However, pre-treatment verification presents several drawbacks: (1) the actual patient geometry is never included in pre-treatment measurements, making it difficult to estimate the effect of observed dosimetric deviations on the actual patient; (2) deviations from intended treatment occurring during the dose delivery cannot be detected [9; 10]; (3) pre-treatment verification requires additional measurement sessions, taking up valuable linac time and increasing workload. The first issue is usually addressed in commercial systems by reworking observed deviations on the patient anatomy (using the planning CT scan) [11; 12; 13]; however, the last two issues cannot be circumvented by using pre-treatment verification. To solve these questions, in vivo dosimetry systems (IVDs) that perform dose measurements during the treatment and compare them to the intended dose are sometimes introduced into the clinical routine. In this work, a study of uncertainties affecting breast irradiation was carried out. Three main error sources were analyzed: delivery errors, setup errors and human errors. Delivery errors are systematic errors affecting delivery parameters which can lead to output variations (e.g., incorrect linac dose calibration or incorrect calibration of the multi-leaf collimator or of the jaws). Inter-fraction setup errors instead are more difficult-to-control errors tightly related to the patient alignment procedure followed by operators, to anatomical modifications and to treatment preparation imaging devices. Regrettably, human mistakes could also occur, e.g., incorrect arrangement of the patient, perhaps due to clothes or objects accidentally located inside the treatment field, or incorrect identification of the patient occurs. The effectiveness of IVDs in detecting systematic and random treatment errors has been investigated in several review papers and reports [9; 14; 15; 16; 17; 18; 19]. In particular, all authors agreed that standard quality pre-treatment checks are not able to detect a large number of treatment errors, highlighting the need for IVDs. Recently, new international recommendations, following the suggestions indicated in many incident reports [20; 21; 22; 23], have encouraged the introduction of IVDs as good practice in radiotherapy [24; 25; 26; 27; 28; 29]. For example, article 63 of the European Directive 2013/59/Euratom [24] clearly states that the member countries have to include a risk analysis in their quality management systems for radiotherapy practice. In some European countries, such as Sweden, Denmark, Norway, the Czech Republic, and France, an IVD is mandatory [27; 28; 29]. Commercial systems for in vivo dose monitoring are available with different objectives. Several devices measure the patient's dose by using detectors placed on the patient (for example, on the patient's skin) or next to the patient (in a known position), or by processing the radiation transmitted through the patient. Other commercial systems measure the photon fluence at the linac output. Real-time in vivo devices, such as diodes, metal-oxide semiconductor field effect transistors (MOSFETs) and electronic portal imaging devices (EPID) [30; 31], belong to the first group. In particular, EPID systems use back projection methods on measured portal images to obtain 2D [32; 33; 34] or 3D [35; 36] patient dose distributions directly comparable with the planned dose. Transmission radiation detectors (TRDs), consisting usually of an array located on the linac head, belong instead to the second group. However, these devices monitor the delivery but not the patient dose; therefore, they cannot be classified as IVDs [37; 38; 39; 40; 41; 42]. Our project aims to implement a new quality assurance (QA) procedure for patients undergoing highly conformal radiotherapy treatments that combines a transmission detector to monitor the fluence entering the patient and in vivo dosimetry with an EPID to measure the dose actually released into the patient during the treatment.