# Characterization of a Modified Clinical Linear Accelerator for Ultra-High Dose Rate Beam Delivery
## 4 Discussion  
In this study, a LINAC Elekta SL 18 MV was upgraded to deliver a 10 MeV electron beam under UHDR conditions, and a characterization of the beam was performed. The main modifications consisted of removing the primary and secondary scattering foils, increasing the gun current and magnetron power and adjusting some parameters related to beam transport. Moreover, a custom PCC was developed to count the pulses delivered by the LINAC and stop the irradiation after the requested number of pulses, due to the unreliability of the IC chamber after the addition of an attenuator. A silicon diode sensor, whose response has already been proven to be linear with the DPP [17], was used to verify the stability of the pulses within the same beam delivery and their repeatability across different beam deliveries and days. A variation of 3.7% in the charge per pulse was measured between single and consecutive pulse deliveries. Different irradiations within the same day showed good reproducibility with a maximum charge variation from the median of 2% for the LP mode and 4% for the HP mode. It is important to note that the verification of the UHDR pulse stability of the LINAC was performed when considering all pulses, including the first one, which, as shown, can be up to 4% smaller than the stable pulses. For this reason, the percentage variation shown in this study was slightly higher than that of [PERSON] et al. [26], who modified a similar Elekta LINAC and reported deviations of less than 3% when the first pulse was excluded from the analysis. Since it is known that standby periods may impact the performance of the LINAC [26], the reproducibility was assessed on different days over a period of three months. The deviation of the charge per pulse measured by the silicon diode was less than 8% from the median value and is deemed acceptable for in vitro radiobiology studies. The beam energy distribution was not affected by the LINAC modifications, as demonstrated by the PDD curves and the related parameters. The DPPs and the dose rates achieved in this study are comparable to those obtained for other modified LINACs [10; 11; 12], providing from a minimum 0.6 Gy/pulse (3.6\(\cdot\)10\({}^{5}\) Gy/s instantaneous dose rate) at the isocenter position (i) to a maximum 27 Gy/pulse (136\(\cdot\)10\({}^{5}\) Gy/s instantaneous dose rate) at the wedge tray position (iv). Average dose rate values over 300 Gy/s were reached at the crosshair foil position (iii) with a PRF of 100 Hz. These values allow planning both radiobiology experiments and test of instrumentation for the comparison of conventional and UHDR regimes, although the limited space available in the wedge irradiation setting needs to be carefully considered to design proper arrangements. Several improvements of the reached performances will be investigated in the next months. The results reported in this study are referred to a PRF of 100 Hz and to LINAC software and hardware modifications corresponding to a maximum of 6 kW power drawn from the magnetron. Considering that the maximum PRF reachable for this LINAC is 400 Hz and that the magnetron power could be increased to 7 kW, further optimization of the delivery parameters will be studied, aiming at reaching higher DPP and average dose rates. Moreover, a correlation study between the signal generated in silicon sensors and the dose delivered in the PMMA (or solid water) phantom, measured with the AM chamber or GaFChromic films, will be performed. This could lead to the design of a dose-based beam monitoring approach, e.g., positioning silicon sensors in the edge of the irradiation field and replacing or complementing the beam delivery control based on the sole counting of the number of beam pulses by the PCC. The characterization of the LINAC beam for different settings reported in this study is preliminary for its future usage in studies of spatial fractionation.