title: Evaluation of fractionation regimens in stereotactic radiotherapy using a mathematical model of repopulation and reoxygenation.
abstract: OBJECTIVE
  Various forms of stereotactic fractionation regimens have been used in the clinic. The purpose of this article is to evaluate the influence of the number of fractions on tumor control in stereotactic radiotherapy.
  
  
  METHODS
  The 50% curative dose (D50) is calculated using a mathematical model of reoxygenation and repopulation. A review of the literature of stereotactic fractionation regimens is also presented.
  
  
  RESULTS
  The number of fractions required to achieve the best therapeutic ratio is strongly influenced by the pattern of reoxygenation and the potential doubling time of the tumor. In the case of rapid repopulation, well-oxygenated clonogenic cells present a greater danger than hypoxic cells in a prolonged fractionated treatment schedule. In a slowly growing tumor, repopulation is relatively unimportant and the response is dominated by the pattern of reoxygenation. For a small number of large dose fractions, the potential doubling time and the time interval between fractions have a negligible effect on the D50 values.
  
  
  CONCLUSIONS
  If a small number of large dose fractions is used in stereotactic radiotherapy and if reoxygenation occurs more efficiently with the passage of time, the therapeutic ratio can be enlarged by a sufficient time interval between fractions.
authors.affiliation: Department of Radiology, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Nakamura
authors.forename: A
authors.initials: A
authors.lastname: Brahme
mesh_terms: D001932:Brain Neoplasms / Q000532:radiotherapy*; D015687:Cell Hypoxia; D002470:Cell Survival; D019583:Dose Fractionation, Radiation; D006801:Humans; D008962:Models, Theoretical; D011829:Radiation Dosage; D013238:Stereotaxic Techniques