title: Spot Weight Adaptation for Moving Target in Spot Scanning Proton Therapy.
abstract: OBJECTIVE
  This study describes a real-time spot weight adaptation method in spot-scanning proton therapy for moving target or moving patient, so that the resultant dose distribution closely matches the planned dose distribution.
  
  
  METHODS
  The method proposed in this study adapts the weight (MU) of the delivering pencil beam to that of the target spot; it will actually hit during patient/target motion. The target spot that a certain delivering pencil beam may hit relies on patient monitoring and/or motion modeling using four-dimensional (4D) CT. After the adapted delivery, the required total weight [Monitor Unit (MU)] for this target spot is then subtracted from the planned value. With continuous patient motion and continuous spot scanning, the planned doses to all target spots will eventually be all fulfilled. In a proof-of-principle test, a lung case was presented with realistic temporal and motion parameters; the resultant dose distribution using spot weight adaptation was compared to that without using this method. The impact of the real-time patient/target position tracking or prediction was also investigated.
  
  
  RESULTS
  For moderate motion (i.e., mean amplitude 0.5 cm), D95% to the planning target volume (PTV) was only 81.5% of the prescription (RX) dose; with spot weight adaptation PTV D95% achieves 97.7% RX. For large motion amplitude (i.e., 1.5 cm), without spot weight adaptation PTV D95% is only 42.9% of RX; with spot weight adaptation, PTV D95% achieves 97.7% RX. Larger errors in patient/target position tracking or prediction led to worse final target coverage; an error of 3 mm or smaller in patient/target position tracking is preferred.
  
  
  CONCLUSIONS
  The proposed spot weight adaptation method was able to deliver the planned dose distribution and maintain target coverage when patient motion was involved. The successful implementation of this method would rely on accurate monitoring or prediction of patient/target motion.
authors.affiliation: Laboratoire Informatique Gaspard Monge (LIGM), UMR CNRS 8049, Université Paris-Est , Paris , France.
authors.forename: Paul
authors.initials: P
authors.lastname: Morel
authors.affiliation: Department of Radiation Oncology, The University of Iowa , Iowa City, IA , USA ; Department of Electrical and Computer Engineering, The University of Iowa , Iowa City, IA , USA.
authors.forename: Xiaodong
authors.initials: X
authors.lastname: Wu
authors.affiliation: Laboratoire Informatique Gaspard Monge (LIGM), UMR CNRS 8049, Université Paris-Est , Paris , France ; UMR 5800, Laboratoire Bordelais de Recherche en Informatique (LaBRI), Université de Bordeaux , Talence , France.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Blin
authors.affiliation: Laboratoire Informatique Gaspard Monge (LIGM), UMR CNRS 8049, Université Paris-Est , Paris , France.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Vialette
authors.affiliation: Department of Radiation Oncology, The University of Iowa , Iowa City, IA , USA.
authors.forename: Ryan
authors.initials: R
authors.lastname: Flynn
authors.affiliation: Department of Radiation Oncology, The University of Iowa , Iowa City, IA , USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Hyer
authors.affiliation: Department of Radiation Oncology, The University of Iowa , Iowa City, IA , USA.
authors.forename: Dongxu
authors.initials: D
authors.lastname: Wang
keywords: intensity-modulated proton therapy; interplay effect; motion; spot scanning; spot weight adaptation