title: Motion interplay as a function of patient parameters and spot size in spot scanning proton therapy for lung cancer.
abstract: OBJECTIVE
  To quantify the impact of respiratory motion on the treatment of lung tumors with spot scanning proton therapy.
  
  
  METHODS
  Four-dimensional Monte Carlo simulations were used to assess the interplay effect, which results from relative motion of the tumor and the proton beam, on the dose distribution in the patient. Ten patients with varying tumor sizes (2.6-82.3 cc) and motion amplitudes (3-30 mm) were included in the study. We investigated the impact of the spot size, which varies between proton facilities, and studied single fractions and conventionally fractionated treatments. The following metrics were used in the analysis: minimum/maximum/mean dose, target dose homogeneity, and 2-year local control rate (2y-LC).
  
  
  RESULTS
  Respiratory motion reduces the target dose homogeneity, with the largest effects observed for the highest motion amplitudes. Smaller spot sizes (σ ≈ 3 mm) are inherently more sensitive to motion, decreasing target dose homogeneity on average by a factor 2.8 compared with a larger spot size (σ ≈ 13 mm). Using a smaller spot size to treat a tumor with 30-mm motion amplitude reduces the minimum dose to 44.7% of the prescribed dose, decreasing modeled 2y-LC from 87.0% to 2.7%, assuming a single fraction. Conventional fractionation partly mitigates this reduction, yielding a 2y-LC of 71.6%. For the large spot size, conventional fractionation increases target dose homogeneity and prevents a deterioration of 2y-LC for all patients. No correlation with tumor volume is observed. The effect on the normal lung dose distribution is minimal: observed changes in mean lung dose and lung V20 are <0.6 Gy(RBE) and <1.7%, respectively.
  
  
  CONCLUSIONS
  For the patients in this study, 2y-LC could be preserved in the presence of interplay using a large spot size and conventional fractionation. For treatments using smaller spot sizes and/or in the delivery of single fractions, interplay effects can lead to significant deterioration of the dose distribution and lower 2y-LC.
authors: Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA. Grassberger.Clemens@mgh.harvard.edu
authors: Clemens
authors: C
authors: Grassberger
authors: Stephen
authors: S
authors: Dowdell
authors: Antony
authors: A
authors: Lomax
authors: Greg
authors: G
authors: Sharp
authors: James
authors: J
authors: Shackleford
authors: Noah
authors: N
authors: Choi
authors: Henning
authors: H
authors: Willers
authors: Harald
authors: H
authors: Paganetti
mesh_terms: D019583:Dose Fractionation, Radiation; D006801:Humans; D008168:Lung / Q000503:physiopathology; D008175:Lung Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D009010:Monte Carlo Method; D009068:Movement; D061766:Proton Therapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D012062:Relative Biological Effectiveness; D012119:Respiration; D012189:Retrospective Studies; D013997:Time Factors; D047368:Tumor Burden