title: Disregarding RBE variation in treatment plan comparison may lead to bias in favor of proton plans.
abstract: OBJECTIVE
  Currently in proton radiation therapy, a constant relative biological effectiveness (RBE) equal to 1.1 is assumed. The purpose of this study is to evaluate the impact of disregarding variations in RBE on the comparison of proton and photon treatment plans.
  
  
  METHODS
  Intensity modulated treatment plans using photons and protons were created for three brain tumor cases with the target situated close to organs at risk. The proton plans were optimized assuming a standard RBE equal to 1.1, and the resulting linear energy transfer (LET) distribution for the plans was calculated. In the plan evaluation, the effect of a variable RBE was studied. The RBE model used considers the RBE variation with dose, LET, and the tissue specific parameter α/β of photons. The plan comparison was based on dose distributions, DVHs and normal tissue complication probabilities (NTCPs).
  
  
  RESULTS
  Under the assumption of RBE=1.1, higher doses to the tumor and lower doses to the normal tissues were obtained for the proton plans compared to the photon plans. In contrast, when accounting for RBE variations, the comparison showed lower doses to the tumor and hot spots in organs at risk in the proton plans. These hot spots resulted in higher estimated NTCPs in the proton plans compared to the photon plans.
  
  
  CONCLUSIONS
  Disregarding RBE variations might lead to suboptimal proton plans giving lower effect in the tumor and higher effect in normal tissues than expected. For cases where the target is situated close to structures sensitive to hot spot doses, this trend may lead to bias in favor of proton plans in treatment plan comparisons.
authors.affiliation: Medical Radiation Physics, Department of Oncology-Pathology, Karolinska Institutet, Stockholm 17176, Sweden and RaySearch Laboratories, Stockholm 11134, Sweden.
authors.forename: Minna
authors.initials: M
authors.lastname: Wedenberg
authors.affiliation: Medical Radiation Physics, Department of Oncology-Pathology, Stockholm University and Karolinska Institutet, Stockholm 17176, Sweden.
authors.forename: Iuliana
authors.initials: I
authors.lastname: Toma-Dasu
mesh_terms: D001932:Brain Neoplasms / Q000532:radiotherapy; D001933:Brain Stem / Q000528:radiation effects; D006801:Humans; D018499:Linear Energy Transfer; D008954:Models, Biological; D009010:Monte Carlo Method; D009900:Optic Nerve / Q000528:radiation effects; D017785:Photons / Q000627:therapeutic use*; D061766:Proton Therapy / Q000379:methods*; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*