Poster - Thur Eve - 42: Dynamic delivery quality assurance on Elekta linacs

Introduction Recently, Elekta linacs have gained the capability to deliver dynamic fields, such as sliding window IMRT or VMAT fields. Because of the potential complexity of such delivery modes, linac QA and patient-specific QA are of prime importance. The aim of this paper is to explain the fundamental concepts of this new mode of operation on Elekta linacs as well as to introduce the linear-α slit, a novel dynamic QA sequence which allows the performance of the delivery system to be objectively aassessed against dosimetric measurements. In the Elekta dynamic mode, dose rate can only take a limited number of discontinuous values. We have used the uniform slit, a simple slit moving at a constant speed across the field to verify how the linac selects leaf speed and dose rate for a given MU setting. Based on this, we expose the principles behind the linear-α slit, which exhibits two main characteristics: 1) it exercises the linac over a range of dose rates and 2) it produces a dose distribution which is theoretically equivalent to the uniform slit. Discrepancies between measurements of the linear-α slit and the uniform slit directly reflects problems with the delivery. The linear-α slit has been recently introduced in our routine monthly linac QA. We hope it will nicely complement patient-specific QA, and exising linac-QA.