## 1 Introduction  
In vivo dosimetry (IVD) measures how accurately the planned dose is delivered to the patients during radiotherapy treatment. For many years, it has been viewed only as supplementary to a standard clinical QA program, but recently its role in preventing delivery failures and increasing patient safety and treatment quality has been considered. International organizations recommend its use [1] and national and international regulators are starting to require it in national law [2].  
Transit dosimetry, with the Electronic Portal Imaging Device (EPID), is a powerful tool for IVD measurements. It enables large scale acquisitions with a relatively low cost for patients and institutes [3]. Many software programs, some of which are commercially available, have been developed in the last ten years that enable the reconstruction of the dose absorbed in the patient's anatomy (in the planning CT or/and in the daily CBCT) either in a point, a plane, in 3D, or even in 4D [4; 5]. Despite its wide availability and its simplicity of use, the clinical implementation of such a dosimetric technique is still challenging. Currently, only a few centers have reported wide statistics of IVD results [6; 7]. One reason for the IVD implementation difficulty is the lack of guidelines around the validation and acceptance of an EPID IVD algorithm. Moreover, there is no consensus about the dose difference metric and the tolerance levels to use to guarantee the needed level of accuracy and specificity in the errors' detection. The accuracy evaluation of the EPID dose reconstruction algorithm is important for setting the tolerance levels in order to limit the presence of false-positive errors due to algorithm failure. In a recent SBRT study, it was found that, with the standard tolerance levels [8], about 40% of fractions resulted over the tolerance level (OTL), of which approximately half were due to algorithm failure [9]. The presence of tissue inhomogeneity can further increase false-positive errors. [PERSON] et al. found that 34% of OTL but only 1.5% of alerted plans underwent corrective action [10], and the OTL majority was due to EPID algorithm failure in dealing with tissue inhomogeneity.  
EPID reconstruction algorithms have been usually validated against absolute dose measurements only in points [11], and the dose distributions were generally compared to the treatment planning system (TPS) computed dose. Only a few studies have validated the EPID reconstructed dose distributions by comparing them with absolute dose measurements [4; 12; 13]. However, in these studies, the measurements were performed in homogeneous phantoms with simple cylindrical [13] or octagonal [12] geometries.  
To the best of our knowledge, the EPID back-projection algorithms have never been validated in an anthropomorphic phantom against direct two-dimensional measurements.  
In this work, we assessed the accuracy of the new version of the commercial EPID software, Dosimetry Check (DC). The evaluation was conducted in two steps: first, the capability to reconstruct squared field fluence on slab phantoms with air and bone inhomogeneities was evaluated. In the second phase, the dose distributions reconstructed by DC were compared with radiochromic film dosimetry in the anthropomorphic Alderson Rando phantom for volumetric modulated arc therapy (VMAT) plans. To our knowledge, the new algorithm, introduced in version 5.8 of the DC software for dealing with tissue inhomogeneity, has never been evaluated before.  
There is extensive literature about the implementation of EPID software for transit dosimetry in clinical practice. However, this study contains some innovations. In fact, this is the first study comparing EPID based dosimetric results with film dosimetry in an anthropomorphic phantom.  
The methodologies shown in this work are the final step of the commissioning of an EPID transit dosimetry system and can be used to validate any back-projection EPID algorithm [6; 14] before clinical implementation.