2174 - Dosimetric Evaluation of Proton Arc Therapy for Chest Wall Radiation in Patients with Tissue Expanders

Purpose/Objective(s): This study aims to evaluate the feasibility and potential benefits of proton arc therapy for chest wall radiation in patients with tissue expanders by comparing dosimetric values between a conventional three-field IMPT approach and proton arc-based delivery.

Materials/Methods: Five retrospective chest wall cases (Rx=50.4Gy in 28 fx) with single port tissue expanders were planned using dynamic arc in treatment planning system as well as with a traditional three-field IMPT approach for an IBA P-Plus system. One dynamic arc with a path of 150 degrees and a final energy layer count of 200 was generated for each case. For the three-field IMPT approach, beam contributions were limited to 70% per field around the port, and SFO in other regions of the target. All cases used a 7.5 cm range shifter and were robustly optimized and evaluated (setup uncertainty= 5mm, range uncertainty=3.5%) using Monte Carlo v5.5 (0.5% uncertainty). The conventional cases were normalized to match the 95% coverage dose metric for the chest wall target on the arc plans. Volumetric data was extracted for organs at risk and for the CTV CW Opti structure, which is the CTV CW minus expander port plus 5mm.

Results: Proton arc allowed extra dose behind the expander port as shown by the increase in coverage of the CTV CW Opti structure, shifting from an average of 94% to 98% coverage at 47.88 Gy. Using the arc delivery, on average, the LAD D0.03cc decreased from 14 to 9 Gy, the skin rind V50.4Gy decreased from 2.4% to 0.6%, and the contralateral breast V3Gy decreased from 6% to 3.6%. Conversely the average heart mean increased from 1.4 to 1.6 Gy as well as the ipsilateral lung V20Gy from 17.1% to 19.6%. Increased lung dose may be related to the angle trajectory chosen for the arc plan.

Conclusion: Proton arc yielded improved coverage behind the port with increases in lung and heart metrics and reductions in skin rind, LAD, and contralateral breast dose metrics. Future work will focus on evaluating LET_d-weighted dose distributions, RBE evaluations, and delivery time comparisons between dynamic arc and conventional methods.