Differences in Rate of Decline in CT-defined Skeletal Muscle Mass and Physical Performance in Patients With Advanced Non-small Cell Lung Cancer Receiving Chemotherapy and Targeted Therapy/Immunotherapy, Before and After Treatment.

This study tracks changes in muscle mass and physical performance in people with advanced non-small cell lung cancer (NSCLC) who are receiving chemotherapy, targeted therapy, or immunotherapy. Muscle loss (called sarcopenia or cachexia) is common in cancer patients and can affect treatment outcomes and quality of life. Researchers want to understand how different treatments affect muscle tissue over time. **You may be eligible if...** - You are over 18 years old - You have advanced non-small cell lung cancer (stage IIIB, IIIC, or IV) confirmed by biopsy - You are currently receiving chemotherapy, targeted therapy, or immunotherapy **You may NOT be eligible if...** - You are unable to have a CT scan - Your cancer has spread to the L3 vertebral level (a specific part of the spine used for measurement) - You cannot perform a basic physical performance test - You have another primary cancer - You have had definitive surgery for your lung cancer - You are pregnant - You have physical disabilities or neurological conditions limiting your movement Talk to your doctor to see if this trial is right for you.