ESTRO 2021 Abstract Book

S480

ESTRO 2021

EBT3 or EBTXD films were used to measure the delivered dose. A 1%/1mm local Gamma-analysis quantified dose differences between the static reference and tracking cases. Additionally, the dose area histogram (DAH) was determined for the target. Results The VMAT plans had a conformity index (prescribed isodose volume/ PTV) of 1.4-1.5 and an MU-weighted mean-field area of 13-16 cm 2 . Treatment delivery times were: 6.7 min, 13.1 min, and 24.2 min, for the 8x7.5 Gy, 3x18 Gy, 1x34 Gy lung SBRT plans respectively. The plans required an RMS leaf speed of 0.5-0.7 cm/s. Tracking required a maximal additional 2.4 cm/s leaf speed. Each plan was delivered in respectively 2, 4, and 6 arcs. The local gamma analysis for the central delivery shows that MLC-tracking improved the gamma pass-rate from 67.5% to 98.3% for Lujan motion and to 94.2% for the real respiratory trace. For peripheral deliveries with real respiratory motion, the 3x18 Gy delivery had a 97.3% pass-rate and the 1x34 Gy delivery had a 96.8% pass-rate (Fig.1). The DAH (Fig.2) shows that the target dose agrees well between static and tracking deliveries with real respiratory motion. The figure also shows that the minimum dose in the target is well above the prescribed dose.

Conclusion We provided a first experimental demonstration of the technical feasibility of VMAT combined with MR-guided MLC-tracking for central and peripheral lung SBRT.