This study is being done to find out whether features of the thymus gland can predict a common side effect of radiation therapy for lung cancer called radiation-induced lymphopenia.

We will include adults with stage III non-small cell lung cancer (NSCLC) who are scheduled to receive curative radiation therapy to the chest, with or without chemotherapy.

Before, during, and after radiation therapy, we will measure:

The dose of radiation that reaches the thymus gland (based on each patient's treatment plan)

The size and shape of the thymus gland (using CT scans taken for radiation planning)

The number of lymphocytes (a type of immune cell) in routine blood samples

The main goal is to see if certain radiation doses to the thymus, or a smaller thymus size, can predict severe lymphopenia. We will also check whether lymphopenia affects patient outcomes such as infections or survival.

This is an observational study (no added treatment or drug) - patients will receive their standard radiation therapy as planned by their doctor. The study only collects and analyzes data from standard scans and blood tests.

Potential benefits: Findings may help doctors design "immune-sparing" radiation plans in the future to reduce lymphopenia. Potential risks: There are no additional risks beyond standard radiation therapy, as this study does not change your treatment.

Inclusion Criteria:

• Age ≥ 18 years;
• Pathologically or cytologically confirmed primary non-small cell lung cancer (NSCLC) with a clinical stage of III (according to the 8th edition or latest version of the AJCC staging system);
• Eligible for radical radiotherapy or chemoradiotherapy to the chest, with a planned total dose of ≥50 Gy to the chest, using a fractionation schedule of 1.8-2.0 Gy per fraction (conventional fractionation) or a high-dose fractionation regimen permitted by the protocol;
• Planned to undergo periodic blood tests before and after radiotherapy, including absolute lymphocyte count;
• ECOG performance status of 0-2;
• Able to undergo regular follow-up and provide blood samples and imaging evaluations as required by the protocol.

Exclusion Criteria:

• Previous history of thoracic or whole-body radiation therapy;
• A diagnosis of radiation-induced lymphocytopenia or baseline lymphocytopenia graded ≥ Grade 2 (according to the CTCAE criteria) prior to study enrollment;
• Patients with a history of or known primary immunodeficiency, active autoimmune disease, long-term systemic glucocorticoid therapy (prednisone-equivalent dose > 10 mg/day), or long-term use of immunosuppressants following organ transplantation;
• History of concurrent malignancy (excluding basal cell carcinoma of the skin or cervical carcinoma in situ that has been treated curatively and has been recurrence-free for at least 5 years);
• Pregnant or lactating women;
• Any other condition deemed by the investigator to render the subject unsuitable for participation in this clinical study, including severe hepatic, renal, hematologic, or other organ dysfunction;
• Irreversible contraindications to the radiotherapy plan or dose delivery.

This observational study will enroll adult patients with stage III non-small cell lung cancer (NSCLC) who are scheduled to receive definitive thoracic radiotherapy (with or without concurrent/sequential chemotherapy) at participating oncology centers.

Patients are identified consecutively from radiation oncology departments during routine pre-radiotherapy planning visits. No experimental treatment or randomization is applied. The study population includes both treatment-naïve patients and those who may have received prior systemic therapy as long as they have not undergone prior thoracic radiotherapy or have persistent grade ≥3 lymphopenia at baseline.

The target sample size is determined based on event-per-variable criteria. Both inpatients and outpatients are eligible. Recruitment is performed prospectively over an estimated period of 18-24 months. All enrolled patients must meet the full eligibilit.