Implementation of Object Spot Avoidance in Proton Pencil Beam Treatment on Whole Breast with Implant Metal Injector Peng Wang, PhD, DABR, Karla Leach, BS, CMD Texas Center for Proton Therapy-Irving, TX Introduction Proton pencil beam scanning helps reduce unnecessary dose spillage to the lungs and heart when treating left breast cancer patients. Special challenges exist when treating a patient with a breast implant that has a metal injector (Fig.1), since the treatment planning system (TPS) may not accurately calculate the dose from the scattered protons by the metal object. In this study, we present an object spot avoidance technique in the planning process to avoid proton spots traversing, or stopping in the metal injector while maintaining acceptable coverage of the target. Material and Methods A 59 year old female with surgical resection, bilateral mastectomy and expander placement was accepted for proton treatment of the chest wall, axillary and supraclavicular lymph nodes in our clinic. Three fields with gantry 335o, 35o and 95o were used for treatment. The fields at 335o and 35o cover the entire target which is about 25cm long in the superior to inferior aspect and 17cm wide. To avoid extra dose to the arm, the superior border of the field at 95o stops in the axilla region. To cover the target in shallow depth, a range shifter with thickness of 4.0 cm is used for each field. The metal injector dimensions were obtained and contoured on the CT (Fig.2a). Then a 0.5 cm uniform expansion from the metal injector was set as a “spot avoidance contour” (Fig.2b). When defining a spot avoidance, the TPS will prevent proton spots from stopping in or traversing the spot avoidance contour. To achieve coverage around the implant, the PTV in the region around the implant was treated using multi field optimization (MFO), where each beam has a unique dose distribution. Single field optimization (SFO), where each treatment field covers the target with the total dose divided by the number of beams used, is considered more robust and is utilized whenever possible. The PTV regions above and below the implant were treated with SFO. The plan was evaluated by intentionally moving the location of the isocenter +/-0.3 cm in x/y/z direction to simulate setup uncertainty and an additional +/-4% to account for range uncertainty at each isocenter location. For all of the scenarios, no proton spots stop in or traverse through the metal injector contour, and the target dose coverage, as well as the dose received by organs at risk, are within the physician’s specified ranges. The plan passes our patient-specific QA test. Conclusions By implementing the spot avoidance function during the planning process, there are no proton spots stopping in or traversing the spot avoidance contour, which includes a 0.5 cm uniform expansion from the metal injector. Therefore, the possibility of an inaccurate dose calculation of the scattered proton spots due to the metal injector is eliminated. The proton spot avoidance function can also be used in other situations, such as protecting critical organs at risk. The PTV was copied and divided into sections to help control the dose distribution in the areas treated with SFO. The SFO regions did not start until 2cm above and below the implant (Fig.3a) to give adequate room for a transition zone in the MFO and SFO dose distributions. A second SFO region was created for the region above the axilla to control the dose distributions for the 335oand 35o beam angles. Each SFO region was separated with a 2cm gap to give a smooth gradient or transition zone between the two areas (Fig.3b). To avoid errors in the dose calculation from the pencil beam algorithm, the Monte Carlo algorithm was used in both optimization and final dose calculation. Uncertainty of the Monte Carlo algorithm was set to 10000 ions/spot and 1.0% for optimization and final dose calculation respectively. A uniform robustness setting of +/- 0.3 cm shifts in x/y/z directions around isocenter and 4% range uncertainty were used during optimization. a b Fig. 3a Total PTV: pink, SFO PTV: green, distance between metal and SFO PTV measured 2cm Fig. 3b Total PTV: pink, SFO PTV: green, SFO PTV2: blue, distance between SFO PTV’s measured 2cm Results The beam angles that were used allowed adequate coverage of the target around the metal injector (Fig.4). At least two beams contribute dose to every region of the PTV. This spreads out the RBE on the distal end of each beam. There is a uniform dose distribution between the beams in the SFO regions and the dose is sculpted around the implant in the MFO region (Fig.5). Fig. 1 Implant with metal injector Fig. 2a Dimensions and material of the metal injector Fig. 2b Metal injector in yellow, Spot avoidance contour in red Fig. 4 Final dose distribution in axial, saggital and coronal views Fig. 5 Top view: SFO distribution for each individual beam Bottom view: MFO distribution for each individual beam around the implant a b