title: Impact of prolonged fraction delivery times simulating IMRT on cultured nasopharyngeal carcinoma cell killing.
abstract: OBJECTIVE
  To determine the impact of prolonged fraction delivery times (FDTs) simulating intensity-modulated radiotherapy (IMRT) on cultured nasopharyngeal carcinoma (NPC) cell killing.
  
  
  METHODS
  Cultured NPC cell lines CNE1 and CNE2 were used in this study. The biological effectiveness of fractionated irradiation protocols simulating conventional external beam radiotherapy and IMRT (FDT of 15, 36, and 50 minutes) was estimated with standard colony assay, and the differences in cell surviving fractions after irradiation with different protocols were tested by use of the paired t test. The impact degree of prolonged FDTs (from 8 to 50 minutes) on cell killing was also assessed by the dose-modifying factors, which were estimated by comparing the effectiveness of intermittently delivered 2 Gy with that of continuously delivered 1.5 to 2 Gy.
  
  
  RESULTS
  The cell surviving fractions of both CNE1 and CNE2 after fractionated irradiation simulating IMRT were higher than those simulating conventional external beam radiotherapy (p < 0.05). The dose-modifying factors for a fraction dose of 2 Gy increased from 1.05 to 1.18 for CNE1 and from 1.05 to 1.11 for CNE2 with the FDT being prolonged from 15 to 50 minutes.
  
  
  CONCLUSIONS
  This study showed that the prolonged FDTs simulating IMRT significantly decreased the cell killing in both CNE1 and CNE2 cell lines, and these negative effects increased with the FDT being prolonged from 15 to 50 minutes. These effects, if confirmed by in vivo and clinical studies, need to be considered in designing IMRT treatments for NPC.
authors.affiliation: Department of Radiation Oncology, Nanfang Hospital, Southern Medical University, Guangzhou, China. zkn1268@fimmu.com
authors.forename: Xiao-Kang
authors.initials: XK
authors.lastname: Zheng
authors.forename: Long-Hua
authors.initials: LH
authors.lastname: Chen
authors.forename: Wen-Jun
authors.initials: WJ
authors.lastname: Wang
authors.forename: Feng
authors.initials: F
authors.lastname: Ye
authors.forename: Jia-Bing
authors.initials: JB
authors.lastname: Liu
authors.forename: Qi-Sheng
authors.initials: QS
authors.lastname: Li
authors.forename: Hen-Wen
authors.initials: HW
authors.lastname: Sun
mesh_terms: D045744:Cell Line, Tumor; D002470:Cell Survival / Q000528:radiation effects; D019583:Dose Fractionation, Radiation; D006801:Humans; D009303:Nasopharyngeal Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012062:Relative Biological Effectiveness; D013997:Time Factors; D014407:Tumor Cells, Cultured