title: Volumetric modulated arc therapy for nasopharyngeal carcinoma: a dosimetric comparison with TomoTherapy and step-and-shoot IMRT.
abstract: OBJECTIVE
  Volumetric modulated arc therapy (VMAT), a novel technique, employs a linear accelerator to conduct dynamic modulation rotation radiotherapy. The goal of this study was to compare VMAT with helical tomotherapy (HT) and step-and-shoot intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma (NPC) patients with regard to the sparing effect on organs at risk (OARs), dosimetric quality, and efficiency of delivery.
  
  
  METHODS
  Twenty patients with NPC treated by HT were re-planned by VMAT (two arcs) and IMRT (7-9 fields) for dosimetric comparison. The target area received three dose levels (70, 60, and 54 Gy) in 33 fractions using simultaneous integrated boosts technique. The Philips Pinnacle Planning System 9.0 was adopted to design VMAT, using SmartArc as the planning algorithm. For a fair comparison, the planning target volume (PTV) coverage of the 3 plans was normalized to the same level. Dosimetric comparisons between VMAT, HT, and IMRT plans were analyzed to evaluate (1) coverage, homogeneity, and conformity of PTV, (2) sparing of OARs, (3) delivery time, and (4) monitor units (MUs).
  
  
  RESULTS
  The VMAT, HT, and IMRT plans had similar PTV coverage with an average of 96%. There was no significant difference between VMAT and HT in homogeneity, while the homogeneity indices of VMAT (1.06) and HT (1.06) were better than IMRT plans (1.07, p<0.05). HT plans provided a better conformity index (1.17) than VMAT (1.28, p=0.01) and IMRT (1.36, p=0.02). When compared with IMRT, VMAT and HT had a better sparing effect on brain stem and spinal cord (p<0.05). The effect of parotid sparing was similar between VMAT (mean=26.3 Gy) and HT (mean=27.5 Gy), but better than IMRT (mean=31.3 Gy, p<0.01). The delivery time per fraction for VMAT (5.7 min) were much lower than for HT (9.5 min, p<0.01) and IMRT (9.2 min, p<0.01).
  
  
  CONCLUSIONS
  Our results indicate that VMAT provides better sparing of normal tissue, homogeneity, and conformity than IMRT, and shorter delivery time than HT.
authors.affiliation: Division of Radiation Oncology, Departments of Oncology, National Taiwan University Hospital, Taipei, Taiwan.
authors.forename: Szu-Huai
authors.initials: SH
authors.lastname: Lu
authors.forename: Jason Chia-Hsien
authors.initials: JC
authors.lastname: Cheng
authors.forename: Sung-Hsin
authors.initials: SH
authors.lastname: Kuo
authors.forename: Jason Jeun-Shenn
authors.initials: JJ
authors.lastname: Lee
authors.forename: Liang-Hsin
authors.initials: LH
authors.lastname: Chen
authors.forename: Jian-Kuen
authors.initials: JK
authors.lastname: Wu
authors.forename: Yu-Hsuan
authors.initials: YH
authors.lastname: Chen
authors.forename: Wan-Yu
authors.initials: WY
authors.lastname: Chen
authors.forename: Shu-Yu
authors.initials: SY
authors.lastname: Wen
authors.forename: Fok-Ching
authors.initials: FC
authors.lastname: Chong
authors.forename: Chien-Jang
authors.initials: CJ
authors.lastname: Wu
authors.forename: Chun-Wei
authors.initials: CW
authors.lastname: Wang
mesh_terms: D002277:Carcinoma; D006801:Humans; D000077274:Nasopharyngeal Carcinoma; D009303:Nasopharyngeal Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000009:adverse effects / Q000379:methods*; D036542:Tomography, Spiral Computed; D047368:Tumor Burden