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.

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.