Improving reirradiation of recurrent non-small cell lung cancer through non-coplanar beam arrangements.

Background and purpose: Reirradiation for non-small cell lung cancer (NSCLC) is commonly delivered using coplanar techniques. In this study, we investigated whether the selection of favorable non-coplanar beam orientations may limit cumulative doses to critical organs-at-risk (OARs) and thus improve the therapeutic window. Materials and methods: Fifteen cases of challenging high-dose reirradiation for locoregionally recurrent NSCLC were included in this in-silico study. For each patient, the dose distribution from the previous treatment was first mapped to the new patient anatomy using rigid dose registration, and subsequently converted to equivalent dose in 2 Gy fractions (EQD2). A two-arc non-coplanar reirradiation plan was then generated using an EQD2-based direct aperture optimization algorithm, which allows for the simultaneous optimization of dynamic gantry-couch paths and the cumulative EQD2 distribution. Non-coplanar reirradiation plans were benchmarked against two-arc coplanar plans. Results: Considerable reductions of at least 5 Gy in the maximum cumulative EQD2 to critical organs were achieved in 6 out of 15 patients using non-coplanar versus coplanar arcs. In particular, the maximum cumulative EQD2 was reduced by up to -9.0 Gy for the bronchial tree, -5.8 Gy for the esophagus, -5.3 Gy for the trachea and -5.6 Gy for the great vessel. At the same time, target coverage and lung EQD2 metrics were comparable for both methods. Conclusions: The automated selection of favorable non-coplanar beam orientations may reduce the maximum cumulative EQD2 to critical OARs in challenging thoracic reirradiation cases. This allows to explore either better OAR sparing or dose-escalation in future clinical studies.