# Characterization of a Modified Clinical Linear Accelerator for Ultra-High Dose Rate Beam Delivery
## 3 Results
### Dosimetry  
The UHDR PDD curves obtained with GaFChromic films show good agreement with the curves obtained with both the AM chamber and films for the conventional 10 MeV electron beam (Figure 7). The maximum difference between R\({}_{50}\) values is 3.6% and only the initial points (near to the surface) show an underdose that could be caused by some air gaps between the PMMA slabs or by the proximity to the film cut [23]. The dosimetry parameters mentioned in Section 2.3 were calculated and are shown in Table 1.  
Figure 6: (**a**) Collected charge per pulse at the crosshair foil position for different PRFs. Each boxplot represents a total of 20 pulses delivered in HP mode. (**b**) Comparison of the charge per pulse at the crosshair foil position in LP and HP modes considering single pulse irradiations repeated 10 times (LP1p and HP1p) and 30 consecutive pulses (LP30p and HP30p. (**c**) Repeatability of the charge per pulse at the crosshair foil position acquired on different days in HP mode.  
The dosimetry values estimated with different detectors are comparable to each other and confirm that the modification of the LINAC did not affect the energy distribution of the beam, even when the applicator was inserted. The uncertainties of the R\({}_{80}\) and R\({}_{50}\) values are at most 3%, while the R\({}_{\text{p}}\) values are characterized by larger uncertainties of 5% since they are calculated as the intersection between two interpolation lines. The R\({}_{50}\) values reported in Table 1 are 7% smaller than the expected value for 10 MeV electrons [24], indicating that the most probable beam energy is slightly less than 10 MeV.  
From the absorbed dose, measured for the four irradiation settings at the reference depth z\({}_{\text{ref}}\), it was possible to extract the DPP, the average dose rate (Equation (2)) and the instantaneous dose rate in the pulse. Results are reported in Table 2.  
As expected from previous studies [25], the comparison between conventional and UHDR modalities indicates an increase of the three dose quantities of at least three orders of magnitude for all the irradiation settings. At the wedge trap position (_iv_) in the UHDR regime, only one pulse was delivered to avoid exceeding the dose range of EBT XD films (40 Gy); thus, it was not possible to calculate the average dose rate for multiple pulses as in the other settings.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mode** & **Setup** & **R\({}_{\text{80}}\) (cm)** & **R\({}_{\text{50}}\) (cm)** & **R\({}_{\text{p}}\) (cm)** & **z\({}_{\text{ref}}\) (cm)** \\ \hline \multirow{4}{*}{
\begin{tabular}{c}  
The use of the PMMA applicator results in an increase in the DPP at the isocenter of 57% and 28% in conventional and UHDR modes, respectively.  
The beam profiles obtained from the scanned images are shown in Figure 8.  
In the conventional mode, it is possible to note few bumps for the wedge setting profile. This can be attributed mainly to the geometry of the primary filters and of the secondary scattering foils, which have two sloped sections corresponding to specific scattering angles [10].  
The study of the output factor in both irradiation modes is reported in Figure 9. In the conventional mode, there is good agreement between the AM chamber and the GaFChromic films. The analysis of the UHDR mode data show a similar trend with the conventional mode data. For fields larger than 10 \(\times\) 10 cm\({}^{2}\), UHDR values are systematically smaller, as reported by [PERSON] et al. [12], but all the differences are inside the error bars.  
Figure 8: Dose profiles obtained from GaFChromic films for the conventional **(a)** and UHDR modes (**b**). In both figures, the dose profiles at the wedge tray (setting _iv_) were divided by a factor of 10 for ease of comparison.  
Figure 9: Output factor measured with the Advanced Markus and GaFChromic films EBT XD in conventional and UHDR modes for different square field sizes.