title: Exponential Increase in Relative Biological Effectiveness Along Distal Edge of a Proton Bragg Peak as Measured by Deoxyribonucleic Acid Double-Strand Breaks.
abstract: OBJECTIVE
  To quantify the relative biological effectiveness (RBE) of the distal edge of the proton Bragg peak, using an in vitro assay of DNA double-strand breaks (DSBs).
  
  
  METHODS
  U2OS cells were irradiated within the plateau of a spread-out Bragg peak and at each millimeter position along the distal edge using a custom slide holder, allowing for simultaneous measurement of physical dose. A reference radiation signal was generated using photons. The DNA DSBs at 3 hours (to assess for early damage) and at 24 hours (to assess for residual damage and repair) after irradiation were measured using the γH2AX assay and quantified via flow cytometry. Results were confirmed with clonogenic survival assays. A detailed map of the RBE as a function of depth along the Bragg peak was generated using γH2AX measurements as a biological endpoint.
  
  
  RESULTS
  At 3 hours after irradiation, DNA DSBs were higher with protons at every point along the distal edge compared with samples irradiated with photons to similar doses. This effect was even more pronounced after 24 hours, indicating that the impact of DNA repair is less after proton irradiation relative to photons. The RBE demonstrated an exponential increase as a function of depth and was measured to be as high as 4.0 after 3 hours and as high as 6.0 after 24 hours. When the RBE-corrected dose was plotted as a function of depth, the peak effective dose was extended 2-3 mm beyond what would be expected with physical measurement.
  
  
  CONCLUSIONS
  We generated a highly comprehensive map of the RBE of the distal edge of the Bragg peak, using a direct assay of DNA DSBs in vitro. Our data show that the RBE of the distal edge increases with depth and is significantly higher than previously reported estimates.
authors.affiliation: Memorial Sloan Kettering Cancer Center, New York, New York. Electronic address: cuaronj@mskcc.org.
authors.forename: John J
authors.initials: JJ
authors.lastname: Cuaron
authors.affiliation: Texas Center for Proton Therapy, Irving, Texas.
authors.forename: Chang
authors.initials: C
authors.lastname: Chang
authors.affiliation: Memorial Sloan Kettering Cancer Center, New York, New York.
authors.forename: Michael
authors.initials: M
authors.lastname: Lovelock
authors.affiliation: Memorial Sloan Kettering Cancer Center, New York, New York.
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Higginson
authors.affiliation: Procure Proton Therapy Center, Somerset, New Jersey.
authors.forename: Dennis
authors.initials: D
authors.lastname: Mah
authors.affiliation: Memorial Sloan Kettering Cancer Center, New York, New York.
authors.forename: Oren
authors.initials: O
authors.lastname: Cahlon
authors.affiliation: Memorial Sloan Kettering Cancer Center, New York, New York.
authors.forename: Simon
authors.initials: S
authors.lastname: Powell
mesh_terms: D045744:Cell Line, Tumor; D002470:Cell Survival; D003114:Colony-Forming Units Assay; D053903:DNA Breaks, Double-Stranded; D004260:DNA Repair; D004307:Dose-Response Relationship, Radiation; D005434:Flow Cytometry / Q000379:methods; D006657:Histones / Q000032:analysis; D006801:Humans; D018499:Linear Energy Transfer; D017785:Photons; D061766:Proton Therapy; D011522:Protons; D012062:Relative Biological Effectiveness; D013997:Time Factors
chemical_list: C468783:H2AX protein, human; D006657:Histones; D011522:Protons