title: Effect of Anatomic Changes on Pencil Beam Scanned Proton Dose Distributions for Cranial and Extracranial Tumors.
abstract: To estimate the frequency and impact of anatomic changes on the delivered dose in pencil beam scanning proton therapy, to assess the need for repeat CT scanning and adaptive replanning.
  
  
  
  A total of 730 patients treated at Paul Scherrer Institut between 2007 and 2014 were included in this study, for which the number of patients who had control CT scans and who were replanned as a result of anatomic changes was analyzed. For those that were replanned, the nominal dose distributions (originally optimized on the planning CT scan) were recalculated on the replanning CT scan and differences evaluated using standard dose metrics for planning target volumes and clinical target volumes and organs at risk (OARs).
  
  
  
  Control CT studies were acquired for 244 patients (33.5%), and replanning was deemed clinically necessary for 40 (16%) of these (5.5% of the total cohort). The OARs and target dose differences between the nominal and recalculated dose distributions were found to be strongly dependent on the subgroup of patients. Nevertheless, dose differences were found to be ≤ 5% for 88% of all analyzed OARs, and planning target volume/clinical target volume V95% was reduced by ≤5% in 87%/90% of cases.
  
  
  
  Despite anatomic variations, clinically delivered plans have been found to be robust to anatomic changes, with replanning being deemed necessary in only a small number of cases. However, because the dosimetric effect of such changes can be quite large for some cases, they have to be monitored and evaluated on an individual basis.
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, Villigen, Switzerland. Electronic address: lorenzo.placidi@psi.ch.
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Placidi
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, Villigen, Switzerland.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Bolsi
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, Villigen, Switzerland; ETH Zurich, Zurich, Switzerland.
authors.forename: Antony J
authors.initials: AJ
authors.lastname: Lomax
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, Villigen, Switzerland.
authors.forename: Ralf A
authors.initials: RA
authors.lastname: Schneider
authors.affiliation: Radiation Oncology Department, University of Maryland Medical Center, Baltimore, Maryland.
authors.forename: Robert
authors.initials: R
authors.lastname: Malyapa
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, Villigen, Switzerland; Radiation Oncology Department, University Hospital of Zurich, Zurich, Switzerland; Radiation Oncology Department, University Hospital of Bern, Bern, Switzerland.
authors.forename: Damien C
authors.initials: DC
authors.lastname: Weber
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, Villigen, Switzerland.
authors.forename: Francesca
authors.initials: F
authors.lastname: Albertini
mesh_terms: D001932:Brain Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology* / Q000532:radiotherapy*; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology* / Q000532:radiotherapy*; D006801:Humans; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000528:radiation effects; D061766:Proton Therapy / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods* / Q000706:statistics & numerical data; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed