And before it was, what did people think was wrong when someone got a tumor or got sick and died of cancer? Once it was labeled, what did they think the cause was?
I wrote an article (forthcoming) on the history of breast cancer treatments. You would be surprised how far it goes back. Obviously, cancers that develop as tumors or growths have always been recognizable as "abnormal." Internal cancer would have usually gone undetected, at least until the person became symptomatic of whatever cancer they had, not unlike today. But most internal ailments were treated based on the symptoms, so if they showed pain, swelling, etc. those things would have been treated like any other pain or swelling.
To get to the evidence, the Edwin Smith Papyrus which dates to about 1600 BCE includes writings on tumors and abscesses of the breast that were considered incurable. Hippocrates also mentioned what is most likely breast cancer, saying a patient had discharge from her nipple. But because Hippocrates and many other ancients (and physicians throughout the medieval and Early Modern period) used the humoral theory, they did not necessarily consider it a disease, but rather an imbalance of the humors. Many believed black bile was to blame. But surgery of breast cancer and other tumors was not unheard of in antiquity, just rare because of the inherent risk of infection and/or death. Celsus listed 4 stages of breast cancer, but did not include any treatment options in his text. A Greco-Roman physician Leonides, however, was one of the first to document surgical removal of breast cancer. He claims that it was successful.
Essentially, people have always dealt with cancer, but there was little to be done about it. The idea of cancer as a disease really depends on the time and place you're talking about. Physicians have known about it and its symptoms for ages, but the pathology of cancer is really very modern.
Cancer was first identified as a disease 4,600 years ago by Imhotep. According to the book "the Emperor of All Maladies" by Siddhartha Mukherjee, there were no other records of cancer identification until 440 BC, when Greek historian Herodotus wrote of a lump in the breast of Atusa, the Queen of Persia. The tumor was removed by her slave which relieved symptoms temporarily.
Understand though, that their understanding of cancer was not very correct. They did not know anything about cells and such, so their identification of cancer was based on hydraulic metaphors as opposed to the rapidly dividing group of cells that we know them as today.