Stage III (locally-advanced) NSCLC is commonly divided into three sub-categories: IIIA, IIIB and IIIC, defined by how much the cancer has spread locally and the possibility of surgery. The difference between Stage III and Stage IV disease (cancer (metastasized to distant organs)) is that Stage III is treated now with curative intent.

The majority of Stage III NSCLC patients are diagnosed with unresectable tumors. It is recognized that no new treatments beyond chemo-radiation therapy, followed by active surveillance to monitor for progression, have been available to patients for decades.