Room: Exhibit Hall | Forum 7
Purpose: To investigate dosimetric accuracy of RayStation Monte Carlo (MC) for dose perturbation by implanted marker in proton pencil-beam scanning (PBS).
Methods: In this study, three types of markers were tested: Visicoil (gold; 0.35(diameter)mmÃ—10(length)mm, 0.5mmÃ—10mm, 0.75mmÃ—10mm, and 1.1mmÃ—10mm), BiomarC Pro (carbon-coated zirconium; 0.9mmÃ—5mm and 1mmÃ—10mm) and BioZorb (six titanium marker clips in a frame). The markers were placed on 16-cm-thick solid water with buildup of a 3-cm-thick flexible bolus and a 3-cm-thick solid water for CT scan. Markers were contoured to match the exact physical size and proper materials were assigned for markers and surrounding medium. Plans were generated with an anterior 10Ã—10cmÂ² beam (1681 spots, 1 MU/spot, and 0.25cm spot spacing). Dose was calculated with clinically-used 2-mm grid size using RayStation MC (version 8A) commissioned for a Mevion S250i with Hyperscan PBS system using two different energies (E1=70 MeV (~4cm range) and E2=135 MeV (~13cm range) at the marker depth). For dose measurement, Gafchromic EBT3 film was placed at 0, 1, 2, 3, 4cm downstream of the markers.
Results: In film measurement, Visicoils produced -21.5% (1.1mmÃ—10mm; 1cm depth at E1) to +3% (0.75mmÃ—10mm; 3cm depth at E1) dose shadowing. For BiomarC Pro, it was -7.6% (1 cm depth at E1) to 0% (0cm depth at E1) and for BioZorb it was -56.5% due to range pullback (4cm depth at E1) to -2.9% (4cm depth at E2). RayStation MC predicted dose well for Biozorb within 2% for both E1 and E2. For BiomarC Pro, the dose shadowing was well predicted for E2, while the dose difference was higher (up to 7%) for E1. For Visicoil, RayStation MC highly under-estimated dose shadowing especially for E1.
Conclusion: RayStation MC generally underestimated dose perturbation for the markers. Mitigation (i.e., better contouring and multiple angles) should be applied for accurate dose calculation.