# Medical Applications of the GEMPix
## 4 Hadron Therapy  
Hadron therapy is an advanced radiation modality for cancer treatment. The inherent advantage of hadron therapy is its inverted depth dose curve--the so-called Bragg curve--that allows for highly conformal treatment plans with large dose gradients. The dose is well confined in depth with a moderate lateral spread [42]. Therefore, detectors for beam dosimetry and quality assurance should offer a spatial resolution of the order of 1 mm or better. For patient-specific treatment plan verification, arrays of ionization chambers in a water phantom are often used [43]. However, the spatial resolution is limited to the size of each ionization chamber, which is currently around 5 mm. The GEMPix potentially offers a much better spatial resolution due to its highly pixelated readout with a pixel pitch of 55 \(\upmu\)m. Therefore, it provides new information such as 2D images and 3D data representation. As a first step, a triple-GEM detector coupled to an earlier, coarser readout was irradiated with protons and carbon ions in air [44]. Lateral beam profiles of the GEMPix for one carbon ion beam energy were found in agreement with those obtained by radiochromic EBT3 films. Then, the GEMPix was placed inside a watertight box and mounted on the 3D positioning system of a water phantom used for QA at CNAO in order to measure the 3D dose distribution. Bragg curves were calculated by normalizing the integrated GEMPix response at certain depth positions to the respective delivered dose measured by the Dose Delivery System (DDS) of CNAO [45]. Promising results obtained with this setup [22] led to the development of an integrated system consisting of an IBA Scanditronix Wellhofer Blue Phantom type 2001 water phantom, a PTW model 34080 reference ionization chamber, the GEMPix, a trigger system and other auxiliary equipment such as the high-voltage supply, and the control and data acquisition software (Figure 8). The system can be set up relatively fast by keeping all equipment on trolleys. It is a stand-alone system as the normalization to the delivered dose is performed with the reference ionization chamber [23; 24]. Measurements were performed with carbon ions (C\({}^{6+}\)) at one of the fixed horizontal beam lines at CNAO, where a synchrotron delivers scanning proton and carbon ion beams to three treatment rooms. The smallest intensity characterized for clinical applications of 2 \(\times\) 10\({}^{6}\) ions per spill and three beam energies (280, 332 and 380 MeV/u resulting in ranges in water of 150, 200 and 250 mm, respectively) were used. 2D images in the plane perpendicular to the beam axis were acquired at different depths in water. From the 2D images, lateral beam profiles were obtained and compared to radiochromic films showing very good agreement. The Bragg curve was calculated from the 2D images by summing over all pixel values in a  
Figure 7: A dose distribution measured with the GEMPix (**left**) is compared to a gafchromic film measurement with a spatial resolution of 160 \(\upmu\)m (**centre**) for a 6 MV radiation therapy field: the relative dose is color-coded. The gamma index distribution (**right**) obtained is less than one for 90.5% of the data points [21]. single image, normalizing this value to the corresponding reference ionization chamber measurement, then averaging this normalized sum value over all measurements at the same depth and finally plotting this number versus depth. Bragg curves obtained with the integrated system were compared to a dedicated FLUKA Monte Carlo simulation [46; 47] and found to match within \(\pm\)15% (Figure 9). Figure 10 shows a 3D data representation of the measured dose. The average value per pixel per position is calculated and a linear interpolation between positions is used. The figure shows the lateral beam spreading, the Bragg peak and the fragmentation tail of the carbon ion. This information cannot be acquired with commercially available QA detectors. Figure 8: Schematic drawing ((**left**), not to scale) and picture (**right**) of the water phantom setup.