Circulating Tumor Cells (CTC) in Lung Cancer

This study is investigating whether cancer cells found floating in the bloodstream — called circulating tumor cells (CTCs) — can serve as a reliable monitoring tool for patients with advanced lung cancer, potentially providing important information about how the cancer is behaving without requiring invasive biopsies. **You may be eligible if...** - You have been diagnosed with lung cancer — either non-small cell lung cancer (NSCLC) or small cell lung cancer (SCLC) - Your cancer is at an advanced stage (Stage 4 NSCLC or extensive-stage SCLC) - You have newly diagnosed advanced disease and have not yet started systemic treatment (chemotherapy, immunotherapy, or targeted therapy) for advanced disease - If you received prior chemotherapy or combined chemo-radiation for earlier stage disease, it was completed at least 3 months ago - You are 18 years or older and can sign an informed consent form - All adults regardless of gender, race, or ethnic background are welcome **You may NOT be eligible if...** - You have already started systemic treatment for advanced lung cancer - You are under 18 years of age - You are unable or unwilling to sign the consent form Talk to your doctor to see if this trial is right for you.