title: Physical and biological factors determining the effective proton range.
abstract: OBJECTIVE
  Proton radiotherapy is rapidly becoming a standard treatment option for cancer. However, even though experimental data show an increase of the relative biological effectiveness (RBE) with depth, particularly at the distal end of the treatment field, a generic RBE of 1.1 is currently used in proton radiotherapy. This discrepancy might affect the effective penetration depth of the proton beam and thus the dose to the surrounding tissue and organs at risk. The purpose of this study was thus to analyze the impact of a tissue and dose dependent RBE of protons on the effective range of the proton beam in comparison to the range based on a generic RBE of 1.1.
  
  
  METHODS
  Factors influencing the biologically effective proton range were systematically analyzed by means of treatment planning studies using the Local Effect Model (LEM IV) and the treatment planning software TRiP98. Special emphasis was put on the comparison of passive and active range modulation techniques.
  
  
  RESULTS
  Beam energy, tissue type, and dose level significantly affected the biological extension of the treatment field at the distal edge. Up to 4 mm increased penetration depth as compared to the depth based on a constant RBE of 1.1. The extension of the biologically effective range strongly depends on the initial proton energy used for the most distal layer of the field and correlates with the width of the distal penumbra. Thus, the range extension, in general, was more pronounced for passive as compared to active range modulation systems, whereas the maximum RBE was higher for active systems.
  
  
  CONCLUSIONS
  The analysis showed that the physical characteristics of the proton beam in terms of the width of the distal penumbra have a great impact on the RBE gradient and thus also the biologically effective penetration depth of the beam.
authors: Department of Biophysics, GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt 64291, Germany; Institute of Medical Physics and Radiation Protection, University of Applied Sciences Gießen, Gießen 35390, Germany; and Medical Faculty of Philipps-University Marburg, Marburg 35032, Germany.
authors: Rebecca
authors: R
authors: Grün
authors: Thomas
authors: T
authors: Friedrich
authors: Michael
authors: M
authors: Krämer
authors: Klemens
authors: K
authors: Zink
authors: Marco
authors: M
authors: Durante
authors: Rita
authors: R
authors: Engenhart-Cabillic
authors: Michael
authors: M
authors: Scholz
mesh_terms: D000818:Animals; D016466:CHO Cells; D045744:Cell Line, Tumor; D006224:Cricetinae; D003412:Cricetulus; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D018499:Linear Energy Transfer; D008297:Male; D009369:Neoplasms / Q000532:radiotherapy*; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D061766:Proton Therapy / Q000295:instrumentation* / Q000379:methods*; D011522:Protons; D011878:Radiotherapy / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted; D012062:Relative Biological Effectiveness; D012984:Software
chemical_list: D011522:Protons