Adjuvant Pembrolizumab vs Observation Following Curative Resection for Stage I Non-small Cell Lung Cancer (NSCLC) With Primary Tumors Between 1-4 cm

This study is testing whether pembrolizumab (an immunotherapy drug) given after surgery can prevent early-stage lung cancer from coming back, compared to no additional treatment (observation only), in patients with stage I NSCLC whose tumor was between 1 and 4 cm. **You may be eligible if...** - You are at least 18 years old - You have had complete surgical removal of stage I NSCLC between 4 and 12 weeks ago - Your tumor was between 1 and 4 cm in size - Your surgical margins were clear (no cancer at the edges) - You are in good physical condition (ECOG performance status 0 or 1) - Your organ function is within normal ranges **You may NOT be eligible if...** - You have active autoimmune disease requiring treatment - You are on immune-suppressing medications - You have had radiation therapy to the chest - You are pregnant or breastfeeding - You have had another cancer in the past 3 years (with some exceptions) Talk to your doctor to see if this trial is right for you.