ESTRO 2024 - Abstract Book

S5484

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

Since November 2022, new type of wedges (Macromedics, The Netherlands) have been used in combination with the same individual headrest employed in the standard setup. The wedges are fixed to the baseplate using the mask, the second baseplate is no longer used in this setup (Fig. 1 right). The GTV-PTV margin is increased to 2 mm when wedges are used due to the expected less stability of the setup compared to the standard setup. The aim of this study is to find out if it is possible for patients lying on wedges to use a GTV-PTV margin of 1 mm.

Material/Methods:

To investigate whether it is possible to reduce the margin from 2mm to 1mm when the patient is positioned on wedges, the infraction motion of 3 different setups was compared: positioned without a wedge (NoW), positioned with the old wedge and standard headrest (OW), and positioned on the new wedges with individual headrest (NW). In each group 12 patients were included. All the included patients received radiosurgery between 2020 and 2023 at our department. In order to assess the intrafration motion, the post-CBCT to CT registration results, translations and rotations, were considered. In particular, it was assessed whether the translations are ≤1mm and the rotations are ≤1 degree (tolerance per institutional protocol). The statistical difference between the three considered setup was assessed by a Wilcoxon Signed ranks test.

Fig. 1: Left: old situation with wedges, standard headrest and second baseplate (OW); right: new situation with wedges and individual headrest (NW).

Results:

In the NoW group, the intrafraction motion of 1 patient was outside the tolerance with a rotation >1 degree. In the OW group, 2 patients exceeded the tolerance: 1 had a translation >1mm and 1 patient had a rotation >1 degree (both patients lies on a single wedge). In the NW group, 2 patients were outside the tolerance: 1 patient with a translation >1mm (lies on four wedges) and 1 patient with a rotation >1 degree (lies on a single wedge). In the NoW and the NW groups the same amount patients have an intrafraction motion >1 mm and/or >1 degree. No statistical significant difference was found between the groups except for the longitudinal (p=0.016) and the height (p=0.04) translation of the OW with respect to NoW.