ESTRO 2024 - Abstract Book

S3384

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

instantaneous dose-rate within a single delivered pulse become relevant for the complete understanding of the FLASH effect. In this framework, the use of ultra-thin silicon carbide (SiC) membranes, which are recently emerging for their suitability in UHDR applications, for the real time beam monitoring and instantaneous dose rate measurements in FLASH radiotherapy was experimentally investigated.

Material/Methods:

A new configuration of SiC detectors of different active thicknesses (0.2- 20 µm thick) where the bulk substrate is electrochemically removed was recently realized. Such configuration allows to improve the transparency of the sensor and obtain a negligible energy loss and angular distribution spread of the primary beam and has a promising potentiality for the beam monitoring in many applications [1]. The time response of such innovative detectors named “free standing membranes” together with the standard detectors provided with a thick (370 µm) substrate placed behind the active layer, were studied with 9 MeV ultra-high dose rate (UHDR) electron beams accelerated with an innovative research LINAC fully dedicated to preclinical studies. The dose rate independence of the SiC detectors in terms of charge collected per single pulse as a function of the DPP was already demonstrated in previous experimental investigations [1], highlighting their potentiality for the dosimetry in FLASH radiotherapy. In the study here presented, a 3x7 mm2 active area, 10 um thick membrane (figure 1a) was placed at the position of the maximum depth dose distribution of the 9 MeV electron beam, by using a 13 mm thick solid water slab. A final 30 mm diameter applicator was employed to assure a homogeneous transversal dose distribution at the SiC surface. In order to measure the waveform of the single electron pulse, the SiC detector was directly connected to a 50 ohm terminated fast oscilloscope (Picoscope) and to a Keithley 6517A for supplying the bias voltage. Comparisons with one AC- Current Transformer (ACCT) mounted along the Linac and currently considered as the reference monitor system in FLASH radiotherapy [2], have been also performed.

Results:

Figure 1b shows the good agreement in terms of pulse width and temporal shape of the single electron pulse of the normalized current signals measured with the SiC detector and the ACCT and acquired with the same oscilloscope. As shown, both detectors can detect the instantaneous increase of the beam current happening for this measurement at the beginning of the pulse. By applying a dedicated dose calibration previously performed with Radiochromic films and the Flash diamond detector, the signal amplitudes acquired with the oscilloscope were then converted in instantaneous dose rate for pulse widths ranging from 0.2 up to 4 us and DPP ranging from few cGy up to 10 Gy/pulse (figure 1c, 1d).