title: Evidence for adaptive response and implication in pulse-simulated low-dose-rate radiotherapy.
abstract: OBJECTIVE
  Pulsed-dose-rate (PDR) brachytherapy as a substitute for continuous low-dose-rate (LDR) brachytherapy has a number of clinical advantages. However, early results show that some cells can exhibit an adaptive response to radiation and in PDR where many pulses are given such an adaptive response may play an important role in the outcome.
  
  
  METHODS
  Nine human cell lines (two normal fibroblast and seven tumor) were evaluated for an adaptive response. Cells were given either a single adapting dose before a challenge dose or given PDR sequences for which the average dose rate matched the LDR dose rate. Response was assessed using the colony survival assay.
  
  
  RESULTS
  Five of the nine cell lines showed an adapting response to single small doses of radiation. Three of these cell lines were further investigated for adapting response to PDR and two of the three lines (one ovarian carcinoma and one glioma) showed an adaptive response which was dependent on pulse size and interval.
  
  
  CONCLUSIONS
  The data show that an adaptive response can occur in human cells and that it can vary among cell lines. In addition, PDR sequences also produced an adaptive response which could have an affect on PDR therapy if such a response is found in tissues.
authors.affiliation: Medical Physics Department, Ottawa Regional Cancer Centre, Ottawa, Ontario, Canada. graaphorst@orcc.on.ca
authors.forename: G P
authors.initials: GP
authors.lastname: Raaphorst
authors.forename: C E
authors.initials: CE
authors.lastname: Ng
authors.forename: D
authors.initials: D
authors.lastname: Smith
authors.forename: M
authors.initials: M
authors.lastname: Niedbala
mesh_terms: D000222:Adaptation, Physiological; D001918:Brachytherapy / Q000379:methods*; D019008:Drug Resistance, Neoplasm; D006801:Humans; D011836:Radiation Tolerance; D011853:Radiobiology; D011879:Radiotherapy Dosage; D014407:Tumor Cells, Cultured / Q000502:physiology / Q000528:radiation effects*; D014410:Tumor Stem Cell Assay