Baltimore surgeon William Halsted pioneers a new approach to removing breast tumors, radical mastectomy, in which the entire breast and the surrounding lymph nodes and chest muscles are removed. This helps reduce recurrences of the disease, which was previously nearly always fatal. Halsted's work also leads to similar approaches for other cancers, in which both the tumor and surrounding tissue are removed. These techniques are still an important part of treatment for some cancers today. For breast cancer, however, surgeries have become far more conservative and effective, enabling many women to avoid mastectomy altogether.

In October 1846, a Boston dentist named William T.G. Morton provides the first public demonstration of ether as a general anesthetic, allowing surgeons to remove a tumor from a patient's jaw without pain. This advance gains immediate and widespread attention, ultimately eliminating the excruciating pain experienced by surgical patients until that time, and making the modern era of surgery possible.