Session Item

The aim of this work was to characterise the attenuation of the couch and coils structure for two Unity MR-Linacs. The Patient Positioning System (PPS) consists of a moveable couch top with a comfort mattress for patient support, a bridge rigidly mounted into the bore, and the anterior and the posterior coils. The same model in the treatment planning system (TPS) Monaco is used for all Unity machines. Therefore, correct attenuation measurements of the PPS are important for the validation of the dose calculation in the TPS.

A cylindrical PMMA phantom of outer diameter of 40 mm, wall thickness 3.0 mm, length 110 mm with a mounting hole for a Farmer chamber insert was constructed, see Figure 1. Magnetic field effects due to air cavities were minimized by filling the volume around the chamber inside the phantom with water and screwing the chamber into the cylinder. A tripod was designed to support the phantom and careful adjustment of the effective point of measurement to the isocentre of the Linac. The PTW Farmer chamber was connected to a PTW UNIDOS electrometer. Software was developed for the communication and automatic collection of measured data. The correct position of the Farmer chamber at the isocentre was validated using the megavoltage imager controller at two orthogonal gantry angles. The full PPS was in place to simulate the treatment situation. The 5x5 cm² beams were delivered automatically in the step-and-shot mode with 100 MU in each segment,at 214 gantry angles in step of 1º in the range 90º to 270º and in 5º steps outside this range. The measurements were repeated after rotating the phantom 180º around its cylindrical axis to cancel out asymmetry in the Farmer response.

The data were normalised to gantry 90º and the effect of the cryostat was removed by dividing the data with the cryostat characterisation measurements. A minor sinusoidal effect from residual misalignment was extracted from the measured doses using D’=(D+a×sin ϕ)/k with a and k being fitting parameters to ensure that D’(90º)=D’(270º) and ϕ is the gantry angle. Eight measurements were carried out on the UAS Linac and two at the OUH Linac and the mean attenuation is shown in Figure 2. The differences in attenuation between the two PPSs are generally within 0.5%. The largest differences are 1% and can be seen around angles 115º and 245º where the attenuation pattern is very complex.

This work shows that the two MR-Linac PPSs have only minor differences in attenuation. Differences of around 1% was observed where the attenuation pattern is complex. The peak attenuation is at gantry angles at 122º and 238º degrees. The strong gradients in the attenuation may contribute to dose delivery uncertainties in the situation of a patient misalignment. An evaluation of the TPS against the measured PPS attenuation is the next step to quantify the uncertainties in dose deliveries to patients irradiated through the PPS.