Exploiting the peculiar features of organic semiconductors to target specific applications is the root to envisage novel functionalities of this class of devices, with the aim to fill the gap left by technologies based on inorganic materials. This is the case of ionizing radiation detectors, where the requirements of large-area, conformability and portability, lightweight, and low-power operation could be fulfilled by the development of all-organic detecting systems, paving the way for actual placement on the market as alternative and complementary devices with respect of the currently available detectors.
In our article1 we propose a novel fully organic detector, designed to target real-time and in-situ dose monitoring during proton therapy and characterized by mechanical flexibility and low power operation, assessing its potential employment as a personal dosimeter with high comfort and low risk for the patient. Charged particle therapy is in fact at the cutting-edge of the medical treatments for fighting cancer, as testified by the numerous particle irradiation facilities for clinical therapy spreading in Europe. The clinical advantage of charged particle therapy with respect to conventional photon-based treatments, where the delivered dose of radiation decreases exponentially with depth, is the capability to control the depth-dose distribution. In fact, the depth where the charged particles release their peak of energy (Bragg peak) can be tuned and focussed on the selected cancer volume and its immediate surroundings, avoiding the radiation spreading to healthy tissues. The main challenge in particle therapy thus is to deliver a prescribed radiation dose (energy per unit of mass) to the tumour to be treated while limiting the dose delivered to surrounding healthy organs and tissues. In particular, the sparing of organs at risk is a crucial indication in the planning strategy, and dose on these structures should be minimized and computed with utmost precision. Therefore, there is an increasing demand of systems capable of recording and mapping the dose delivered during a treatment plan. Commercially available personal dosimeters and diagnostic detectors, based on inorganic materials (e.g. Silicon based solid-state devices for dosimeters, a-Si, a-Se, or poly-CZT for large-area flat panels) are difficult to grow in large, pixelated matrices with limited costs and by means of easy and low temperature fabrication techniques. Furthermore, they are heavy, bulky, rigid and uncomfortable to be worn or placed in the proximity of the irradiated region for real-time in-situ dosimetry.
The motivation of our work was therefore to develop a novel full-organic detector to be assessed for the challenging application of the in-situ and real-time monitoring of the radiation dose impinging onto healthy tissues during a session of proton therapy, e.g., in prostate tumour treatment.
The innovative detectors we propose have been conceived and realized in the framework of the FIRE (Flexible organic Ionizing Radiation dEtectors) project, funded by the 5th Scientific Committee (CSN5) of the Italian National Institute for Nuclear Physics (INFN). The work involved 6 different sections of such institution (Bologna, Roma Tre, Napoli, Padova, Florence, Legnaro National Laboratories), 6 Italian Universities (Bologna, Roma Tre, Napoli, Trento, Padova), the Italian National Research Council (CNR) and the Trento Institute for Fundamental Physics and Applications (INFN-TIFPA), demonstrating the broad interest for the development of organic technology for ionizing radiation detection in different fields, spanning from experimental nuclear physics to material science and medical physics.
The bendable fully-organic detector operates in the indirect mode, by coupling a polysiloxane-based flexible scintillator to a flexible organic phototransistor (OPT) detecting the scintillation light during the irradiation (Fig.1A and Fig.1B).
Polysiloxane-based scintillating films have been produced with an emission wavelength matching the optical absorption of the OPT active layer. The scintillator also presents stable mechanical and optical properties, since its response under proton exposure does not show degradation of light yield or any other evident radiation damage effects. The OPTs are based on dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) as the active layer and were fabricated on 100 μm thick polyethylene-naphthalate (PEN) foil, which allows combining flexibility with a carrier-free large-area compatible process. Transistors exhibit high field-effect mobility, μ = (1.1 ± 0.2) cm2 V-1 s-1, and high stability under bias stress. They were proved to be suitable for flexible applications, being their response unaffected by repeated bending cycles. The encapsulation layer efficiently protects the active layer from the environment and aging, while granting an efficient coupling with the scintillator.
The full device shows response stability and reproducibility under proton irradiation at 5 MeV, the typical energy of particles scattered in proximity of the prostate during a proton therapy session, as defined in accordance with Monte Carlo simulation (Fig.1C and Fig.1D ). As shown in Fig. 1E the detector response trend is unaffected by bending stress and the proton-induced photocurrent shows only a slight increase in bent devices. The detector operates at a very low bias voltage (1 V), hence it is compliant with basic safety constraints for electrical hazards and with a low power supply. Fig. 1F reports the variation of the detector response obtained mimicking a typical operating condition during a proton therapy session. The results show that the device response is reliable and stable until the end of the session and beyond.
Further, we present a kinetic model able to precisely reproduce the dynamic response of the device under proton irradiation and to provide further insight into the physical processes underlying its behaviour. The response in the operating range of the detector, the rise and fall dynamics, and the progressive build-up of a persistent photocurrent are correctly reproduced by the proposed model. In particular, two components in the response under proton flux were identified (a swift and a persistent one) and ascribed to two kinds of defects with different mean distributions of the recovery activation energies. Both experimentally and computational analyses confirmed that the fast response is recurring independently from the persistent current drift, thus assessing the suitability of the here proposed devices as a real-time proton detector. Finally, the quantification of the two components in the detector response can be exploited to operate in two modes simultaneously, as recently proposed for direct organic proton detectors with a similar behaviour2: (i) the real-time monitoring of proton irradiation and (ii) the monitoring of the total received dose.
In conclusion, this work demonstrates the potential of fully organic thin-film flexible devices for a variety of applications within the proton detection field, from experimental scientific research to innovative theranostics.
- Calvi, S. et al. Flexible fully organic indirect detector for megaelectronvolts proton beams. npj Flex Electron 7, 1–11 (2023).
- Fratelli, I. et al. Direct detection of 5-MeV protons by flexible organic thin-film devices. Science Advances 7, eabf4462 (2021).
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