## 4 Discussion  
In this work, the accuracy of a novel EPID back-projection transit algorithm (DC 5.8) was assessed, to our knowledge, for the first time, against direct measurements with film dosimetry in an anthropomorphic phantom. The novel DC algorithm was tested in realistic geometries with inhomogeneous tissue densities. The tests performed in the slab phantom aimed to determine the accuracy of the entrance fluence reconstructed by the deconvolution kernel. For all the field sizes, the differences between the reconstructed MUs and the planned MUs are within 2%. The main finding of our work came from the direct comparison of the film measurements and EPID transit reconstructed doses. The GAI between the two measurements was very high, showing a high accuracy of the EPID reconstructed doses in anthropomorphic geometries. The PB algorithm of DC was already tested in a homogeneous phantom against the 2D ionization chamber array for  
Figure 8: Comparison of computed dose distribution for the VMAT plans: green lines are the doses computed by the treatment planning system; pink lines are the doses reconstructed by the Dosimetry check with the Collapsed cone convolution algorithm. Isodoses curves displayed are the 100%, 95%, 90%, 80% and 50% of the prescribed dose. Figure 7: Gamma agreement index at 3%/3 mm local normalization with 10% dose threshold, in the axial slice passing through the isocenter, between the Dosimetry Check doses reconstructed with the two algorithms, pencils beam (PB) and Collapsed Cone Convolution (CCC), and the planned doses computed by the treatment planning system (TPS). VMAT plans [4], and the agreement of the two measurements was high. However, the PB algorithm was not able to accurately reproduce the dose distributions in the lung, and a strategy of in-water re-computation was needed. The same \"in aqua-vivo\" solution was previously applied by [PERSON] et al. [17] to the EPID transit algorithm developed at NKI [18]. The novel CC algorithm of DC was otherwise shown to be able to accurately reproduce the dose distribution also in the lung. The dose reconstructed by DC with the CC algorithm can be directly compared with the TPS doses in the lung target without any need for in-water re-computation. [PERSON] and co-workers [19] have reported the accuracy of the homemade PDR EPID back-projection algorithm in an anthropomorphic phantom against TPS computation. The dose distributions reconstructed by the PDR algorithm were also accurate in the presence of tissue inhomogeneity. However, in the work of [PERSON], the EPID doses were only compared to the TPS doses computed by the AAA Eclipse algorithm. Even if the TPS computation could be highly accurate, after proper system commissioning, it should be viewed as only an estimate of the actual dose delivered by the linac. In fact, residual inaccuracies of the TPS implementation, and/or variability of the linac output due to leaves and collimators movement and positioning inaccuracy and mis-calibration, and to monitor unit mis-calibration, limit the use of the TPS computation as a gold standard. We believe that only a direct measure with film could be considered as a gold standard for evaluating the accuracy of an in vivo EPID system before clinical implementation. Radiochromic dosimetry is challenging, but currently, it is consolidated that proper handling, together with the application of a multichannel approach for dose calibration, allows for reliable results [16; 20; 21]. Our work has some limitations: the first is the phantom used. The Anderson Rando phantom was not originally designed for film dosimetry and offers the only possibility of measuring dose distributions with film between axial slices. This could create a certain space between slices that needs to be taken into account during the plan computation. Moreover, the presence of holes inside the phantom, designed for point dosimetry with thermoluminescent detectors, could add unnecessary complexity to the EPID algorithm.