title: A method for evaluation of proton plan robustness towards inter-fractional motion applied to pelvic lymph node irradiation.
abstract: BACKGROUND
  The benefit of proton therapy may be jeopardized by dose deterioration caused by water equivalent path length (WEPL) variations. In this study we introduced a method to evaluate robustness of proton therapy with respect to inter-fractional motion and applied it to irradiation of the pelvic lymph nodes (LNs) from different beam angles. Patient- versus population-specific patterns in dose deterioration were explored.
  
  
  METHODS
  Patient data sets consisting of a planning computed tomography (pCT) as well as multiple repeat CT (rCT) scans for three patients were used, with target volumes and organs at risk (ORs) outlined in all scans. Single beam spot scanning proton plans were optimized for the left and right LN targets separately, across all possible beam angle configurations (5° angle intervals). Isotropic margins of 0, 3, 5 and 7 mm from the clinical target volume (CTV) to the planning target volume (PTV) were investigated. The optimized fluence maps for the pCT for each beam were applied onto all rCTs and the dose distributions were re-calculated. WEPL variation for each beam angle was computed by averaging over beams eye view WEPL distributions.
  
  
  RESULTS
  Similarity in deterioration patterns were found for the investigated patients, with beam angles delivering less dose to rectum, bladder and overall normal tissue identified around 40° and around 150°-160° for the left LNs, and corresponding angles for the right LNs. These angles were also associated with low values of WEPL variation.
  
  
  CONCLUSIONS
  We have established and explored a method to quantify the robustness towards inter-fractional motion of single beam proton plans treating the pelvic LNs from different beam configurations and with different CTV to PTV margins. For the patients investigated we were able to identify beam orientations that were robust to dose deterioration in the target and ORs.
authors.affiliation: a Department of Medical Physics , Aarhus University Hospital/Aarhus University , Aarhus , Denmark.
authors.forename: Andreas G
authors.initials: AG
authors.lastname: Andersen
authors.affiliation: a Department of Medical Physics , Aarhus University Hospital/Aarhus University , Aarhus , Denmark.
authors.forename: Oscar
authors.initials: O
authors.lastname: Casares-Magaz
authors.affiliation: a Department of Medical Physics , Aarhus University Hospital/Aarhus University , Aarhus , Denmark.
authors.forename: Ludvig P
authors.initials: LP
authors.lastname: Muren
authors.affiliation: a Department of Medical Physics , Aarhus University Hospital/Aarhus University , Aarhus , Denmark.
authors.forename: Jakob
authors.initials: J
authors.lastname: Toftegaard
authors.affiliation: b Department of Oncology , Aarhus University Hospital/Aarhus University , Aarhus , Denmark.
authors.forename: Lise
authors.initials: L
authors.lastname: Bentzen
authors.affiliation: c Section of Medical Physics, Haukeland University Hospital , Bergen , Norway.
authors.forename: Sara
authors.initials: S
authors.lastname: Thörnqvist
authors.affiliation: a Department of Medical Physics , Aarhus University Hospital/Aarhus University , Aarhus , Denmark.
authors.forename: Jørgen B B
authors.initials: JB
authors.lastname: Petersen
mesh_terms: D006801:Humans; D008207:Lymphatic Metastasis; D008297:Male; D009038:Motion; D009068:Movement; D058958:Organs at Risk; D010388:Pelvis; D011471:Prostatic Neoplasms / Q000473:pathology* / Q000532:radiotherapy*; D061766:Proton Therapy / Q000379:methods*; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012007:Rectum; D001743:Urinary Bladder