ESTRO 2022 - Abstract Book

S1305

Abstract book

ESTRO 2022

5mm apart, with 10mm spacing elsewhere. Because the Delta4 records the received dose every 25ms, and the logged target position is known within each dose interval, we can deduce how far off-centre the dose was delivered. To artificially move the phantom with the target, we shifted the measured incremental dose by the same amount but in opposite direction as the target had moved w.r.t the iso-centre. Dose shifts were carried out by re-gridding the data using cubic spline interpolation. We created three VMAT treatment plans for lung SBRT with 3mm GTV-to-PTV margins: a central plan (8x7.5Gy) and two peripheral plans (3x18Gy and 1x34Gy). A 1%/1mm local Gamma-analysis quantified dose differences between a static delivery and tracking cases.

Results The 1x34Gy dose maps (Fig 2) show the effect of a virtual HexaMotion platform. W/o tracking (no re-grid) differences with the static delivery are up to 10Gy, while this reduces to <1.5Gy with tracking. Following a single diode over time shows superimposed dose lines for static and tracking, while w/o tracking it receives only 60% of the dose. Gamma pass-rates improved from 74.0% to 98.4% (8x7.5Gy), 34.7% to 98.1% (3x18Gy) and 17.8% to 99.7% (1x34Gy).