# Medical Applications of the GEMPix
## 3 Conventional Radiation Therapy  
The daily QA and treatment plan verification measurements in radiation therapy facilities are typically performed with gafchromic films or a matrix of small ion chambers [39]. The first method shows very good spatial resolution but needs relatively long time for data analysis, while the second has an online data acquisition but comes with dead space (i.e., non-active detection area) in the treatment area. The GEMPix is a detector that could be used for this application offering advantages on both spatial resolution without dead space and online monitoring capabilities. Several measurements have been performed with GEM and GEMPix detectors at the Tor Vergata Radiotherapy Centre in Italy (Figure 6) using two electric field configurations [21]: the GEMPix in a standard configuration with all electric fields on, and the triple GEM operated with zero electric drift field and the first GEM foil as a gamma ray converter. In the latter case, the spatial resolution is expected to be better, as the gap between the first and the second GEM now serving as the drift gap is only 1 mm and, therefore, the charge spread is reduced. The conversion efficiency was certainly very low but still reasonably good to obtain a 2D image in 10 times less time as compared with gafchromic films. Moreover, with the GEMPix used in ToT mode, the measured charge is correlated more precisely to the dose released in the gas. To compare the results obtained with the GEMPix and with gafchromic films quantitatively, the gamma index [40,41] was used, one of the most common quantities for comparison of 2D dose distributions. The gamma index takes into account the dose difference and the distance difference of the two dose distributions to calculate a dimensionless metric for each point. It is commonly accepted that the first must be maximum 3% and the second maximum 3 mm. When the gamma index is less than one, both criteria are satisfied and this condition must be valid for at least 90% of the reference dose points to which a measured dose is compared. Figure 5: Measurement with an \({}^{55}\)Fe source over nine days. The plots show the time evolution of the ToT counts of the 5.9 keV peak from \({}^{55}\)Fe (**top plot**), the ratio temperature/pressure (T/P) (**middle plot**), and the High Voltage (HV, **lower plot**). The ToT counts are within 6000 \(\pm\) 200 if measurements in the heat-up phase of the detector (‘fast rising T’) and after the HV was off are discarded. In case of the first, standard GEMPix configuration with three active GEMs, the gamma index shows the best results with the lower energy spectrum (6 MV, gamma index <1 in 96.6% of the points). For higher energy spectra, the percentage of the gamma index decreases (69 and 40% for 10 and 18 MV spectra, respectively). An analogous result is obtained for an Intensity-Modulated Radiation Therapy (IMRT) field. High energy fields are affected by large halos which have no counterpart on the gafchromic films. A significant optimization of the results has been obtained working in a double-GEM configuration with zero voltage applied to the first GEM foil, i.e., with zero drift field; in this way only two GEMs are used as charge amplifiers. The gamma index percentage improves to better than 90% as shown in Figure 7. Some tests with IMRT fields have shown an excellent agreement, especially when a threshold cut is applied. The GEMPix presents some advantages with respect to the traditional methods used in radiation therapy:  
* It is more sensitive and the gain has been even reduced working with only two GEM foils. The lower limit is represented by the single dose pulse, a limit that gafchromic films cannot reach [5]. The upper limit is the storage bit limit of the read-out register. * It has an optimal linearity with dose rate and this result is confirmed by the currents measured on the GEM electrodes. * Compared to gafchromic films, it does not need a scanning process to read the measured doses. This also means no fading and no UV sensitivity problems.