title: Relative Biologic Effectiveness (RBE) of 50 kV X-rays measured in a phantom for intraoperative tumor-bed irradiation.
abstract: OBJECTIVE
  Intraoperative radiation therapy (IORT) with low-energy x-rays is used to treat the tumor bed during breast-conserving surgery. The purpose was to determine the relative biologic effectiveness (RBE) of 50-kV x-rays for inactivation of cells irradiated in a tumor-bed phantom.
  
  
  METHODS
  The RBE was determined for clonogenic inactivation of human tumor and normal cells (MCF7, human umbilical vein endothelial cells, normal skin fibroblasts), and hamster V79 cells. The 50-kV x-rays from the Intrabeam machine (Carl Zeiss Surgical) with a spherical 4-cm applicator were used. Cells were irradiated in a water-equivalent phantom at defined distances (8.1-22.9 mm) from the applicator surface. The 50-kV x-rays from a surface therapy machine (Dermopan, Siemens) were included for comparison; 6-MV x-rays were used as reference radiation.
  
  
  RESULTS
  At 8.1-mm depth in the phantom (dose rate 15.1 Gy/h), mean RBE values of 50-kV x-rays from Intrabeam were 1.26 to 1.42 for the 4 cell types at doses yielding surviving fractions in the range of 0.01 to 0.5. Confidence intervals were in the range of 1.2 and 1.5. Similar RBE values were found for 50-kV x-rays from Dermopan for V79 (1.30, CI 1.25-1.36, P=.74) and GS4 (1.42, CI 1.30-1.54, P=.67). No significant dependence of RBE on dose was found for Intrabeam, but RBE decreased at a larger distance (12.7 mm; 9.8 Gy/h).
  
  
  CONCLUSIONS
  An increased clinically relevant RBE was found for cell irradiation with Intrabeam at depths in the tumor bed targeted by IORT. The reduced RBE values at larger distances may be related to increased repair of sublethal damage during protracted irradiation or to hardening of the photon beam energy.
authors.affiliation: Department of Radiation Oncology, Universitätsmedizin Mannheim, Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany.
authors.forename: Qi
authors.initials: Q
authors.lastname: Liu
authors.forename: Frank
authors.initials: F
authors.lastname: Schneider
authors.forename: Lin
authors.initials: L
authors.lastname: Ma
authors.forename: Frederik
authors.initials: F
authors.lastname: Wenz
authors.forename: Carsten
authors.initials: C
authors.lastname: Herskind
mesh_terms: D001943:Breast Neoplasms / Q000473:pathology / Q000532:radiotherapy* / Q000601:surgery; D002470:Cell Survival / Q000502:physiology / Q000528:radiation effects*; D004260:DNA Repair; D005260:Female; D005347:Fibroblasts / Q000528:radiation effects; D061307:Human Umbilical Vein Endothelial Cells / Q000528:radiation effects; D006801:Humans; D007430:Intraoperative Care; D015412:Mastectomy, Segmental; D019047:Phantoms, Imaging; D012062:Relative Biological Effectiveness; D023041:Xenograft Model Antitumor Assays / Q000379:methods