ETD

• dosimetry
• flash radiotherapy
• minibeam radiotherapy
• plastic scintillator

Radiotherapy is one of the most widely used treatment modalities for cancer. Continuous technological developments have significantly improved the precision of dose delivery and treatment outcomes. Nevertheless, the irradiation of healthy tissues surrounding the target volume remains one of the main limitations of conventional radiotherapy. For this reason, considerable research efforts are currently focused on the development of innovative irradiation techniques capable of widening the therapeutic window.
Among the most promising approaches are FLASH Radiotherapy and MiniBeam Radiotherapy. FLASH Radiotherapy relies on the delivery of radiation at ultra-high dose rates (UHDR), typically exceeding 40 Gy/s, with irradiation times shorter than 100--200 ms. Under these conditions, the so-called FLASH effect has been observed, consisting of a reduction of radiation-induced damage in healthy tissues while maintaining tumor control. The dosimetric characterization of FLASH beams is particularly challenging because conventional detectors may suffer from saturation, recombination, and dose-rate-dependent effects when exposed to such extreme irradiation conditions. As a consequence, detectors employed for FLASH dosimetry must provide a stable and linear response over a wide range of dose-per-pulse and dose-rate values.
MiniBeam Radiotherapy is a spatially fractionated irradiation technique in which the dose is delivered through submillimetric or millimetric beamlets, producing characteristic patterns composed of alternating high-dose regions (peaks) and low-dose regions (valleys). Although the radiobiological mechanisms differ from those involved in FLASH Radiotherapy, experimental studies have shown that spatial fractionation may also enhance healthy tissue sparing while preserving tumor control. The dosimetric characterization of MiniBeam fields requires the accurate measurement of parameters such as the peak-to-valley dose ratio (PVDR), center-to-center distance (CTC), and full width at half maximum (FWHM), thus demanding detectors with high spatial resolution.
In this context, several alternative detectors have been proposed for FLASH and MiniBeam dosimetry, including solid-state detectors, modified ionization chambers, and plastic scintillators. Among these, plastic scintillators represent a particularly promising solution.
They provide real-time signal readout, high spatial resolution, linear response under UHDR irradiation conditions, water-equivalent response, relatively low cost, and can be manufactured in different geometries. In particular, thin scintillator sheets enable two-dimensional dosimetry and the direct reconstruction of dose maps.
In this work, 0.5 mm thick EJ-262 plastic scintillator sheets were characterized under both electron and proton irradiation conditions. Experimental investigations were performed using 9 MeV electron beams at the Centro Pisano FLASH Radiotherapy (CPFR) facility and proton beams at the Trento Proton Therapy Center (TPC).
A common optical detection system was employed throughout the study, consisting of a CMOS camera and a 45 degrees mirror.
Specific modifications were subsequently introduced according to the requirements of the different experimental campaigns.
For the electron-beam investigations, the system was optimized to achieve high spatial resolution. Optical filters were also employed to investigate and reduce the contribution of background luminescence, particularly Cherenkov radiation. MiniBeam patterns were obtained using a tungsten collimator characterized by 1 mm wide slits separated by 2 mm tungsten septa. Measurements were performed both with and without a PMMA block positioned downstream of the scintillator to evaluate backscattering effects. Additional reference measurements were carried out using EJ-212 scintillators and Radiochromic films.
CTC and FWHM, measured with EJ-262, were in good agreement with the collimator geometry, confirming the correct reconstruction of the MiniBeam pattern, although slightly larger FWHM values were observed when the scintillator was coupled to the PMMA block.
Conversely, the PVDR showed a strong dependence on the detector configuration, increasing from 3.1 +- 0.2 for EJ-262 coupled to PMMA to 5.1 +- 0.2 in the absence of the PMMA block. This substantial increase highlights the significant impact of the PMMA backing material on the measured peak-to-valley ratio, likely due to additional optical transport, diffuse light, and backscattering effects.
Regarding the application to proton beams, the energy dependence of the scintillator was investigated using proton beam energies ranging from 70 to 228 MeV. In air, the scintillation signal exhibited a linear dependence on the deposited energy, with no measurable quenching effects observed.
When the scintillator was immersed in water, deposited energies between 0.234 and 1.050 MeV were obtained after accounting for energy losses in the traversed materials. Under these conditions, quenching effects became evident, and a Birks coefficient of 0.012 cm/MeV was extracted, in agreement with values reported in the literature.
The linearity of the scintillator response under UHDR conditions was also investigated by varying the beam current, demonstrating a linear behaviour over the range 5–500 nA.
The scintillator system demonstrated the capability to reconstruct proton depth-dose distributions. The measured distance between the Bragg peaks of the 112 MeV and 148 MeV proton beams was (5.9 +- 0.1) cm, in agreement with the value expected from the beam data provided by the TPC.
Finally, MiniBeam measurements were performed using Radiochromic films as reference dosimeters, and characteristic parameters such as PVDR, CTC and FWHM were evaluated at different solid-water depths.
The PVDR measured with the EJ-262 scintillator decreased from approximately 1.80 +- 0.03 at 0 cm depth to about 1.14 +- 0.03 at 3 cm, reflecting the progressive reduction of the peak-to-valley ratio due to multiple scattering within the medium. In comparison, the Radiochromic film exhibited a PVDR of approximately 3.3 +- 0.1 at 0 cm. This discrepancy can be mainly attributed to the different thicknesses of the two dosimeters: 0.5 mm for the scintillator and approximately 0.2 mm for the Radiochromic film, thus providing a signal integrated over a different thickness. Conversely, the measured CTC and FWHM values were found to be consistent with the collimator geometry and the film measurements.
Overall, the investigated systems exhibited a linear response under UHDR irradiation conditions and proved capable of reconstructing both MiniBeam spatial patterns and proton depth-dose distributions.
These findings support the use of plastic scintillators as promising tools for the dosimetric characterization of emerging radiotherapy techniques such as FLASH and MiniBeam Radiotherapy, while highlighting the importance of optical transport effects, detector geometry, and light-collection conditions in quantitative measurements.