Abstract
Intensity modulated particle therapy (IMPT) with carbon ions can generate highly conformal treatment plans; however, IMPT is limited in robustness against range and positioning uncertainty. This is particularly true for moving targets, even though all motion states of a 4DCT are considered in 4D-IMPT. Here, we expand 4D-IMPT to include robust non-linear RBE-weighted optimization to explore its potential in improving plan robustness and sparing critical organs. In this study, robust 4D-optimization—based on worst-case optimization on 9 scenarios—was compared to conventional 4D-optimization with PTV margins using 4D dose calculation and robustness analysis for 21 uncertainty scenarios. Slice-by-slice rescanning was used for motion mitigation. Both 4D-optimization strategies were tested on a cohort of 8 multi-lesion lung cancer patients with the goal of prioritizing OAR sparing in a hypofractionated treatment plan. Planning objectives were to keep the OAR volume doses below corresponding limits while simultaneously achieve CTV coverage with D95% ≥ 95 %. For the conventional plans, average D95% was at 98.7% which fulfilled the target objective in 83.2% of scenarios. For the robust plans, average D95% was reduced to 97.6% which still fulfilled the target objective in 80.7% of cases, but led to significantly improved overall OAR sparing: Volume doses were below the limits in 96.2% of cases for the conventional and 99.5% for the robust plans. When considering the particularly critical smaller airways only, fulfillment rates could be increased from 76.2% to 96% for the robust plans. This study has shown that plan robustness of 4D-IMPT could be improved by using robust 4D-optimization, offering greater control over uncertainties in the actual delivered dose. In some cases, this required sacrificing target coverage for the benefit of better OAR sparing.
Highlights
Advanced stage lung cancer treatment remains a challenge in spite of the relative success of stereotactic hypofractionated radiotherapy for non-small cell lung cancer (NSCLC) (Baumann et al 2001, Kong et al 2005, Rosenzweig et al 2005, Fakiris et al 2009, Grutters et al 2010, Greco et al 2011)
When a sufficient compromise could not be achieved, OAR sparing was favored over target coverage
The axial view of the resulting dose distribution can be seen in figures 1(a) and 1(c), where for both the conventional and robust treatment plan, the dose distributions showed a cold spot in target coverage for the medial area of the left CTV, which will further be referred to as ‘low dose area’
Summary
Advanced stage lung cancer treatment remains a challenge in spite of the relative success of stereotactic hypofractionated radiotherapy for non-small cell lung cancer (NSCLC) (Baumann et al 2001, Kong et al 2005, Rosenzweig et al 2005, Fakiris et al 2009, Grutters et al 2010, Greco et al 2011). In a recent simulation study (Anderle et al 2018), we have shown that scanned carbon ion therapy can maintain or increase the dose to a tumor while significantly reducing the radiation exposure of critical organs In this previous study, while different motion patterns were assessed, only nominal target conditions were analyzed without considering further uncertainties. We introduce robust 4D-optimization of carbon ion treatment plans with RBE-weighted (relative biological effectiveness) doses and test it on a cohort of complex patients with stage IV NSCLC and multiple lesions in both lung lobes This was assessed by performing a robustness analysis (RA) on a variety of uncertainty scenarios in order to test the following hypotheses: 1. This was assessed by performing a robustness analysis (RA) on a variety of uncertainty scenarios in order to test the following hypotheses: 1. Robust 4D-optimization decreases the variation of both target coverage and OAR exposure over multiple uncertainty scenarios, including those not contained within the optimization
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