Characterization of an organic semiconductor diode for dosimetry in radiotherapy

Abstract

The development of novel detectors for dosimetry in advanced radiotherapy modalities requires materials that have a water-equivalent response to ionizing radiation such that characterization of radiation beams can be performed without the need for complex calibration procedures and correction factors. Organic semiconductors are potentially an ideal technology in fabricating devices for dosimetry due to tissue-equivalence, mechanical flexibility and relatively cheap manufacturing cost. The response of a commercial organic photodetector (OPD), coupled to a plastic scintillator, to ionizing radiation from a linear accelerator and orthovoltage x-ray tube has been characterized to assess its potential as a dosimeter for radiotherapy. The radiation hardness of the OPD has also been investigated to demonstrate its longevity for such applications.Methods: Radiation hardness measurements were achieved by observing the response of the OPD to the visible spectrum and 70 keV x-rays after pre-exposure to 40 kGy of ionizing radiation. The response of a pre-irradiated OPD to 6 MV photons from a linear accelerator in reference conditions was compared to a non-irradiated OPD with respect to direct and indirect (RP400 plastic scintillator) detection mechanisms. Dose-rate dependence of the OPD was measured by varying the surface-to-source distance between 90 cm and 300 cm. Energy dependence was characterized from 29.5 keV to 129 2 keV with an x-ray tube. The percentage depth dose (PDD) curves were measured from 0.5 cm to 20 cm and compared to an ionization chamber. Results:The OPD sensitivity to visible light showed substantial degradation of the broad 450nm to 600 nm peak from the donor after irradiation to 40 kGy. After irradiation, the spectral shape has a dominant absorbance peak at 370 nm, as the acceptor better withstood radiation damage. Its response to x-rays stabilized to 30% after 35 kGy, with a 0.5% difference between 770 Gy increments. The OPD exhibited reproducible detection of ionizing radiation when coupled with a scintillator. Indirect detection showed a linear response from 25 cGy to 500 cGy and constant response to dose-rates from 0.31 Gy/pulse to 3.4 x10 -4 Gy/pulse. However, without the scintillator, response increased by 100% at low dose rates.Energy independence between 100 keV and 1.2 MeV advocates their use as a dosimeter without beam correction factors.A dependence on the scintillator thickness used during a comparison of the PDD to the ionizing chamber was identified.A 1 mm thick scintillator coupled with the OPD demonstrated the best agreement of ±3%. Conclusions:The response of OPDs to ionizing radiation has been characterized, showing promising use as a dosimeter when coupled with a plastic scintillator. The mechanisms of charge transport and trapping within organic materials varies for visible and ionizing radiation, due to differing properties for direct and indirect detection mechanisms and observing a substantial decrease of sensitivity to the visible spectrum after 40 kGy. This study proved that OP...

Towards high spatial resolution tissue-equivalent dosimetry for microbeam radiation therapy using organic semiconductors

“…suggested to be the reason for such divergence from the expected results (Posar et al, 2020b). The maximum photocurrent from the OPD for the Al-Al filter was 100Â larger than that of the Cu-Al and Cu-Cu filters.…”

“…At doses of 35 kGy or higher, the variation of any subsequent changes in charge output was observed to be <0.5%, suggesting that the radiation effects stabilize past this point and the detector can be used in such high-dose-rate radiation fields for MRT applications. Our previous work has shown that such exposure to X-rays in the energy range of interest for MRT degrades the absorption spectrum of the acceptor and donor materials of the sensitive volume above wavelengths of 450 nm (Posar et al, 2020b).…”

Electron Beam Induced Tailoring of Electrical Characteristics of Organic Semiconductor Films

“…The flexibility makes organic film based electronic devices superior over its inorganic counter parts. For flexible device applications such as power sources [74,75], displays [76], electronics [77], super capacitors [78], gas sensors [79][80][81], thermoelectric power generators [82,83], organic light emitting diodes [84,85], radiation dosimeters [86][87][88] etc., organic semiconductors are promptly used. The nature of interaction between high energy EB and organic materials depends on the energy as well as dose of the incident EB [7].…”

Polymer Photodetectors for Printable, Flexible, and Fully Tissue Equivalent X‐Ray Detection with Zero‐Bias Operation and Ultrafast Temporal Responses

“…Consequently, their use for medically relevant dosimetry or personal health monitoring requires complex, often unreliable calibrations limiting their applications in radiotherapy or food sterilization. [ 8 ] A new generation of materials for X‐ray sensing must therefore be envisaged, combining a tissue equivalent response with other ideal properties such as mechanical flexibility, low operating power requirements, and real‐time response rates.…”