title: Hydrogel rectum-prostate spacers mitigate the uncertainties in proton relative biological effectiveness associated with anterior-oblique beams.
abstract: OBJECTIVE
  Anterior-oblique (AO) proton beams can form an attractive option for prostate patients receiving external beam radiotherapy (EBRT) as they avoid the femoral heads. For a cohort with hydrogel prostate-rectum spacers, we asked whether it was possible to generate AO proton plans robust to end-of-range elevations in linear energy transfer (LET) and modeled relative biological effectiveness (RBE). Additionally we considered how rectal spacers influenced planned dose distributions for AO and standard bilateral (SB) proton beams versus intensity-modulated radiotherapy (IMRT).
  
  
  METHODS
  We studied three treatment strategies for 10 patients with rectal spacers: (A) AO proton beams, (B) SB proton beams and (C) IMRT. For strategy (A) dose and LET distributions were simulated (using the TOPAS Monte Carlo platform) and the McNamara model was used to calculate proton RBE as a function of LET, dose per fraction, and photon α/β. All calculations were performed on pretreatment scans: inter- and intra-fractional changes in anatomy/set-up were not considered.
  
  
  RESULTS
  For 9/10 patients, rectal spacers enabled generation of AO proton plans robust to modeled RBE elevations: rectal dose constraints were fulfilled even when the variable RBE model was applied with a conservative α/β = 2 Gy. Amongst a subset of patients the proton rectal doses for the planning target volume plans were remarkably low: for 2/10 SB plans and 4/10 AO plans, ≤10% of the rectum received ≥20 Gy. AO proton plans delivered integral doses a factor of approximately three lower than IMRT and spared the femoral heads almost entirely.
  
  
  CONCLUSIONS
  Typically, rectal spacers enabled the generation of anterior beam proton plans that appeared robust to modeled variation in RBE. However, further analysis of day-to-day robustness would be required prior to a clinical implementation of AO proton beams. Such beams offer almost complete femoral head sparing, but their broader value relative to IMRT and SB protons remains unclear.
authors.affiliation: a Department of Radiation Oncology , Massachusetts General Hospital, Harvard Medical School , Boston , MA , USA.|b Department of Medical Physics and Bioengineering , University College London , London , UK.
authors.forename: Tracy S A
authors.initials: TS
authors.lastname: Underwood
authors.affiliation: a Department of Radiation Oncology , Massachusetts General Hospital, Harvard Medical School , Boston , MA , USA.
authors.forename: Justin C
authors.initials: JC
authors.lastname: Voog
authors.affiliation: a Department of Radiation Oncology , Massachusetts General Hospital, Harvard Medical School , Boston , MA , USA.
authors.forename: Maryam
authors.initials: M
authors.lastname: Moteabbed
authors.affiliation: c ProCure Proton Therapy Center , Somerset , NJ , USA.
authors.forename: Shikui
authors.initials: S
authors.lastname: Tang
authors.affiliation: c ProCure Proton Therapy Center , Somerset , NJ , USA.
authors.forename: Edward
authors.initials: E
authors.lastname: Soffen
authors.affiliation: c ProCure Proton Therapy Center , Somerset , NJ , USA.
authors.forename: Oren
authors.initials: O
authors.lastname: Cahlon
authors.affiliation: a Department of Radiation Oncology , Massachusetts General Hospital, Harvard Medical School , Boston , MA , USA.
authors.forename: Hsiao-Ming
authors.initials: HM
authors.lastname: Lu
authors.affiliation: a Department of Radiation Oncology , Massachusetts General Hospital, Harvard Medical School , Boston , MA , USA.
authors.forename: Anthony L
authors.initials: AL
authors.lastname: Zietman
authors.affiliation: a Department of Radiation Oncology , Massachusetts General Hospital, Harvard Medical School , Boston , MA , USA.
authors.forename: Jason A
authors.initials: JA
authors.lastname: Efstathiou
authors.affiliation: a Department of Radiation Oncology , Massachusetts General Hospital, Harvard Medical School , Boston , MA , USA.
authors.forename: Harald
authors.initials: H
authors.lastname: Paganetti
mesh_terms: D006801:Humans; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate; D008297:Male; D058958:Organs at Risk; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D061766:Proton Therapy / Q000295:instrumentation* / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012007:Rectum; D012062:Relative Biological Effectiveness
chemical_list: D020136:Hydrogel, Polyethylene Glycol Dimethacrylate