Room: AAPM ePoster Library
Purpose: To compare the results of the cumulative signal for patient-specific SBRT quality assurance performed with an electronic portal imaging device (Varian TrueBeam aS1200) and an integral quality monitor (iRT Systems IQM).
Methods: Eight SBRT plans, consisting of sixteen arcs in total, were selected from our clinical database. These plans included two lung, two spine, two prostate, one kidney, and one pelvic SBRT plan. Selected plans featured small field segments and relatively high modulation (i.e. MU / dose delivered). The cumulative signal for the EPID was evaluated using the ratio of the measured to predicted portal dose. The cumulative signal for the IQM was evaluated using the ratio of the measured to calculated reference signal.
Results: Values for the cumulative signal of the EPID range from 1.020 to 1.034 (mean 1.026 ± 0.005). Values for the cumulative signal of the IQM range from 0.963 to 0.998 (mean 0.984 ± 0.012). Statistically significant similarity is not found between the two datasets (p < 0.05).
Conclusion: The greater range and larger standard deviation of the IQM’s cumulative signal reinforces that the IQM is more sensitive to dose differences than the EPID. This is of particular interest for SBRT treatments where small fields and large modulation are prominent. Preliminary results show no statistically significant correlation; however, a larger sample size per treatment site may elucidate correlations. While the IQM cannot provide information regarding spatial dose distribution like the EPID, deviations from the measured reference signal can be observed along each arc’s control points. This presents an avenue in determining the position in an arc where a treatment field is failing. Systematic error detection capability will be evaluated by introducing errors in the treatment plans such as MU and MLC position offsets, which will also contribute to enlighten the origin of the arc deviations.
Quality Assurance, Small Fields