title: Reirradiation of spinal column metastases: comparison of several treatment techniques and dosimetric validation for the use of VMAT.
abstract: BACKGROUND
  For reirradiation of spinal column metastases, intensity-modulated radiation therapy (IMRT) reduces the dose to the spinal cord, while allowing longer treatment times. We analyzed the potential of volumetric modulated arc therapy (VMAT) to reduce treatment time and number of monitor units (MU).
  
  
  METHODS
  In CT datasets of 9 patients with spinal column metastases, the planned target volume (PTV) encompassed the macroscopic tumor including the spinal cord or medullary cone, respectively. The prescribed dose for the target was 40 Gy, but median spinal cord dose was intended to be < 26 Gy. We compared a posterior (3D-PA) static field technique, a two-field wedge technique (3D-wedge) and 5-/7-beam IMRT with VMAT. Conformity index (CI), homogeneity index (HI40), dose volume histogram (DVH) parameters, treatments delivery time (T), and MU were analyzed. Dosimetry was validated with EDR2-film/ionization chambers.
  
  
  RESULTS
  PTV coverage was insufficient for 3D-conformal radiotherapy (3D-CRT) when spinal cord tolerance was respected. The IMRT approach provided excellent results but has the longest treatment time. VMAT produced dose distributions similar to IMRT with shorter treatment times (VMAT: mean 4:49 min; IMRT: mean 6:50 min) and fewer MU (VMAT: 785; IMRT: 860). Reduced conformity and increased homogeneity for VMAT when compared to IMRT were observed. An absolute deviation between measured and calculated dose of +0.70 ± 3.69% was recorded. γ-Index analysis showed an agreement of 91.33 ± 3.53% for the 5%/5 mm criteria.
  
  
  CONCLUSIONS
  For this paradigm, VMAT produces high quality treatment plans with homogeneity/conformity similar to static IMRT, shorter treatment times, and fewer MU. Verification measurements showed good agreement between calculation and delivered dose, leading to clinical implementation.
authors.affiliation: Department of Radiation Oncology, University Medical Center Mannheim, University Heidelberg, Germany. florian.stieler@umm.de
authors.forename: Florian
authors.initials: F
authors.lastname: Stieler
authors.forename: Dirk
authors.initials: D
authors.lastname: Wolff
authors.forename: Linda
authors.initials: L
authors.lastname: Bauer
authors.forename: Hans-Jörg
authors.initials: HJ
authors.lastname: Wertz
authors.forename: Frederik
authors.initials: F
authors.lastname: Wenz
authors.forename: Frank
authors.initials: F
authors.lastname: Lohr
mesh_terms: D004307:Dose-Response Relationship, Radiation; D006801:Humans; D011874:Radiometry; D011878:Radiotherapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted; D020266:Radiotherapy, Conformal / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D019233:Retreatment; D013116:Spinal Cord / Q000528:radiation effects*; D013125:Spinal Neoplasms / Q000532:radiotherapy* / Q000556:secondary*