SU‐C‐108‐03: Investigation of MLC Leaf Tolerance and Dose Accuracy in VMAT Delivery

Abstract

Purpose: VMAT employs a multi‐parameter modulation methodology. It tends to perform less reliably than the one‐dimensional IMRT. Therefore, setting a suitable leaf positioning tolerance on the machine is crucial. In this study, we investigated the relationship between the leaf tolerance and the dose delivery accuracy for VMAT SBRT lung plans. The goal was to determine the optimal leaf tolerance for daily VMAT SBRT treatments. Methods: The study was performed on a VMAT‐enabled Trilogy LINAC. To minimize the systematic bias in leaf and gantry speeds, all the VMAT SBRT plans had a prescription dose of 1800 cGy × 3. Prior to each plan delivery, the leaf positioning tolerance was reset to a new value in Machine Hardware Configuration in Treatment Administration. Upon completion of the plan delivery, the Dynalog Files and, more importantly, the Treatment Log File, i.e., the Dlog File, were saved and exported for post‐processing. For our initial investigation, we tested leaf tolerance values of 1, 2, 3, 4, 5, and 6 mm, respectively. Results: We found that when the leaf tolerance was set to a tighter value, such as 1, 2, and 3 mm, a good delivery accuracy was preserved without any MLC interlock occurrences. However, when the leaf tolerance was increased to be greater than a threshold value, such as 3 mm in this case, the delivery accuracy deteriorated almost linearly beyond that point. We also found that except for the leaf tolerance = 1 mm, the gantry angle accuracy virtually remained unchanged as the leaf tolerance was increased, indicating a decoupling between the leaf tolerance and the gantry angle uncertainty. Conclusion: We recommend that for VMAT SBRT lung plans with a prescription dose of 1800 cGy × 3, the optimal leaf tolerance should be set to 3 mm.