title: Impact of geometric variations on delivered dose in highly focused single vocal cord IMRT.
abstract: OBJECTIVE
  To investigate the robustness of single vocal cord intensity modulated radiation therapy (IMRT) treatment plans for set-up errors, respiration, and deformation.
  
  
  METHODS
  Four-dimensional computed tomography (4D-CT) scans of 10 early glottic carcinoma patients, previously treated with conventional techniques, were used in this simulation study. For each patient a pre-treatment 4D-CT was used for IMRT planning, generating a reference dose distribution. Prescribed PTV dose was 66 Gy. The impact of systematic set-up errors was simulated by applying shifts of ± 2 mm to the planning CT scans, followed by dose re-calculation with original beam segments, MUs, etc. Effects of respiration and deformation were determined utilizing extreme inhale and exhale CT scans, and repeat scans acquired after 22 Gy, 44 Gy, and 66 Gy, respectively. All doses were calculated using Monte Carlo dose simulations.
  
  
  RESULTS
  Considering all investigated geometrical perturbations, reductions in the clinical target volume (CTV) V95%, D98%, D2%, and generalized equivalent uniform dose (gEUD) were limited to 1.2 ± 2.2%, 2.4 ± 2.9%, 0.2 ± 1.8%, and 0.6 ± 1.1 Gy, respectively. The near minimum dose, D98%, was always higher than 89%, and gEUD always remained higher than 66 Gy. Planned contra-lateral (CL) vocal cord DMean, gEUD, and V40 Gy were 38.2 ± 6.0 Gy, 43.4 ± 5.6 Gy, and 42.7 ± 14.9%. With perturbations these values changed by -0.1 ± 4.3 Gy, 0.1 ± 4.0 Gy, and -1.0 ± 9.6%, respectively.
  
  
  CONCLUSIONS
  On average, CTV dose reductions due to geometrical perturbations were very low, and sparing of the CL vocal cord was maintained. In a few observations (6 of 103 simulated situations), the near-minimum CTV-dose was around 90%, requiring attention in deciding on a future clinical protocol.
authors.affiliation: Department of Radiation Oncology, Erasmus MC, Daniel den Hoed Cancer Center , Rotterdam , the Netherlands.
authors.forename: Sarah O S
authors.initials: SO
authors.lastname: Osman
authors.forename: Eleftheria
authors.initials: E
authors.lastname: Astreinidou
authors.forename: Peter C
authors.initials: PC
authors.lastname: Levendag
authors.forename: Ben J M
authors.initials: BJ
authors.lastname: Heijmen
mesh_terms: D002277:Carcinoma / Q000532:radiotherapy*; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D007822:Laryngeal Neoplasms / Q000532:radiotherapy*; D009010:Monte Carlo Method; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012189:Retrospective Studies; D014827:Vocal Cords