Session Item

The combination of magnetic resonance imaging and a linear accelerator in MR-linacs has made it possible to online adapt treatments through daily MR images. Furthermore, some MR-linacs can dynamically turn the beam on and off (gating) based on online organ tracking. In dose per pulse measurements with organic plastic scintillators, we have previously found that the Viewray MRidian 0.35 T MR-linac has a dose transient at the start of every beam-on event, also during repeated events in gating. We have now investigated the effect of the transients on accumulated dose profiles (i.e. the total dose a point will receive) during a simulated treatment delivered under different gating conditions.

The measurement system consisted of four BCF-60 plastic scintillation detectors (PSD), 1 mm in diameter (1, 2 or 5 mm long) that were connected to two synchronized ME40 systems via 9 m (1.5 mm diameter) long PMMA optical fibers. The four PSDs were placed in a 3D printed plastic rod that fits into the moving part of a dynamic MRI compatible phantom (CIRS, US). A treatment plan was applied which had an isodose center with a diameter of 3 cm. The PSDs were placed with one in the center and the others 15, 20, and 25 mm from the center (Figure 1). The treatment was repeated with different movement patterns (sinusoidal 12.75 mm amplitude and 5 s or 10 s periods) and different gating borders (5 mm or 3 mm). A static treatment served as reference.

Furthermore, a setup with a single detector placed outside the movable piston, but beam-on was still controlled by gating was also used.  This was done to remove dose gradients due to movement.

Figure [1]

The gated treatment showed a broadening of accumulated dose profiles compared to the static setup, with a higher dose delivered in the outer penumbra region (20 mm) and a slightly lower dose in the inner penumbra region (15 mm). A simulation based on the movement of the scintillator determined that the dose transients had little effect on the broadening of the penumbra.
Furthermore, it was found that the central detector received less dose (up to 3%) for the fastest motion while being close to the static for the slowest motion. These results are shown in figure 2 along with modelled dose profiles based on the piston movement.

For the setup without detector movement, but still gated. 1% less dose was measured for the fastest motion, while again the slowest movement was close to the static. It was noted that the two inbuilt monitor chambers were not in agreement for the treatment with the fastest motion while they agreed for the static treatment.

Figure [2]

The dose transient was present every time the beam turned on but had little effect on the distribution in the penumbra region compared to the detector movement. Of more concern is the drop in dose to the center detector of up to 3%. We encourage consideration of the dosimetric and clinical relevance of these effects when new treatments are implemented.