The "Stage IIIA" classification is the dramatic sweet spot. It is the precipice. It isn't localized enough to be easily cut out (Stage I or II), but it hasn't yet metastasized wildly to distant organs like the brain or bones (Stage IV). It is the diagnosis of "probable doom with a sliver of hope."

The tumor is considered "inoperable" at the start, meaning surgery is not an immediate option because of the location or size of the mass.

The cancer has spread to the lymph nodes on the same side of the chest as the primary tumor.

In the pilot, the tumor is deemed inoperable due to its location and the involvement of lymph nodes. This sentence— inoperable —is what drives Walt to theRV. With nothing to lose, he chooses to break bad.

In a twisted bit of narrative irony, the treatment works too well. Walt undergoes a risky surgery (a pneumonectomy or lobectomy) and enters remission. The audience expects a cancer drama where the disease kills the protagonist. Instead, the disease creates a monster, and then recedes, leaving the monster to rampage unchecked. The cancer didn't kill Walter White; it just cleared the way for Heisenberg.