Abstract
Radiotherapy (RT) plays an important role in the curative treatment of lung cancer; however, longitudinal changes in regional pulmonary function after treatment remain incompletely understood. In this case report, we present a single-patient, hypothesis-generating analysis evaluating the association between lung dose and regional functional changes using deformable image registration (DIR)-based four-dimensional computed tomography (4DCT) ventilation imaging (Jacobian-derived) obtained at three time points: pre-RT, six months post-RT, and one year post-RT. An 83-year-old male with clinical stage IIIA right upper lobe lung cancer underwent definitive chemoradiotherapy. Consolidation durvalumab was initiated post-treatment, and the patient developed pneumonitis consistent with radiation-induced lung injury (Common Terminology Criteria for Adverse Events (CTCAE) grade 2), which was managed with oral prednisolone. SpO₂ was 94% on room air, and oxygen therapy was not required. Dynamic analysis of 4DCT images showed that the volume of the highly irradiated right upper lobe decreased over time, while the rate of contraction increased. The volume of the non-irradiated left lower lobe increased by 11% at one year post-RT, whereas the rate of contraction decreased slightly. 4DCT-derived ventilation suggested a dose-related decline in ventilation that persisted at one year, including within low-dose regions. This single-case observation supports the potential value of integrating functional information with anatomic CT findings during post-RT follow-up and in efforts to preserve functional lung, while acknowledging the limitations related to a single patient, DIR-dependent ventilation estimates, and the absence of post-treatment pulmonary function testing.