From my assistant Claude.
Stage IV NSCLC is normally defined by distant metastasis. But one specific subtype — M1a — covers cases where cancer has spread within the thoracic cavity but not to distant organs: things like malignant pleural or pericardial effusion, or nodules in the opposite lung. That still counts as Stage IV because the disease has crossed outside the primary tumor site and lymph nodes, but it hasn’t gone systemic.