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Particle therapy

Particle therapy

Organic thin-film devices show promise as proton dosimeters

26 May 2021
Organic dosimeter
Schematic of an organic semiconductor-based device for direct detection of 5 MeV protons. The co-planar sensor is fabricated on a flexible plastic substrate by thermal evaporation of two interdigitated gold electrodes and the deposition from solution of an organic thin film of TIPGe-Pn. (Courtesy: Ilaria Fratelli)

Researchers in Italy have developed organic detectors that can quantitatively and reliably measure proton radiation dose, both in real time and integration mode. The organic devices, based on semiconductor thin films, demonstrated direct detection of 5 MeV protons. The team suggest that this new class of material has the potential to create flexible, portable and tissue-equivalent proton detectors for use in applications such as proton therapy.

Organic semiconductors have been demonstrated previously to be reliable detectors of ionizing radiation; but the multi-institutional team, led by Beatrice Fraboni of the University of Bologna, believes that this is the first study to evaluate the detector’s responsivity to proton beams.

Organic detection devices have unique advantageous features for developing flexible, large-area, direct proton dosimeters, according to the researchers. Organic semiconductors can be deposited from solution using low-cost techniques that are easily scalable onto large areas. Low-temperature fabrication processes (below 180°C) allow for fabrication of flexible devices onto plastic substrates. The devices operate at very low bias (less than 1 V), are portable and wearable. Finally, their composition and density make them human-tissue equivalent in terms of proton absorption. Thus they can be employed as medical dosimeters without requiring complex calibration procedures.

The detectors fabricated by the team have a photoconductor structure in which the active semiconducting layer is an organic thin film of microcrystalline TIPGe-Pn. This 150 nm film is deposited from solution onto two interdigitated gold electrodes on a plastic substrate, which ensures the mechanical flexibility of the system.

The researchers tested the detector’s response to proton irradiation, both in real time and in integration mode, reporting their findings in Science Advances. They irradiated the detectors using a 5 MeV proton beam from the 3 MV Tandetron accelerator at the LABEC ion beam centre in Firenze.

The best sensitivity obtained by the detectors was 5.15±0.13 pC/Gy, with a calculated limit of detection of down to 30±6 cGy/s. The sensors demonstrated a stable and reproducible response to proton beams with fluences between 3.5 x109 and 8.7×1011 protons/cm2, and maintained a linear response up to a total dose of 28 kGy.

The researchers note that while the energy of therapeutic beams is commonly above 70 MeV, the proton energy tested in this work is similar to end-of-range values, in particular the energies of scattered protons reaching internal tissues surrounding the target. The detector could therefore be employed to monitor dose to healthy tissues during treatments, such as the dose delivered to the rectal wall during proton therapy of prostate cancer, for example. The organic sensors could fit into a rectal balloon to help ensure the safety of surrounding healthy tissues.

Another potential application is as a medical dosimeter to measure radiation absorbed by astronauts during long-duration space missions.

“Our work demonstrates the possibility to operate simultaneously the same sensor in real-time mode and in integration mode, exploiting the interface coupling of the organic semiconductor with the plastic substrate,” first author Ilaria Fratelli tells Physics World. “While the energy released by the proton beam in the organic semiconductor is registered by an instantaneous increase of current, the energy released in the plastic substrate generates an accumulation of trapped charges, which induces an increase of the device conductivity proportional to the total radiation dose absorbed by the system.”

The team is now planning further tests in proton therapy centres to investigate the effect of higher energy proton beams. “We are also continuing our research through two parallel pathways,” says Fratelli. “We are studying the fundamental mechanism of interaction between protons and the materials forming the device, to reach full control and optimization of the sensing system. And we are working on the geometry and architecture of the device, on the interface effects that rule the integrative response of the detector, and on the semiconducting material employed as the real-time active layer.”

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