Robotically Guided Prostatic Fiducial Marker Insertion Under Direct MRI Guidance: A Proof of Concept in a Live Canine Subject

Materials/Methods: We calculated VHEE treatment plans for a lung cancer case using the EGSnrc/DOSxyznrc code implemented in an inhouse MATLAB based GUI developed by our group. We evaluated the use of beam energies: 80 MeV, 100 MeV, and 120 MeV; number of equidistant beams: 16 or 32; and beamlets size: 3 mm, 5 mm or 7 mm. Inverseplanning optimization was performed in a research version of a commercially available treatment planning software using identical objective functions and constraints for all VHEE plans. We used the corresponding clinical 6MV photon VMAT, calculated in an Eclipse Treatment Planning System, for comparison purposes. The prescription dose (Dp) was 5400 cGy to 95% of the 99 cm planning target volume (PTV). We calculated the conformity index defined as the ratio of the 50% (CI50) and 100% (CI100) isodose volume to the PTV volume. Results: PTV coverage was matched and indexes CI50 and CI100 were equivalent for all the VHEE plans. Average dose to organs at risk (OARs) was equivalent for the 100 MeV and 120 MeV plans. The 80 MeV plan showed up to 8% higher average doses than the 100 MeV plan. Based in these results we selected 100MeV as optimum energy. The study to determine the optimum number of beams (16 or 32) showed a mean difference in average dose of 2.4% (0%-7.7%) between the two plans. For simplicity and due to the small differences found we chose 16 beams as the optimum number of beams. The study of the beamlet size showed that a 3 mm beamlet size systematically reduces the dose to all the OARs. Based on these results we selected the 100 MeV-16 beams-3 mm beamlet size as our best VHEE plan to compare it against VMAT. Average Doses were lower for the VHEE plan. The larger differences were found in the low dose region (<25%Dp). Heart (125% lower dose) and Trachea (177% lower dose) were the structures that benefited the most from using VHEE. Average dose to the body was 11% higher for VMAT. VHEE plan was more conformal than VMAT (CI50 index was 14% higher for the VMAT). Conclusions: Using 100 MeV-16 beams-3 mm beamlet size of VHEE for the planning of a lung cancer case provided the highest dose sparing of OARs, improving the actually delivered VMAT plan. These parameters could be used for other targets with similar size, shape, and location. For larger targets a larger beamlet size such as 5mm might be used without significantly increasing the dose. Author Disclosure: B.A. Palma: None. M. Bazalova: None. H. Bjorn: A. Employee; RaySearch Americas. E. Hynning: A. Employee; RaySearch Americas. B. Qu: None. P.G. Maxim: E. Research Grant; Varian, RaySearch. F. Honoraria; Varian. B.W. Loo: E. Research Grant; Varian, RaySearch. F. Honoraria; Varian.