D005931:Glottis; D006801:Humans; D007822:Laryngeal Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D009068:Movement; D009367:Neoplasm Staging; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000528:radiation effects; D056888:Patient Positioning; D011832:Radiation Injuries / Q000517:prevention & control; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061106:Radiotherapy Setup Errors; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D014827:Vocal Cords