Session Item

Some MR-linacs have the ability to perform gated treatments based on continuous tracking on 2D images acquired during dose delivery (ciné). This potentially reduces the treatment margins required for tumor coverage, but at the same time adds complexity to the dosimetric QA. To characterize the gating performance of a MR-linac with respect to dose, we have developed a dosimetry system with dose-per-pulse time resolution that can be used without distorting the MR images. The system is based on one or more plastic scintillation point detectors (PSD) coupled to optical PMMA fibers. This all-plastic system has a high degree of water equivalence. In this work, the system was applied on the Viewray MRIdian 0.35 T MR-linac and was able to identify a systematic transient in dose rate at the start of every beam-on period. 

The PSD in this study was a BCF-60 scintillator coupled to a 15 m long 1 mm diameter PMMA optical fiber connected to the readout equipment (ME40, DTU). The PSD was placed in a water-filled plastic tube that was inserted into a dynamic MRI compatible phantom (CIRS, US). To avoid large dose gradients near the penumbra region, the PSD was placed at the beam axis and irradiated with a 20 cm x 14 cm field. The piston of the CIRS phantom performed a sinusoidal movement with a period of 4 s and an amplitude of 12.83 mm. The gating window boundary was set ± 5 mm from the center of the sinusoidal movement. Figure [1] shows the three different gating phases of a half sinusoidal cycle (the piston position in the lower panel corresponds the red part of the motion curve).

 Figure [1]

Due to the high time resolution of the detector system, it was possible to identify two different transient (warm-up) effects in the MRIdian. The first was a reduced dose rate during the initial part of every gating event when the beam turned on. The reductions were up to 10 % of the final (stable) dose rate. The transients gradually decreased with time and vanished completely after 1.5 s or less.  The second effect was a slow but systematic increase of the measured dose rate, at the reference point, of about 2% throughout the whole treatment (figure [2]). Finally, a latency of the gating mechanism was observed resulting in dose delivery even when the piston was outside the gating area.  This most likely reflected the limited frame rate (4 fps) used in this treatment.

Figure [2]

The dose-per-pulse scintillator system revealed temporal effects in the MRIdian beam gating system using. The most significant being the systematic transient each time the beam was turned on (up to 10% reduction compared to the final dose rate). It is unknown whether this phenomenon is specific for the accelerator tested. Furthermore, the clinical relevance of these effects is not known and are likely depending on the exact gating procedure. For challenging cases with low duty cycle it should be considered to what extend warm-up or latency effects could perturb the delivered dose.