The influence of the geometry of the irradiated optical fibre on the Cherenkov radiation during small-field dosimetry using Exradin W2 scintillators

Abstract Stereotactic Body Radiation Therapy (SBRT) and Stereotactic Radiosurgery (SRS) pose significant dosimetric challenges due to their small field sizes and high fractional doses. Plastic scintillation detectors (PSDs) offer promising potential as non-reference dosimeters, owing to their water equivalency, high spatial resolution, and linear response. However, accurate dose measurement with PSDs is complicated by Cherenkov radiation emitted during irradiation, which can interfere with scintillation signals and require correction. The most widely used method for such correction is the spectral method as described by Frelin et al., which is also recommended by the manufacturer for the Exradin W2 PSD. The study evaluates the effectiveness of the spectral method for PSD calibration under varying conditions, specifically addressing inconsistencies observed in the manufacturer’s calibration bracket that failed to align with composite data. Two modified brackets were developed, enabling calibration of length and orientation to assess the influence of fibre length on both calibration factors and the background-to-signal (B/G) ratio. Contrary to Frelin’s findings, we observed variability in the B/G signal ratio with changes in fibre length and orientation, suggesting a need to further refine calibration methods. Under selected calibration conditions, the spectral correction method implemented in the W2 system provided output factors close to the reference values. However, the results indicate that the accuracy of stem-signal correction is sensitive to the length and orientation of the irradiated optical fibre relative to the beam direction. Therefore, this study highlights the need to carefully control and report the calibration geometry when applying this method in small-field dosimetry. This experimental study suggests that a theoretical approach may further elucidate the relationships among fibre length, B/G signal ratio, and calibration accuracy, potentially leading to more robust correction protocols for SBRT and SRS dosimetry.