ESTRO 2022 - Abstract Book

S725

Abstract book

ESTRO 2022

by the planned 50% isodose volume (VID50). For in vivo measurements, we selected 12 prostate plans (5 × 9Gy) treated with 10 FFF beams, yielding 32 fractions in vivo . Average DVH differences ( Δ D2, Δ D50 and Δ D98) for the VID50, targets and OARs were determined. Results Table 1.A shows our pre-treatment and in phantom results. Substantial dose differences were found for mdMLC <1cm with highly modulated fields, yielding an average Δ D50 for VID50 of -4.7 ± 2.8% and -2.4 ± 1.7%, respectively, which can go (partly) unnoticed when using gamma passing rates. Geometric and dosimetric calibration of these very small leaf distances is complicated in many aspects (LINAC, dosimeters, TPS, ECMP, etc.), so for further analysis we concentrated on 12 hypo- fractionated prostate treatments with mdMLC of 1-2cm. Table 1.B shows smaller systematic dose differences (<1% underdose) for in phantom prostate measurements, still explainable by minimal errors in leaf settings like the dosimetric leaf gap (DLG). For in vivo measurements, mean underdosage increases up to 5%, except for the urethra wall that is enclosed by the PTV (Fig.1). Apart from tissue inhomogeneities, anatomical changes and set-up errors that take effect during patient’s treatments, we are currently investigating whether the extra underdose might stem from residual uncertainties in the transit EPID dose conversion itself and from differences in 3D dose calculation algorithms (XVMC vs AAA).

Conclusion