ESTRO 2021 Abstract Book

S1616

ESTRO 2021

original clinical treatment goal unless tolerance doses had been substantially exceeded in the clinical treatment plan. To what extent dose to mastication structures could be lowered without violating existing clinical treatment goals were quantified by group and by patient. Results Altogether, there were 334 clinical treatment goals in the clinical treatment plans (median=15, range: 7-24 per patient, depending on tumour location), which easily could be met in the corresponding MCO plans. Mean doses to the mastication structures were in most cases below proposed tolerance doses in the clinical plan but could on average be further reduced between 3-5 Gy in the MCO plans ( Table 1 ). Of the 25/88 patient reductions below 5 Gy (28%), 18/25 (72%) were for the masseter (n=8) and medial pterygoid (n=10) muscles ( Figure 1 ). Table 1. Mean dose characteristics for mastication structures (bilateral representation) in HNC patients*

Figure 1.

Conclusion With modern RT, it seems possible to reduce the dose to mastication structures below proposed trismus dose tolerance thresholds for most HNC patients without violating clinical treatment goals. Focusing on masseter and medial pterygoid muscle doses may prove to give the largest benefit in individual cases. PO-1897 Prone vs. supine neoadjuvant accelerated partial breast irradiation on an MR-linac: a planning study M. Groot Koerkamp 1 , F. van der Leij 1 , T. van 't Westeinde 1 , G. Bol 1 , V. Scholten 1 , R. Bouwmans 1 , S. Mandija 1 , D. van den Bongard 2 , A. Houweling 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands; 2 Amsterdam UMC, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective Neoadjuvant accelerated partial breast irradiation (NA-APBI) has the advantage of smaller treatment volumes and better target definition compared to adjuvant APBI. Due to the excellent soft tissue contrast of MRI, NA-APBI is a treatment that may benefit from the MR-linac for tumor localization. Our aim was to investigate whether prone or supine treatment position was dosimetrically beneficial for NA-APBI on a 1.5T MR-linac. Materials and Methods