## 4 Discussion  
In this study deviations in treatment delivery and in patient setup were simulated. Both types of errors happen in clinical routine. Scenarios can be various: small, but even large deviations can occur during the treatment, for example, when a wrong linac dose calibration is done or when patient is not correctly identified or prepared for the therapy. The strength of the combined use of the two systems in detecting all different kinds of errors, even when very small, has been evidenced in this work. Separate use of IQM or SoftDiso devices has been widely discussed for different irradiation modalities [43; 44; 45; 46; 47; 48; 49], but to our knowledge, no study exists of their combined use, in particular for breast irradiation. A clinical study of the SoftDiso-based quality assurance procedure for post-mastectomy IMRT and VMAT radiation therapy is reported in reference [50], where specific thresholds for R-value, \(\gamma_{\text{PR}}\) and \(\gamma_{\text{mean}}\) indexes were proposed, based on the clinical experience and validated with a phantom chest. The authors confirmed the feasibility and the importance of a QA procedure based on EPID-based IVD to detect inter-fraction setup errors to overcome the weakness of a setup control based only on weekly cone beam CT acquisitions. The high IQM sensitivities to simulated delivery errors have been assessed by several authors [43; 44; 45; 46] and confirmed in our work. However, to our knowledge, studies of the application of IQM to breast irradiation have not been reported elsewhere. From our measurements, the IQM device demonstrates high sensitivity to small MU variations and jaw position deviations. The SoftDiso R-value was also found to be able to detect MU variations with a sensitivity slightly smaller than for IQM. However, the R-value is much less sensitive to collimator position variations, as expected given the stability of output factors for small modifications of the field size. The IQM system is obviously insensitive to patient setup errors given its positioning upstream the patient itself. However, in this work we have demonstrated the ability of the EPID-based SoftDiso to detect simulated setup errors. The SoftDiso R-value was shown to have good sensitivity to small displacements in the anterior direction--the error most critical for the particular simulated plan used. To recover full sensitivity to 3D displacements a gamma-index analysis was needed. This analysis, with 2%/2 mm criteria, yielded alert thresholds of about 90% for \(\gamma_{\text{PR}}\) and 0.4 for \(\gamma_{\text{mean}}\), in agreement with those proposed in the literature [50]. The \(\gamma_{\text{mean}}\) threshold was also demonstrated to be able to detect a phantom rotation of 2.8\({}^{\circ}\). It is worth noticing here that the gamma-index analysis, while being fairly sensitive, cannot distinguish between patient positioning errors and beam setup errors. In this study we have therefore shown that the combined and simultaneous use of IQM and an EPID-based system provides a complete monitoring of beam and patient setup errors. Both devices allow one to set alert thresholds, which can be suitably adjusted according to the specific treatment, providing a warning message or sound when the measurement is out of tolerance. The IQM system can generate a real time warning when the delivery is incorrect. On the other hand, the R-values provided by the SoftDiso immediately after the sessions can be saved in a table, showing not only whether the tolerance has been exceeded but also whether a trend over a series of sessions appears, pointing to the need for a setup correction. Finally, the response of gamma analysis comparison is quickly available after a very fast data analysis (few minutes). Consequently, by combining the IQM and the SoftDiso, delivery and/or setup errors are detected quickly during or shortly after the delivery, allowing one to correct the inaccuracies during or immediately following treatment sessions.