ESTRO 2023 - Abstract Book

S391

Sunday 14 May 2023

ESTRO 2023

plans with the remaining RSs. Differences in dose in the centre of SOBP plans with varying ranges and widths (fig.2) was within 3% for all algorithms, with Eclipse underestimating the dose for all cases.

Conclusion The performance of the four PBS-PT models was benchmarked against experimental data. GATE provided an accurate independent dose calculation tool to support the commissioning and quality assurance workflows of clinical proton facilities.

Poster Discussion: Patient management strategies

PD-0483 Skin markers are no longer a necessity in prostate MR-Linac treatments R. Rutgers 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands

Purpose or Objective Because the 1.5T MR-linac allows for online plan adaptation to the daily anatomy, the exact positioning of the patient on the couch is less relevant than in conventional non-adaptive radiotherapy. Therefore, skin marks can be omitted to improve patient experience. Because we apply full plan adaptation in SBRT prostate treatments (adapt-to-shape, ATS) we have stopped the use of tattoos in this group. However, in conventionally fractionated treatment for high risk disease, translation only corrections (adapt-to-position, ATP) are applied and so pelvic rotations are more critical. We therefore investigated pelvic rotations with and without tattoos. Materials and Methods Clinical MR scans (T2 3D TSE) of 26 prostate patients treated on a 1.5T MR-linac (Unity system, Elekta AB) were used to retrospectively measure the pelvic bone rotations differences relative to the pretreatment MR scans. Pelvic rotations were measured in 20 patients treated with SBRT (100 fractions, ATS) without tattoos and 6 patients (92 fractions, ATP) with