ESTRO 2021 Abstract Book

S1306

ESTRO 2021

the source. Small-size circular fields dosimetric performances were assessed through output factor (OF) measurements carried out with an Elekta Synergy linear accelerator in the 4-12 MeV electron beam energy range. A PTW MP3 motorized water phantom and three different types of detectors were employed, during the same measurement session: a novel PTW microSilicon detector, a PTW diode E and a PTW microDiamond 60019, used in parallel configuration. A PTW plane–parallel ionization chamber Advanced Markus was added only for the 5 cm aperture diameter. For detector centering, two orthogonal profiles were acquired in the 3D water scanner and each detector was positioned at the maximum detector signal point for each energy smallest field size. OFs were obtained as the ratio of the signal at the reference depth for the field of interest to the 10 × 10 cm 2 reference field at SSD of 100 cm with the same number of MU. The reference detector choice was different according to applicator aperture diameter: for 5 cm the ionization chamber was considered, for 2, 3 and 4 cm the average between the diode E and the microDiamond measurements was considered. Results The OF values measured by all dosimeters are shown in the figure. The overall uncertainty in detectors response is about 1.5% for the solid-state dosimeters and 1% for the ionization chamber. OFs deviation from the reference value for each energy and beam diameter are reported in table. OFs measured by the solid- state detectors agree with those measured with the ionization chamber within 1.7% for the 5 cm diameter. To note that the best agreement with the ionization chamber was achieved by the microSilicon, with a maximum deviation of 0.8%. OFs measured by the diode E and the microDiamond well agree with each other: differences are typically within ± 0.5% with a maximum value of 2.4% for 4 MeV and the smallest aperture. All differences between solid-state detectors are still consistent with the measurement uncertainty. For the microSilicon, the OFs agreement with the reference values is always within ±1%, except for the smallest aperture with 4 and 6 MeV energies, where is within 2%.

Conclusion The good agreement, within the stated uncertainties, between the microSilicon OFs measurements and the reference values for all aperture diameters and beam energies confirms the reliability of this detector in OFs assessment for small radiation therapy electron beams collimated by tubular applicators.