ESTRO 2025 - Abstract Book

S2876

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

respectively [2], [3]. Modified radiobiological parameters for the hypoxic sub region using an Oxygen Enhancement Ratio of 1.5 have also been investigated following [4]. Results: In all the 13 patients treated so far the radio-hyperthermia treatment resulted in an increase in EQD2 D 95 doses to the GTV, compared to the corresponding RT-only treatment . In the hypoxic sub region, the increase was almost always more pronounced. Conclusion: A radiobiological assessment of the combination of surface hyperthermia and external beam radiotherapy on the soft tissue sarcoma patients treated with our protocol confirm the therapeutic dose scaling effect of adding hyperthermia to a conventional radiation treatment. The results have been qualitatively confirmed by post operative histology.

Keywords: Thermoradiotherapy, Hyperthermia, Equivalent dose

References: [1] Jakob Öden et al. Int J Radiat Oncol Biol Phys. 119(5):1530-1544 (2024) [2] M van Leeuwen et al. International Journal of Hyperthermia, 34:1, 30-38 (2018)

[3] R L Haas et al. Radiation Research 196, 23-30 (2021) [4] DJ Carlson et al. Med Phys. 33(9), 3105-15 (2006)

3538

Proffered Paper Online dose calculations on the MR-linac to enable dosimetrically determined intra-fraction drift correction Peter R.S. Stijnman, Pim T.S. Borman, Stijn Oolbekkink, Martin F Fast, Bas W Raaymakers Radiotherapy, UMC Utrecht, Utrecht, Netherlands Purpose/Objective: During radiation therapy the CTV can move away from the initial treatment setup, potentially causing underdosage of the CTV and overdosage of OARs. To minimize this effect, the MR-linac can be used to track the movement of the CTV using cine MRI and the treatment plan can be adapted with a baseline shift correction [1]. Currently, this decision is based on geometric rather than dosimetric considerations. Here, we show a decision-making support tool for online guidance on when to perform baseline shift corrections based on dosimetric constraints. Material/Methods: The accumulated dose during treatment using a 1.5T Unity MR-linac (Elekta AB, Sweden) is estimated by streaming the current state of the machine (at 5Hz) to a Monte Carlo dose engine (i.e. GPUMCD, Elekta) to calculate clinical grade dose distributions. Movement is taken into account through a rigid translation of the daily anatomy. Furthermore, we predict the effect the remaining part of the dose delivery, given the current rigid translation, has on the accumulated dose [2]. This is compared to the planned dose. An overview can be seen in Figure 1.