ESTRO 2023 - Abstract Book

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ESTRO 2023

mutagenesis. Such a pattern has been observed in NPC survivors receiving IMRT, with an increase in thyroid cancer risk that was not present in 2DRT cases.

Conclusion The NPC survivors with second primary EACSCC exposed to an equally high radiation dose to both EACs in previous 2DRT. With IMRT/VMAT, the mean dose to the EACs could be reduced by half, but the wider dose range implied an exposure of a low and heterogeneous dose to the EACs. Further study to evaluate the dominating factors that increase the risk of second cancer after IMRT/VMAT is warranted when sufficient follow-up is available.

PO-2347 Bladder wall dose in PCa patients treated with stereotactic, daily adaptive MR-guided radiotherapy

E. de Groot - van Breugel 1 , J. de Boer 1 , T. Willigenburg 1 , J. Hes 1 , J. van der Voort van Zyp 1

1 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands

Purpose or Objective Recently, we have clinically applied a sub-fractionation workflow for intrafraction re-planning during prostate SBRT on a 1.5 Tesla MR-linac (Unity, Elekta AB) in over 80 patients. This workflow counters intrafraction prostate motion and 2-3 mm planning margins were applied. Before sub-fractionation, we applied more generous planning margins. We investigated if the current sub-fractionation workflow results in a relevant decrease in bladder wall dose. Materials and Methods Since 2019, we have been treating prostate cancer patients with 5 x 7.25 Gy on our MR-linacs, using daily imaging, re- contouring and re-planning (‘adapt to shape’ workflow). In 130 of these patients, Willigenburg et al (Rad Onc 2022, 171:182-