ESTRO 2024 - Abstract Book

S4775

Physics - Quality assurance and auditing

ESTRO 2024

calibration of the MLCs. The OUH data, however, shows great agreement. The actual sphere position represents its setup location.

Table 1 shows the difference between the MR-to-MV isocentre shift found here and the values applied by the vendor. The most significant deviation was found to be 0.2-0.3 mm in the z -direction consistent with earlier findings. The consequence of this is that the isocentre in the TPS is located above the radiation isocentre of the MRI-linac. In the x- and y- directions, the differences were not recognised as significant. The voxel size in the MRI scanning protocol compared to the volume of the zirconium ball was recognised as critical for sphere position determination in the MRI scan. Table 1. Translations of the MV radiation isocentre relative to the planned isocenter in the xyz -room coordinate system, ∆ R =( ∆X , ∆Y , ∆Z )=∆ r TPS -∆ r found via phantom irradiation and T1 body coil scan. The results refer to the xyz -room coordinate system, IEC61217[4].

Parameter

OUH

UUH

(mm)

(mm)

ΔX

0.05

-0.08

ΔZ

0.04

0.03