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@ARTICLE{Hfel:288730,
      author       = {S. Höfel and F. Zwicker$^*$ and M. K. Fix and M. Drescher},
      title        = {{T}owards liquid {EPR} dosimetry using nitroxides in
                      aqueous solution.},
      journal      = {Physics in medicine and biology},
      volume       = {69},
      number       = {5},
      issn         = {0031-9155},
      address      = {Bristol},
      publisher    = {IOP Publ.},
      reportid     = {DKFZ-2024-00439},
      pages        = {055026},
      year         = {2024},
      abstract     = {Objective. Water-equivalent dosimeters are desirable for
                      dosimetry in radiotherapy. The present work investigates
                      basic characteristics of novel aqueous detector materials
                      and presents a signal loss approach for electron
                      paramagnetic resonance (EPR) dosimetry.Approach. The
                      proposed principle is based on the radiation dose dependent
                      annihilation of EPR active nitroxides (NO·) in aqueous
                      solutions. Stable nitroxide radicals
                      (3-Maleimido-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (MmP),
                      3-Carbamoyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (CmP)) in
                      aqueous solutions containing dimethyl sulfoxide (DMSO) as an
                      additive were filled in glass capillaries for irradiation
                      and EPR readout. Radiation doses ranging from 1 to 64 Gy
                      were applied with a clinical 6 MV flattening filter free
                      photon beam. EPR readout was then performed with a X-band
                      benchtop spectrometer. The dose response, temporal stability
                      and reproducibility of the samples' EPR signal amplitudes as
                      well as the influence of the nitroxide concentration between
                      10 and 160μM on the absolute signal loss were investigated
                      using MmP. CmP was used to examine the dependence of the
                      dose response on DMSO concentration between 0 and 10
                      $vol\%.$ An indirect effect model was fitted to the
                      experimental data assuming irradiation induced radical
                      reactions as the underlying mechanism.Main results. For an
                      initial MmP concentration of 20μM, absolute EPR signal loss
                      is linear up to a dose of 16 Gy with a yield G(-NO·) of
                      approximately 0.4μmol J-1. Within five weeks upon sample
                      irradiation to doses between 0 and 32 Gy relative EPR signal
                      fluctuations were on average (126 readouts) below $1\%$
                      (1σ). For c(MmP) ≥ 20μM, absolute signal loss is only
                      weakly dependent on c(MmP), whereas it increases strongly
                      with increasing c(DMSO) in the range 0-5 $vol\%.$ An
                      indirect effect model is applicable to describe the reaction
                      mechanism resulting in the observed dose response
                      curve.Significance. Liquids consisting of nitroxides in
                      aqueous solution and small amounts of DMSO (2 $vol\%)$ show
                      promising basic characteristics for application as
                      water-equivalent EPR dosimeter materials in radiotherapy.
                      The EPR signal loss is based on an indirect effect mediated
                      by diffusing radicals originating from the radiolysis of the
                      water/DMSO mixture.},
      keywords     = {Electron Spin Resonance Spectroscopy: methods /
                      Reproducibility of Results / Dimethyl Sulfoxide /
                      Radiometry: methods / Water / Nitrogen Oxides / EPR
                      dosimetry (Other) / dimethyl sulfoxide (Other) / nitroxide
                      (Other) / radiotherapy (Other) / water equivalence (Other) /
                      nitroxyl (NLM Chemicals) / Dimethyl Sulfoxide (NLM
                      Chemicals) / Water (NLM Chemicals) / Nitrogen Oxides (NLM
                      Chemicals)},
      cin          = {E055},
      ddc          = {530},
      cid          = {I:(DE-He78)E055-20160331},
      pnm          = {315 - Bildgebung und Radioonkologie (POF4-315)},
      pid          = {G:(DE-HGF)POF4-315},
      typ          = {PUB:(DE-HGF)16},
      pubmed       = {pmid:38306975},
      doi          = {10.1088/1361-6560/ad25c4},
      url          = {https://inrepo02.dkfz.de/record/288730},
}