Towards liquid EPR dosimetry using nitroxides in aqueous solution.

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 - 64 Gy were applied with a clinical 6 MV FFF 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 - 160 µM 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 - 10vol%. 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 20 µM, absolute EPR signal loss is linear up to a dose of 16 Gy with a yield G(NO∙) of approximately 0.4 µmol/J. Within five weeks upon sample irradiation to doses between 0 Gy and 32 Gy relative EPR signal fluctuations were on average (126 readouts) below 1% (1σ). For c(MmP) ≥ 20 µM, absolute signal loss is only weakly dependent on c(MmP), whereas it increases strongly with increasing c(DMSO) in the range 0 - 5vol%. 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 (2vol%) 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.