The etymology of the word in "cancer" in Old English suggests that tumours were recognised for a fairly long time, although it is unlikely they had a very precise identification between cancerous tumours and similar growths that might be lumped in with them. By the turn of the 17th century, it was given an increasingly modern medical sense and distinguished from the general term which had become "canker".

https://www.etymonline.com/word/cancer#etymonline_v_654

According to John. M. Riddle in "Ancient and Medieval Chemotherapy for Cancer" (1985) cancer was predominately believed to caused by excessive black bile in classical and medieval medicine. John Demaitre in "Medieval Notions of Cancer: Malignancy and Metaphor" (1998) catalogues some of the tangled history with definitions and understandings of cancer as well as the use of terminology. Suffice to say, it was not understood particularly clearly or with modern insights like the causal relationship with forms of radiation. Some descriptions of what cancer looks like are more literal and accurate than others. Fun fact; bear's meat is listed as a food that might cause black bile by medieval authors, no idea why (who even eats bear meat?).

Riddle and Demaitre also mention treatments for cancer, mostly focusing on the idea of rebalancing the humours. Spurge, referred to as elaterion was considered one of the strongest treatments by the 11th century physician Constantinus Africanus for its capacity to purge black bile which would dissolve tumours, also recommended by Galen for malignant tumours. Papaverine alkaloids, derived from the opium poppy, and tropane alkaloids, which come from henbane and deadly nightshade, were also used. Spurge is probably effective at fighting skin cancers, and at least some papaverine alkaloids may also have some utility in fighting other cancers. The squirting cucumber (Ecballium elaterium) and Narcissus were also used and may also have some genuine effectiveness (Riddle, 1985). Furthermore, surgery was also used, although as with drugs there were frequent concerns about the potential to exacerbate the pain or kill the patient (Demaitre, 1998 and Riddle).

For an individual case, I refer to that of Robert Cecil, 1st Earl of Salisbury in 1612, a little later than Medieval but there were largely similar conditions in medicine at the time. As well as scurvy, he had one or two large tumours in his chest during his final illness (there may also have been dropsy see "Europe's Physician: The Various Life of Theodore de Mayerne" by Hugh Trevor-Roper, 2006). Mayerne's first recommended treatment was a lifestyle one; getting certain dietary needs he didn't have enough of (more soft meat, vegetables and herbs, less fat, spices and milk), moderate exercise and taking the air for relief. He later proposed various medicines and he appears to have used Salisbury's case as an experimental subject; many of the doctors of this time relying heavily on trial and error with frequent theories and new medicines proposed.

Unfortunately, although the lifestyle suggestions appear to have eased the symptoms he could not prevent the earl's death, which would probably be difficult even for modern doctors due to the severity of his illness (perhaps limes and modern surgery could treat the scurvy and tumours?). For some reason, when looking at the high class of time who are more likely to live to an older age and die of natural illness, I often see them become overwhelmed by various symptoms and ailments in a rather horrifying way. That's probably an artefact of the limited medicine of the time; you can see this in modern medical cases where treatment is delayed or the wrong treatment is initially given, and symptoms and ailments snowball.

Hope this gives you a bit of a clearer picture.