Lymph Nodes and Staging & Prognosis
When we think about the prognosis of lung cancer, it really all hinges on staging. So what are the important things for staging? One of the most important things in the T and M staging system is that middle letter, which is N. N stands for lymph nodes. Lymph nodes are tricky because they’re typically small, they can be difficult to characterize radiographically. PET scans can be helpful, but again, because of the size criteria of lymph nodes, it’s not always a perfect test for that. So what are we talking about when we think about lymph nodes? So typically a lung cancer will start somewhere in the actual tissue of the lungs as a small nodule that grows. At some point during that growth cycle, a lung cancer will metastasize to lymph nodes. There’s a usual order of which lymph nodes they hit first as they go through that process. And if we think of the tracheobronchial tree as a literal upside down tree, the trachea would be the trunk of that tree. The bigger branches come out from there, those bigger branches branch into smaller branches and so far and so forth. So if you think of the leaves of that tree being the actual lung tissue, a nodule grows in one of those leaves, the first lymph nodes that cancer will spread to, if it begins to spread, is typically at the branch point where the two branches will split. We term those our Hilar lymph nodes. They are in the lung tissue themselves, where the branches are splitting. From there, we’ll move up to mediastinal lymph nodes. Those are the ones that are going along the trachea. So your lung cancer that starts out in the periphery will again first move toward the Hilar lymph nodes, then up toward the mediastinal lymph nodes. Now obviously the mediastinum representing the central part of the chest. You’ve got a right mediastinum and a left mediastinum. If the lung cancer itself is on the right side, then the right mediastinal lymph nodes are termed ipsilateral, meaning they’re on the same side as the nodule in question. If the lymph nodes on the other side of the chest are affected, that’s termed contralateral mediastinal lymphadenopathy. That’s all very important for staging, because Hilar lymph nodes on the same side as the nodule are termed N 1 disease. Mediastinal lymph nodes that are ipsilateral to the lung nodule itself are termed N 2. And those contralateral mediastinal lymph nodes are termed N 3. As your N number goes up, essentially so does the stage. We ideally want to find these lung cancers when we have N 0, meaning that the cancer has not spread to any of the lymph nodes and still resides in the lung itself.