Dosimetric Evaluation of a Novel Non-Metallic Dual Port Expander for Proton Therapy

Abstract

This study evaluates a novel non-metallic (plastic) dual port expander and clinically utilized single/dual port metallic expanders for intensity modulated proton therapy (IMPT) in order to quantify the dosimetric implications in clinical treatment plans. Four previously treated breast cancer patients (3 left sided, 1 right sided) with tissue expanders were retrospectively planned for IMPT utilizing a treatment planning system. Utilizing the existing CT data sets, models of three Sientra tissue expanders (dual port plastic (DPP), dual port metal (DPM) and single port metal (SPM)) were inserted into the 3D dataset and aligned using the existing patient anatomy. Each patient was planned to a total dose of 50.4 Gy in 28 fractions using 3 incident proton fields and Monte Carlo calculation algorithm. A 5 mm expansion was placed around the metallic portion of the expander to define a beam avoidance region which no proton spots traversed or were placed to minimize dosimetric error. Robust optimization (5mm setup uncertainty, 3.5% range uncertainty) was employed to a total target volume which comprised the CTV chest wall, IMN, axilla and supraclavicular nodes. The subsequent treatment plans were evaluated for target and OAR coverage. Due to the non-metallic construction of the DPP expander, the metallic structure avoidance region was significantly smaller at 4.4cc on average than either the SPM (45cc average) or DPM (63.5cc average). This allowed the IMPT plans to be significantly less modulated, delivering a more uniform dose to the implant volume (SPM 47.1Gy; DPM 45.2 Gy; DPP 50.9 Gy) and thus enabling these plans to be inherently more robust to setup uncertainties. The additional benefit of the reduced avoidance region size is that the hinge angle of the lateral proton beams can be reduced for DPP plans thus reducing dose to the heart and contralateral breast. Average mean heart dose across the four cases presented is 0.68 Gy, 1.03 Gy, and 0.93 Gy, for the DPP, DPM and SPM cases respectively, while average contralateral breast/CW dose is 0.38 Gy, 0.58 Gy and 0.63 Gy. The V20 for the ipsilateral lung was, on average, reduced for the DPP expander at 14.3% across the 4 cases, while 18.8% and 17.0% for the DPM and SPM cases respectively. The minimization of metal in the DPP expander allows for a significantly smaller beam avoidance region, in-turn enabling creation of a more homogeneous plan that can be potentially more robust while also reducing dose to surrounding OARs. Additional work is underway to expand the study to include a larger patient cohort, while also evaluating the impact of different robustness goals on OAR sparing.