title: Relative biological effectiveness of photon energies used in brachytherapy and intraoperative radiotherapy techniques for two breast cancer cell lines.
abstract: BACKGROUND
  Partial breast irradiation (IORT or brachytherapy) differ from external radiation of whole breast in terms of irradiated volumes, fractionation, radiation energy and dose rate; all factors influencing the treatment outcome in a complex manner. Theoretically obtained RBE values comparing effects of radiation used in IORT and external therapy are published, but experimental studies are required to confirm these data. The aim of this study is to establish such RBE values for two breast cancer cell lines.
  
  
  METHODS
  Colony formation of breast cancer cell lines (MCF-7 and T-47D) were studied after photon irradiation with qualities and dose rates used in IORT, brachytherapy and external radiation. RBE values from survival data were used to compare effects.
  
  
  RESULTS
  Increasing the photon energy (dose rate 0.2 Gy/min) from 50 kV (Intrabeam) to 380 keV (¹⁹²Ir source) and 6 MV (linear accelerator) yielded an increase in the cell survival, whereas increasing the dose rate to 6 Gy/min had minor effect. Average RBE values for 50 kV with 6 MV as reference radiation varied from about 1.4 (for doses < 5 Gy) to > 1.9 (for doses < 0.02 Gy) for MCF-7 cells and from about 1.4 to > 3.1 for T-47D cells for the same dose levels. Corresponding RBE values for 380 keV radiation were about 1.4 for MCF-7 cells and 1.3-2.3 for T-47D cells.
  
  
  CONCLUSIONS
  RBE data for breast cancer cells exposed to radiation used in IORT, brachytherapy or external radiation differ among the cell lines tested. The values are in agreement with published theoretical and experimental work.
authors.affiliation: Department of Oncology, St. Olavs Hospital, Trondheim University Hospital, Norway. Anne.Marthinsen@stolav.no
authors.forename: Anne B L
authors.initials: AB
authors.lastname: Marthinsen
authors.forename: Ragnhild
authors.initials: R
authors.lastname: Gisetstad
authors.forename: Signe
authors.initials: S
authors.lastname: Danielsen
authors.forename: Jomar
authors.initials: J
authors.lastname: Frengen
authors.forename: Trond
authors.initials: T
authors.lastname: Strickert
authors.forename: Steinar
authors.initials: S
authors.lastname: Lundgren
mesh_terms: D001918:Brachytherapy / Q000379:methods; D001943:Breast Neoplasms / Q000473:pathology / Q000532:radiotherapy* / Q000601:surgery*; D045744:Cell Line, Tumor / Q000528:radiation effects; D002470:Cell Survival / Q000528:radiation effects; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006801:Humans; D007432:Intraoperative Period; D009367:Neoplasm Staging; D017785:Photons / Q000627:therapeutic use*; D011879:Radiotherapy Dosage; D018714:Radiotherapy, Adjuvant / Q000379:methods*; D012062:Relative Biological Effectiveness