# Radiochromic Films for the Two-Dimensional Dose Distribution Assessment
## 1 Introduction  
Accurate knowledge of the characteristics of the radiation field is crucial in medical applications of particle physics. As an example, medical cyclotrons, either for hadron-therapy or for radioisotope production, are equipped with beam instrumentation and detection systems to assure effective cancer treatments, as well as functional and safe production. Ionization chambers and Faraday Cups are usually installed in beam lines to measure the beam current. Beam profiles are measured by means of beam viewers, plastic scintillators, optical fiber-based detectors, scintillation screens, and secondary electron emission detectors [1; 2; 3; 4; 5; 6; 7]. High resolution two-dimensional measurements of the dose distribution can be achieved with radiochromic films (RCFs). These instruments are mainly used in the dose verification and quality assurance in radiation therapy with photon, electron and proton beams [8; 9; 10; 11]. The remarkable interest of the scientific community to RCF dosimetry is confirmed by the high number of publications on this topic. By the end of 2015, nearly 1000 papers on RCFs have been published on peer-reviewed journals with a growing trend [12]. Several comprehensive works, including reviews and protocols, as well as a book on RCF dosimetry, have been published so far [12; 13; 14; 15; 16; 17; 18]. Such a wide use is due to the number of their unique features compared to other detectors and dosimeters. The main strengths are the sub-millimeter spatial resolution, near water-equivalence, sensitivity to many radiation types and to a wide dose range, weak dependency on the energy, and dose rate. In addition, the relative ease of use and cost are considerable advantages. The first radiochromic materials were developed to be used only in high dose applications, such as radiation processing, including sterilization of food and medical devices [19; 20]. Successively, new radiochromic media were developed with an improved sensitivity to doses of medical interest [21; 22]. Today, many RCF types are available, covering a sensitivity dose range from a few mGy to hundreds of kGy; this allows their use in applications from the low dose radiological diagnostic to Quality Assurance (QA) in radiotherapy, brachytherapy up to high dose radiation processing [23; 24; 25; 26]. Because of this versatility and of the aforementioned characteristics, RCFs are used in applications for which an in-depth characterizationof the particle beam profile and homogeneity is required, such as in the monitoring of the irradiation for radiation hardness [27; 28; 29; 30; 31], as well as in the characterization of charged particle beam lines for radiobiology studies [32; 33; 34; 35]. RCFs are used in the dosimetric characterization of proton and heavy ion beams for hadron-therapy [36; 37; 38; 39]. Beam profile measurements with RCFs are performed at cyclotrons for radioisotope production mainly for guaranteeing the beam positioning on target materials [40; 41]. As an example, RCFs are used for this purpose, as well as for the accurate beam characterization in studies of the radiation damage for high energy physics and spatial applications at the 18 MeV cyclotron in operation at the Bern University Hospital (Inselgital) [31; 42; 43]. The independence of RCFs on the dose rate resulted in their application in ultra-high dose beam diagnostics studies, such as laser-plasma proton beams and FLASH radiotherapy [44; 45; 46; 47]. In addition to the conventional two-dimensional dose distribution measurements, stacks of RCFs have been successfully used to measure energy spectra in these challenging applications [48]. In order to exploit all the features of this interesting radiation detector, an in-depth knowledge of its main features is necessary. The use of a specific reading technique allows, e.g., measurements in dynamic and sensitivity ranges other than those obtained by using another reading technique.