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@ARTICLE{Hartmann:176969,
      author       = {G. Hartmann$^*$ and P. Andreo and R.-P. Kapsch and K. Zink},
      title        = {{C}ema-based formalism for the determination of absorbed
                      dose for high energy photon beams.},
      journal      = {Medical physics},
      volume       = {48},
      number       = {11},
      issn         = {2473-4209},
      address      = {College Park, Md.},
      publisher    = {AAPM},
      reportid     = {DKFZ-2021-02202},
      pages        = {7461-7475},
      year         = {2021},
      note         = {#EA:E040# /2021 Nov;48(11):7461-7475},
      abstract     = {Determination of absorbed dose is well-established in many
                      dosimetry protocols and considered to be highly reliable
                      using ionization chambers under reference conditions. If
                      dosimetry is performed under other conditions or using other
                      detectors, however, open questions still remain. Such
                      questions frequently refer to appropriate correction
                      factors. A cema-based approach to formulate such correction
                      factors offers a good understanding of the specific response
                      of a detector for dosimetry under various measuring
                      conditions and thus an estimate of pros and cons of its
                      application.Determination of absorbed dose requires the
                      knowledge of the beam quality correction factor kQ,Qo, where
                      Q denotes the quality of a user beam and Q0 is the quality
                      of the radiation used for calibration. In modern Monte Carlo
                      (MC) based methods, kQ,Qo is directly derived from the MC
                      calculated dose conversion factor which is the ratio between
                      the absorbed dose at a point of interest in water and the
                      mean absorbed dose in the sensitive volume of an ion
                      chamber. In this work, absorbed dose is approximated by the
                      fundamental quantity cema. This approximation allows the
                      dose conversion factor to be substituted by the cema
                      conversion factor. Subsequently, this factor is decomposed
                      into a product of cema ratios. They are identified as the
                      stopping power ratio water to the material in the sensitive
                      detector volume, and as the correction factor for the
                      fluence perturbation of the secondary charged particles in
                      the detector cavity caused by the presence of the detector.
                      This correction factor is further decomposed with respect to
                      the perturbation caused by the detector cavity and that
                      caused by external detector properties. The cema based
                      formalism was subsequently tested by MC calculations of the
                      spectral fluence of the secondary charged particles
                      (electrons and positrons) under various conditions.MC
                      calculations demonstrate that considerable fluence
                      perturbation may occur particularly under non reference
                      conditions. Cema based correction factors to be applied in a
                      6 MV beam were obtained for a number of ionization chambers
                      and for three solid state detectors. Feasibility was shown
                      at a field size of 4 cm x 4 cm and of 0.5 cm x 0.5 cm.
                      Values of the cema ratios resulting from the decomposition
                      of the dose conversion factor can be well correlated with
                      detector response. Under the small field conditions, the
                      internal fluence correction factor of ionization chambers is
                      considerably dependent on volume averaging and thus on the
                      shape and size of the cavity volume.The cema approach is
                      particularly useful at non-reference conditions including
                      the use of solid state detectors. Perturbation correction
                      factors can be expressed and evaluated by cema ratios in a
                      comprehensive manner. The cema approach can serve to
                      understand the specific response of a detector for dosimetry
                      to be dependent on (a) radiation quality (b) detector
                      properties, and (c) electron fluence changes caused by the
                      detector. This understanding may also help to decide which
                      detector is best suited for a specific measurement
                      situation. This article is protected by copyright. All
                      rights reserved.},
      keywords     = {Cema (Other) / Dosimetry (Other) / Monte Carlo (Other)},
      cin          = {E040},
      ddc          = {610},
      cid          = {I:(DE-He78)E040-20160331},
      pnm          = {315 - Bildgebung und Radioonkologie (POF4-315)},
      pid          = {G:(DE-HGF)POF4-315},
      typ          = {PUB:(DE-HGF)16},
      pubmed       = {pmid:34613620},
      doi          = {10.1002/mp.15266},
      url          = {https://inrepo02.dkfz.de/record/176969},
}