The goal of this observational study is to learn about the effects of the timing of radiation therapy and immunotherapy in adults with non-small cell lung cancer (NSCLC) that has spread to the brain. The main questions it aims to answer are:

1. Does the timing of the two treatments change how long the brain tumor stays stable and how long participants live?
2. What medical problems do participants have when receiving these treatments at different times?
3. How does the timing of treatments affect the body's immune system?

Researchers will compare participants who receive radiation and immunotherapy 30 days or less apart to those who receive them more than 30 days apart to see if the timing affects the treatment's success and safety.

Participants already receiving radiation and immunotherapy as part of their regular medical care will:

1. Allow researchers to collect information about their treatment, health, and medical imaging during regular checkups.
2. Give a small blood sample during their routine blood draws.
3. Have standard magnetic resonance imaging (MRI) scans of their brain.

Inclusion Criteria:

• Age ≥ 18 years; no gender restriction;
• Histologically or cytologically confirmed NSCLC, with brain metastases confirmed by contrast-enhanced cranial MRI;
• Scheduled to receive radiotherapy combined with a PD-1/PD-L1 inhibitor, in accordance with real-world clinical treatment plans;
• Negative for driver gene mutations, or positive for mutations but with documented failure of prior targeted therapy;
• ECOG Performance Status score of 0-2, with an estimated life expectancy of ≥ 3 months;
• Voluntarily signs the informed consent form and agrees to cooperate with blood/imaging data collection and follow-up procedures.

Exclusion Criteria:

• History of whole-brain radiotherapy, stereotactic radiotherapy for brain metastases, or brain surgery;
• Presence of contraindications to MRI or inability to tolerate gadolinium-based contrast agents (e.g., severe hepatic or renal insufficiency);
• Presence of active autoimmune disease requiring systemic treatment, or requirement for long-term use of high-dose immunosuppressive agents;
• Pregnant or lactating women;
• Other circumstances deemed by the investigator to involve severe complications or render the patient unsuitable for enrollment.

The study population comprises adult patients (aged 18 years and older) histologically or cytologically diagnosed with non-small cell lung cancer (NSCLC) accompanied by brain metastases, confirmed via contrast-enhanced head MRI. These patients are seeking treatment at Xiangya Hospital and are scheduled to receive a combination of radiotherapy and PD-1/PD-L1 immune checkpoint inhibitors according to their routine, real-world clinical care plans. The cohort specifically represents a real-world NSCLC population with relatively good performance status (ECOG 0-2), intact local brain anatomy (no prior history of whole-brain radiotherapy, stereotactic radiosurgery, or brain surgery), and who are negative for actionable driver mutations or have experienced disease progression following prior targeted therapies.

IPD will not be made publicly available on open-access platforms to safeguard patient privacy. However, de-identified and aggregated datasets, or specific subsets of IPD underlying the published results, might be shared conditionally. Such data will only be provided upon reasonable request from qualified academic researchers, subject to the approval of the Institutional Review Board (IRB) of Xiangya Hospital and the execution of a formal Data Use Agreement (DUA).