Functional Lung Avoidance Radiotherapy Guided by 4DCT Pulmonary Ventilation Function Imaging: A Prospective Single-arm Clinical Study

Inclusion Criteria: * Age 18 to 85 years (inclusive); any sex. * Diagnosed lung malignancy requiring thoracic radiotherapy (e.g., non-small cell lung cancer, small cell lung cancer, or pulmonary metastases). * Planned to receive thoracic radiotherapy using either conventionally fractionated IMRT/VMAT or stereotactic body radiotherapy (SBRT), and able to complete pre-treatment 4DCT simulation suitable for generating a ventilation map. * Adequate baseline organ function and clinically judged able to tolerate radiotherapy; ECOG performance status 0-1. * No absolute contraindication to pulmonary function testing; patients with moderate COPD or impaired lung function may be included if the treating team judges radiotherapy to be tolerable. * Able to comply with treatment and follow-up assessments. * Written informed consent provided. Exclusion Criteria: * Pregnant or breastfeeding women. Women of childbearing potential must have pregnancy excluded prior to radiotherapy per institutional practice. * Age \<18 or \>85 years. * Uncontrolled severe cardiopulmonary disease or other severe comorbidities that make radiotherapy excessively high risk (e.g., decompensated heart failure, unstable cardiopulmonary status, active severe pulmonary infection). * Prior high-dose thoracic radiotherapy to the same region such that safe re-irradiation is not feasible. * Concurrent other active/advanced malignancy requiring priority treatment that would confound study outcomes. * Recent participation in another interventional clinical trial or concurrent investigational therapy judged likely to interfere with this study's endpoints. * Severe psychiatric, cognitive, or other conditions that prevent cooperation with radiotherapy and/or scheduled follow-up. * Inadequate 4DCT image quality or inability to generate/validate the 4DCT ventilation map required for functional-lung-guided planning. * Any other condition deemed unsuitable by the investigators in the interest of participant safety.