Robustness Evaluation for Proton Therapy with a Spirometry Based Deep Inhalation Breath Hold System

Abstract

<h3>Purpose/Objective(s)</h3> Deep inhalation breath hold (DIBH) treatment allows for greater sparing of healthy tissue by minimizing target motion due to respiration. However, the breath hold level achieved during simulation and throughout treatment is not perfectly reproducible. DIBH techniques such as spirometry define an allowed variation in the nominal breath hold level to facilitate treatment. This variation represents an uncertainty which must be incorporated into the robust evaluation which is essential in proton therapy treatment planning. The aim of this study is to evaluate the dosimetric impact of this uncertainty for DIBH patients treated with proton therapy using a spirometry system with visual feedback and to determine the ideal clinical workflow to ensure robustness. <h3>Materials/Methods</h3> A dedicated workflow involving simulation, treatment planning and quality assurance was developed and implemented in our clinic to assess the potential dosimetric impact of this uncertainty. The workflow involved a multi-scan simulation procedure in which patients were simulated under their nominal breath hold level and two additional breath hold levels at ± 100 mL tidal lung volume with respect to the nominal. This variation was chosen as it represents twice the allowed variation during treatment in order to be conservative. The additional scans were used to assess motion near the target and to determine the best techniques to be used during plan optimization to ensure robustness. In particular a 5 mm motion threshold was used by physics to recommend the use of simultaneous optimization across all three image sets. The final treatment plan was forward calculated on the additional image sets to guarantee robustness. Patient data, including on-treatment imaging and verification CT scans (QA-CT), were analyzed to assess plan robustness for 30 patients treated under this workflow over two years. <h3>Results</h3> Out of 30 DIBH patients treated under this workflow the simultaneous optimization tool was recommended in three cases but ultimately not utilized in planning at MD/dosimetry discretion balancing dose to OARs and robustness. All thirty patients passed the robust evaluation procedure and there were no cases for which QA-CT indicated a need to revise a treatment plan due to a variation in the breath hold level. <h3>Conclusion</h3> Our initial experience indicates that in proton therapy utilizing spirometry based DIBH with visual feedback the uncertainty associated with the variation in breath hold level does not have a significant dosimetric impact on plan robustness. Given these results, the decision was made to reduce the workload and simplify the workflow by eliminating the additional scans and utilizing a single breath hold level during simulation.