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@ARTICLE{Behrends:186674,
      author       = {C. Behrends and C. Bäumer$^*$ and N. Verbeek and J. Wulff
                      and B. Timmermann$^*$},
      title        = {{O}ptimization of proton pencil beam positioning in
                      collimated fields.},
      journal      = {Medical physics},
      volume       = {50},
      number       = {4},
      issn         = {0094-2405},
      address      = {College Park, Md.},
      publisher    = {AAPM},
      reportid     = {DKFZ-2023-00043},
      pages        = {2540-2551},
      year         = {2023},
      note         = {2023 Apr;50(4):2540-2551},
      abstract     = {The addition of static or dynamic collimator systems to the
                      pencil beam scanning delivery technique increases the number
                      of options for lateral field shaping. The collimator shape
                      needs to be optimized together with the intensity modulation
                      of spots.To minimize the proton field's lateral penumbra by
                      investigating the fundamental relations between spot and
                      collimating aperture edge position.Analytical approaches
                      describing the effect of spot position on the resulting spot
                      profile are presented. The theoretical description is then
                      compared with Monte Carlo simulations in TOPAS and in the
                      RayStation treatment planning system, as well as with
                      radiochromic film measurements at a clinical proton therapy
                      facility. In the model, one single spot profile is analyzed
                      for various spot positions in air. Further, irradiation
                      setups in water with different energies, the combination
                      with a range shifter, and two-dimensional proton fields were
                      investigated in silico.The further the single spot is placed
                      beyond the collimating aperture edge ('overscanning'), the
                      sharper the relative lateral dose fall-off and thus the
                      lateral penumbra. Overscanning up to 5 mm reduced the
                      lateral penumbra by about $20\%$ on average after a
                      propagation of 13 cm in air. This benefit from overscanning
                      is first predicted by the analytical proofs and later
                      verified by simulations and measurements. Corresponding
                      analyses in water confirm the benefit in lateral penumbra
                      with spot position optimization as observed theoretically
                      and in air. The combination of spot overscanning with
                      fluence modulation facilitated an additional improvement.The
                      lateral penumbra of single spots in collimated scanned
                      proton fields can be improved by the method of spot
                      overscanning. This suggests a better sparing of proximal
                      organs at risk in smaller water depths at higher energies,
                      especially in the plateau of the depth dose distribution.
                      All in all, spot overscanning in collimated scanned proton
                      fields offers particular potential in combination with
                      techniques such as fluence modulation or dynamic collimation
                      for optimizing the lateral penumbra to spare normal tissue.
                      This article is protected by copyright. All rights
                      reserved.},
      keywords     = {Monte Carlo (Other) / lateral penumbra (Other) / pencil
                      beam scanning with aperture (Other) / proton therapy (Other)
                      / radiobiology (Other)},
      cin          = {ED01},
      ddc          = {610},
      cid          = {I:(DE-He78)ED01-20160331},
      pnm          = {899 - ohne Topic (POF4-899)},
      pid          = {G:(DE-HGF)POF4-899},
      typ          = {PUB:(DE-HGF)16},
      pubmed       = {pmid:36609847},
      doi          = {10.1002/mp.16209},
      url          = {https://inrepo02.dkfz.de/record/186674},
}