Dosimetric characterization of a new plastic scintillator and other detectors in standard and small field photon beams from a medical linear accelerator

Radiotherapy consists in the use of ionizing radiation beams of various nature (usually photons, electrons, or protons/heavy ions) to treat tumor diseases. That is achieved by killing tumor cells while sparing as much as possible the surrounding healthy tissues. Focusing on photon beams, over the years, their clinical production shifted from the use of gamma sources, i.e. 60Co, to the usage of Linear Accelerators (LINAC) that generates an electron beam of different energies, then producing a photon beam through bremsstrahlung emission. Recently, research has advanced considerably regarding the tools and techniques adopted in radiotherapy. For instance, it is now possible to account for irregular shapes of a tumor with the help of a Multi-Leaf Collimator (MLC), and furthermore to modulate the beam intensity to perform advanced treatment techniques like Intensity Modulated Radiation Therapy (IMRT) or Volumetric Modulated Arc Therapy (VMAT). Those techniques are enabled also thanks to the optimization of algorithms used in Treatment Planning Systems (TPS) which permit to precisely determine the dose delivered to each irradiated voxel and thus optimize the beam intensity dispensed during treatments. To be ready for clinical use, it is then necessary to construct a model of the LINAC machine to be uploaded into the TPS, so that the software can simulate the beam delivery, allowing the algorithms previously cited to work properly. This is part of a complex process called commissioning, which encompasses a series of dosimetric measures to collect enough data to characterize the LINAC behaviour during ordinary therapeutic activity. Those measures must be taken following the international regulations written by the International Atomic Energy Agency (IAEA), particularly the IAEA TRS-398 Code of Practice. The main measures to be collected are some sets about the Percentage Depth Dose (PDD) and the Off Axis Ratio (OAR) distributions, followed by Field Output Factors (FOF) values. Regarding the measures, it is possible to perform the acquisition with different types of detectors, such as solid-state detectors, scintillators, ionization chambers, etc. All the sets must be collected for all the allowed field sizes, which may vary from large field dimensions (tenths of centimetres per side) to the so called small field sizes (some millimetres). Small fields are characterized by some issues that larger fields dimensions don’t show. There exist other international protocols and instructions for dealing with small fields, listed in the IAEA TRS-483 Code of Practice, where specific procedures and correction factors are reported. The main goal of this thesis work is to evaluate the performances of different detectors during the commissioning of the VERSA HD LINAC recently installed at the Radiotherapy Department of AULSS 3 "Ospedale dell’Angelo" located in Mestre (Venice) by using two different photon energies (6 MV WFF and 6 MV FFF). To this purpose, PDDs, OARs and FOFs have been subject of comparison with suitable detectors such as PTW microdiamond, IBA Razor diode, IBA EFD3G unshielded-diode and the new Blue Physics Plastic Scintillator Detector (BP-PSD). In particular, the features and properties of the BP-PSD were investigated, as well as challenges and issues about dosimetry in small fields.