###### Abstract  
High-energy small electron beams, generated by linear accelerators, are used for radiotherapy of localized superficial tumours. The aim of the present study is to assess the dosimetric performance under small radiation therapy electron beams of the novel PTW microSilicon detector compared to other available dosimeters. Relative dose measurements of circular fields with 20, 30, 40, and 50 mm aperture diameters were performed for electron beams generated by an Elekta Synergy linac, with energy between 4 and 12 MeV. Percentage depth dose, transverse profiles, and output factors, normalized to the 10 \(\times\) 10 cm\({}^{2}\) reference field, were measured. All dosimetric data were collected in a PTW MP3 motorized water phantom, at SSD of 100 cm, by using the novel PTW microSilicon detector. The PTW diode E and the PTW microDiamond were also used in all beam apertures for benchmarking. Data for the biggest field size were also measured by the PTW Advanced Markus ionization chamber. Measurements performed by the microSilicon are in good agreement with the reference values for all the tubular applicators and beam energies within the stated uncertainties. This confirms the reliability of the microSilicon detector for relative dosimetry of small radiation therapy electron beams collimated by circular applicators. r 2022 20220111 December 20215 January  
However, the dosimetry of small and/or irregular electron fields is challenging. Electron beam side deflections, due to multiple Coulomb scattering, occur in small circular collimators, resulting in reduced output and, therefore, to a lack of lateral-scatter equilibrium. This fact plays an important role in small electron beams dosimetry: the depth of maximum dose moves toward the surface, the central axis percentage depth dose decreases, and the output dose/monitor unit decreases when electron field sizes are smaller than or comparable to the radius required for lateral scattering equilibrium [1; 2]. Moreover, the size of the detector, with respect to the field size, has to be considered. The detector volume averaging effect, which is dependent on detector size, affects small electron beam dose measurements as well as in narrow photon beams. Therefore, a detector with high spatial resolution capability and high sensitivity is required, which is also tissue equivalent. Previous studies have largely investigated various parameters such as detector type and volume that affect the dosimetry of photon beams in small fields, leading to the recommendation of the TRS483 on small field dosimetry [3]. However, little attention has been given in literature to small electron beams [2; 4; 5]. Plane-parallel ionization chambers may be not adequate for small electron beam dosimetry because of their too large collecting electrode and consequent beam non-uniformity over the chamber area. Both silicon diodes and diamond dosimeters were found to be suitable to perform relative electron dosimetry [1; 6; 7], including the Output Factors (OFs) determination. However, only a couple of studies reported dosimetric data for beams shaped by tubular applicators [4; 5]. The novel PTW (PTW-Freiburg, Germany) microSilicon detector may be a suitable detector for measurements in high-energy small-field electron beams. The performance of this small-volume detector, for dosimetry in small photon beams, was already studied, showing very good dosimetric properties [8; 9; 10]. In addition, a three-dimensional characterization of the active volumes of the PTW microSilicon has been performed for accurate Monte Carlo simulations of the detector [11]. Recently, this detector was characterized for electron beam dosimetry of the reference 10 cm \(\times\) 10 cm field [12]. However, to our knowledge, the dosimetric performances of the novel PTW microSilicon detector, under small electron beams, have not been assessed.