Why do we care about lymph nodes when staging lung cancer?
The simple answer is that they give us prognostic information (insight into the likely future behavior, the “natural history”, of the cancer). When you think of a cancer’s growth, there are a few potential ways for it to progress. It might progress right where it started, staying local. It might travel through the lymphatic system, kind of like debris swept off a roof and into a home’s gutter, and it might spread distantly to other parts of the body. We’re interested in getting a sense of how aggressive a cancer is likely to be overall, the engine behind its growth, but also whether it’s more likely to stay local or travel distantly.
The staging system for lung cancer and most others is called the TNM system, where T stands for Tumor stage (the primary cancer, where it started), N stands for Nodal stage (are lymph nodes involved, and if so, where?), and M stands for Metastasis stage (distant metastases, yes or no?). The M stage overrides the others — if a cancer has spread to distant parts of the body, it’s an advanced cancer, no matter how big or small the primary tumor or whether there are lymph nodes involved or not. But N stage is also quite important.
Essentially, the distance from the primary cancer to any involved lymph nodes, along with the number of lymph nodes that have cancer within them, help predict the probability that the cancer is able to get out of the area in which it started and spread to distant parts of the body, where cure is generally not something we can expect to achieve. Lymph nodes within the same lung as the primary cancer are called N1, and those in the mid-chest (mediastinum) between the lungs are called N2 if on the same side as the primary tumor, or N3 if on the other side or above the collarbone (lymph nodes outside of the chest are unfortunately considered metastatic sites, because they almost always occur in the setting of more widespread progression).
This numeric staging reflects the distance the cancer had to travel to get there, so higher number for N stage means greater distance and greater chance of spread to other parts of the body, and the stage is often dictated by the highest N stage seen, unless there is distant metastatic disease (which, as noted above, overrides other considerations and denotes metastatic/advanced cancer).
Basically, nodal status assesses the probability of a cancer to travel further, even if you don’t see evidence it has metastasized yet. It’s kind of like when my youngest son had wandered out onto the driveway after figuring out how to unlock the front door, open it, and walk outside. He didn’t quite wander into the street, but he had clearly shown that he had the skills to escape. Imagine that N1 nodal involvement is like finding him having unlocked the door, N2 is like opening it and starting to look outside, and N3 is like him walking down the driveway toward the street. (He is now 7 and at far more risk of video-game induced brain rot.) Continue reading