title: A study of the beam-specific interplay effect in proton pencil beam scanning delivery in lung cancer.
abstract: BACKGROUND
  For lung tumors with large motion amplitudes, the use of proton pencil beam scanning (PBS) can produce large dose errors. In this study, we assess under what circumstances PBS can be used to treat lung cancer patients who exhibit large tumor motion, based on the quantification of tumor motion and the dose interplay.
  
  
  METHODS
  PBS plans were optimized on average 4DCT datasets using a beam-specific PTV method for 10 consecutive patients with locally advanced non-small-cell-lung-cancer (NSCLC) treated with proton therapy to 6660/180 cGy. End inhalation (CT0) and end exhalation (CT50) were selected as the two extreme scenarios to acquire the relative stopping power ratio difference (Δrsp) for a respiration cycle. The water equivalent difference (ΔWET) per radiological path was calculated from the surface of patient to the iCTV by integrating the Δrsp of each voxel. The magnitude of motion of voxels within the target follows a quasi-Gaussian distribution. A motion index (MI (>5mm WET)), defined as the percentage of target voxels with an absolute integral ΔWET larger than 5 mm, was adopted as a metric to characterize interplay. To simulate the treatment process, 4D dose was calculated by accumulating the spot dose on the corresponding respiration phase to the reference phase CT50 by deformable image registration based on spot timing and patient breathing phase.
  
  
  RESULTS
  The study indicated that the magnitude of target underdose in a single fraction plan is proportional to the MI (p < .001), with larger motion equating to greater dose degradation and standard deviations. The target homogeneity, minimum, maximum and mean dose in the 4D dose accumulations of 37 fractions varied as a function of MI.
  
  
  CONCLUSIONS
  This study demonstrated that MI can predict the level of dose degradation, which potentially serves as a clinical decision tool to assess whether lung cancer patients are potentially suitable to receive PBS treatment.
authors.affiliation: a Department of Radiation Oncology , University of Pennsylvania , Philadelphia , PA , USA.
authors.forename: Minglei
authors.initials: M
authors.lastname: Kang
authors.affiliation: a Department of Radiation Oncology , University of Pennsylvania , Philadelphia , PA , USA.
authors.forename: Sheng
authors.initials: S
authors.lastname: Huang
authors.affiliation: a Department of Radiation Oncology , University of Pennsylvania , Philadelphia , PA , USA.|b Department of Radiation Oncology , University of California , San Francisco , CA , USA.
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Solberg
authors.affiliation: c Department of Radiation Oncology , Walter Reed Military Hospital , Bethesda , MD , USA.
authors.forename: Rulon
authors.initials: R
authors.lastname: Mayer
authors.affiliation: c Department of Radiation Oncology , Walter Reed Military Hospital , Bethesda , MD , USA.
authors.forename: Andy
authors.initials: A
authors.lastname: Thomas
authors.affiliation: a Department of Radiation Oncology , University of Pennsylvania , Philadelphia , PA , USA.
authors.forename: Boon-Keng Kevin
authors.initials: BK
authors.lastname: Teo
authors.affiliation: a Department of Radiation Oncology , University of Pennsylvania , Philadelphia , PA , USA.
authors.forename: James E
authors.initials: JE
authors.lastname: McDonough
authors.affiliation: a Department of Radiation Oncology , University of Pennsylvania , Philadelphia , PA , USA.|d Department of Radiation Oncology , University of Maryland Medical Center , Baltimore , MD , USA.
authors.forename: Charles B
authors.initials: CB
authors.lastname: Simone
authors.affiliation: a Department of Radiation Oncology , University of Pennsylvania , Philadelphia , PA , USA.
authors.forename: Liyong
authors.initials: L
authors.lastname: Lin
mesh_terms: D016477:Artifacts; D002289:Carcinoma, Non-Small-Cell Lung / Q000532:radiotherapy*; D019583:Dose Fractionation, Radiation; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D008175:Lung Neoplasms / Q000532:radiotherapy*; D009038:Motion; D009068:Movement; D061766:Proton Therapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*