SU‐E‐T‐170: A Viable Approach to Patient Specific QA for Spine VMAT SRS Using EPID‐Based Dosimetry

Purpose: VMAT involves multi‐parameter modulations, thus presenting multi‐dimensional challenges. Here, we present a viable approach to VMAT patient specific QA using EPID‐based dosimetry for spine VMAT SRS cases Methods: Our spine VMAT SRS plans consisted of two full arcs and were optimized with 6 MV photons for a Varian Trilogy LINAC. The QA plans were computed by resetting all control points to 0° gantry angle while keeping the MLC apertures and corresponding MU weights intact. The dose distribution at the depth of clinical significance was calculated in a solid water phantom. EPIDose (Sun Nuclear Corp) was used to convert MV EPID images into absolute dose in a plane of interest in a homogeneous medium. During measurements, the source‐to‐detector distance (SDD) was set to 105 cm and the VMAT QA plan was delivered with 600 MU/min. For each QA plan, three measurements were made. The average pass rate (thresholds: 2%/2 mm) was then calculated. In addition, the dose at the isocenter or other points of clinical significance was also measured and the mean was calculated. Results: Our current institutional QA acceptance criteria for VMAT plans are: pass rate 90% for 2%/2mm with a dose threshold of 10% and the discrepancy between the measured and planned doses at the isocenter or other points of clinical significance 2%. Our preliminary investigation indicated that the pass rate was normally greater than 95.0% with a total number of valid detection points of ∼30000. The dose difference at the point of interest was, in general, around ±1.0 %. Sharp dose gradients were accurately detected at the PTV‐cord interface. Conclusions: EPID‐ based dosimetry is real‐time and financially viable. It can achieve sub‐ millimeter dosimetry accuracy without extensive inter‐diode interpolations. We believe that EPID‐based VMAT dosimetry offers a competitive alternative to other competing dosimetry technologies.