This prospective, single-center, single-arm study will evaluate the feasibility and safety of 4DCT ventilation functional imaging-guided functional lung avoidance radiotherapy (FLAR) in patients with lung malignancies receiving IMRT radiotherapy. All participants will undergo 4DCT simulation as part of routine radiotherapy preparation. A ventilation map will be generated from 4DCT data, and the top 80% ventilation region will be defined as the high-function lung. This structure will be imported into the treatment planning system to create an FLAR plan that prioritizes sparing of high-function lung while maintaining target coverage (PTV D95%) and meeting standard dose constraints for organs at risk. A conventional anatomic plan (without functional guidance) will also be created for paired, within-patient dosimetric comparison. The primary outcome is improvement in dosimetric sparing of the high-function lung (V10, V20, V30, and mean lung dose). Secondary outcomes include the incidence of grade ≥2 radiation pneumonitis (CTCAE v5.0), changes in pulmonary function (e.g., FEV1 and DLCO), and lung-related quality-of-life scores. Assessments will be performed mid-treatment (after 15 fractions), at the end of radiotherapy (after 30 fractions), and at 1, 3, 6, and 12 months after radiotherapy. The study plans to enroll 100 participants and follow each participant for 12 months.