What does the TNM staging system classify?

The TNM staging system is a widely accepted method for classifying tumors based on three key criteria: the size of the primary Tumor (T), the extent of regional lymph Node involvement (N), and the presence of distant Metastasis (M). This classification helps healthcare providers assess the severity of cancer, determine prognosis, and guide treatment decisions.

In particular, the 'T' category indicates how large the tumor is and whether it has invaded nearby tissue. The 'N' category reflects whether the cancer has spread to nearby lymph nodes, which can be critical for assessing how far the disease has progressed. The 'M' category signifies whether the cancer has metastasized, which means it has spread to other parts of the body. This comprehensive approach enables oncologists to communicate effectively about the cancer stage and tailor treatment plans based on the current extent of the disease.

The other choices do not align with the TNM system's purpose. It does not involve classifying chemotherapy drugs, staging radiation therapy, or assessing the patient's response to treatment. Thus, the correct classification for the TNM staging system is focused on the characteristics of the tumor itself, as described.