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High precision SBRT delivery is possible using MR-guided radiotherapy (MRgRT) with real-time intra-fraction tumor motion monitoring. We implemented a one stop shop (OSS) SBRT service at our department for patients with lung tumors, and report on the different steps and main findings during treatment delivery.

Seven patients with lung cancer underwent an OSS procedure delivering a single fraction dose of either 30  or 34 Gy. Median GTV was 1.7 cm³ (range 1.3 – 22.9). The workflow consisted of four consecutive steps: consultation, simulation, treatment planning and delivery (Figure 1A). The simulation session at the MR-linac included acquisition of MR-planar image acquisitions in sagittal, coronal and axial planes, thereby allowing for individualized assessment of tumor motion for each patient. Tumor contour tracking was also assessed during the simulation procedure, and the most suitable breath-hold phase for treatment was identified.

SBRT plans were generated using step-and-shoot IMRT. A gating boundary of 3mm, and a PTV margin of 5mm around the GTV, were used to account for system-latency during delivery. SBRT was delivered during repeated breath-holds, and during gating 10% of the GTV was permitted to fall outside the tracking boundary before triggering a beam-off. An analysis of the accuracy of SBRT delivery for each individual patient was performed to assess the reproducibility of the GTV position inside the PTV, the effect of system-latency and the duty-cycle.

All 7 patients completed the OSS procedure in a single day. Five patients were treated in inspiration breath-hold; and 2 in expiration as this led to a more stable tumor trajectory during breathing. Figure 1A provides a schematic overview of the OSS workflow. The time needed to complete the OSS procedure decreased ca. 50% from that of the first patient and it could be completed in a half-day for the last patients.

Figure 1B shows a heat map indicating the real-time position of the GTV during treatment delivery for a single patient. During beam-on, the 3 mm tracking boundary encompassed the GTV by 78.4 – 100% across all patients, with the corresponding values for the PTV being 94.4 – 100% (2^th – 98^th percentiles). On average, system-latency due to gating decisions was involved in 17.4% of the treatment delivery time, of which 6.2% were directly related to triggering a beam-off after a gating-off decision. Our use of a tracking boundary inside the PTV minimized the overall impact of system-latency on GTV coverage to only -0.3% on average. Duty-cycle efficiency during treatment delivery ranged from 31.1 – 64.7%.

An MR-linac OSS SBRT service for lung tumors was successfully implemented, using a dedicated simulation procedure and gated breath-hold delivery. All patients completed same-day SBRT delivery and it can now be completed in a half-day. Use of a tracking boundary of 3mm inside the PTV minimized the impact of system-latency on GTV coverage.