Room: Stars at Night Ballroom 4
Purpose: Gamma knife (GK) radiosurgery is a standard for treatment of multiple brain metastases. However, some patients struggle to tolerate this lengthy, frame-based modality. Single-isocenter frameless VMAT SRS potentially improves tolerability. However, misalignment could produce target uncertainty and degrade plan quality. The impact of setup uncertainties on target coverage was investigated.
Methods: Eight patients (2-10 tumors/patient, total 45 tumors) who underwent GK radiosurgery were replanned in Eclipse with a 6X-FFF beam (1400MU/min) on Truebeam using single-isocenter VMAT. A single isocenter was placed at the approximate geometric center of all tumors. Five non-coplanar arcs were used to mimic HyperArc delivery. Prescription was 20 Gy to each tumor. Single-isocenter VMAT plans were compared to GK radiosurgery plans. For the patient with 10 brain metastases, additional plans were generated to simulate clinically realistic translational and rotational setup uncertainties in between [Â±1mm,Â±1o] and [Â±1.5mm,Â±1.5o]. Average gross tumor volume (GTV) was 0.4Â±0.3cc (0.1â€“1.1cc) and planning target volume (PTV) was 0.9Â±0.5cc (0.4â€“1.9 cc). Average isocenter to tumor distance was 8.0Â±2.1cm (3.3â€“10.7cm). Coverages for the GTV and PTV and the whole-brain V12 were evaluated and compared to the original plan.
Results: The single-isocenter technique produced better conformity index, and minimum tumor doses and V12 were comparable to that of GK radiosurgery with significantly less beam on time (p<0.001). However, setup uncertainties significantly decrease target coverage. For the patient with 10 targets, the average loss of target coverage was 15.0Â±3.5% (11.0â€“21.0%) for the GTVs and 27.0Â±3.0% (21.0â€“30%) for the PTVs.
Conclusion: Utilizing single-isocenter VMAT, radiosurgical dose distribution similar to GK radiosurgery are obtained with much faster treatment delivery. However, if set-up uncertainties are unaccounted for, even with an isocenter localization uncertainty of Â±1.0mm/1.0o could deliver clinically unacceptable SRS treatment. Further research includes correction strategies for translational and rotational setup uncertainties and evaluates the dose to surrounding critical structures.