title: Motion mitigation for lung cancer patients treated with active scanning proton therapy.
abstract: OBJECTIVE
  Motion interplay can affect the tumor dose in scanned proton beam therapy. This study assesses the ability of rescanning and gating to mitigate interplay effects during lung treatments.
  
  
  METHODS
  The treatments of five lung cancer patients [48 Gy(RBE)/4fx] with varying tumor size (21.1-82.3 cm(3)) and motion amplitude (2.9-30.6 mm) were simulated employing 4D Monte Carlo. The authors investigated two spot sizes (σ ∼ 12 and ∼ 3 mm), three rescanning techniques (layered, volumetric, breath-sampled volumetric) and respiratory gating with a 30% duty cycle.
  
  
  RESULTS
  For 4/5 patients, layered rescanning 6/2 times (for the small/large spot size) maintains equivalent uniform dose within the target >98% for a single fraction. Breath sampling the timing of rescanning is ∼ 2 times more effective than the same number of continuous rescans. Volumetric rescanning is sensitive to synchronization effects, which was observed in 3/5 patients, though not for layered rescanning. For the large spot size, rescanning compared favorably with gating in terms of time requirements, i.e., 2x-rescanning is on average a factor ∼ 2.6 faster than gating for this scenario. For the small spot size however, 6x-rescanning takes on average 65% longer compared to gating. Rescanning has no effect on normal lung V20 and mean lung dose (MLD), though it reduces the maximum lung dose by on average 6.9 ± 2.4/16.7 ± 12.2 Gy(RBE) for the large and small spot sizes, respectively. Gating leads to a similar reduction in maximum dose and additionally reduces V20 and MLD. Breath-sampled rescanning is most successful in reducing the maximum dose to the normal lung.
  
  
  CONCLUSIONS
  Both rescanning (2-6 times, depending on the beam size) as well as gating was able to mitigate interplay effects in the target for 4/5 patients studied. Layered rescanning is superior to volumetric rescanning, as the latter suffers from synchronization effects in 3/5 patients studied. Gating minimizes the irradiated volume of normal lung more efficiently, while breath-sampled rescanning is superior in reducing maximum doses to organs at risk.
authors.affiliation: Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114 and Center for Proton Radiotherapy, Paul Scherrer Institute, Villigen-PSI 5232, Switzerland.
authors.forename: Clemens
authors.initials: C
authors.lastname: Grassberger
authors.affiliation: Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114.
authors.forename: Stephen
authors.initials: S
authors.lastname: Dowdell
authors.affiliation: Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114.
authors.forename: Greg
authors.initials: G
authors.lastname: Sharp
authors.affiliation: Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114.
authors.forename: Harald
authors.initials: H
authors.lastname: Paganetti
mesh_terms: D015331:Cohort Studies; D003198:Computer Simulation; D066264:Datasets as Topic; D006801:Humans; D008168:Lung / Q000473:pathology / Q000528:radiation effects; D008175:Lung Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D008954:Models, Biological; D009010:Monte Carlo Method; D009038:Motion; D061766:Proton Therapy / Q000379:methods*; D011879:Radiotherapy Dosage; D012119:Respiration; D013997:Time Factors; D047368:Tumor Burden