Yes, benign could be used here also, according to Oxford dictionary, but I guess it's not very common, as it's very very formal.
For medical students, or the ones interested in epistemology:
See Celse here (free book)
Hippocrates (circa. 460 BC) described several kinds of cancer, referring to them by the term karkinos (carcinos), the Greek word for crab or crayfish, as well as carcinoma. This comes from the appearance of the cut surface of a solid malignant tumour, with "the veins stretched on all sides as the animal the crab has its feet, whence it derives its name". Since it was against Greek tradition to open the body, Hippocrates only described and made drawings of outwardly visible tumors on the skin, nose, and breasts. Treatment was based on the humor theory of [the] four bodily fluids (...). According to the patient's humor, treatment consisted of diet, blood-letting, and/or laxatives. Celsus (circa 25 BC ) translated karkinos into cancer, the Latin word for crab or crayfish.
In the 2nd century AD, the Greek physician Galen used oncos (Greek for swelling) to describe all tumours, reserving Hippocrates' term carcinos for malignant tumours. (...)It is from Galen's usage that we derive the modern word oncology [and the suffix "oma" (ome) to mean a tumor)
I've searched the word "benignus" and its derivated, in the Celse's text, but I've dound no occurence.
It doesn't mean they didn't make the difference between bening benign tumor and cancerous tumor (malign, malignant, opposite to benign), they did, but it was not called this way (need more search to know how they called it)
This cancerous meaning was already attested in old French for tumors. I think it's a meaning acquired in old French (and maybe other Romance languages when they became distinct dialects?), and that didn't exist in Latin. (unless I find it in a text). And it was borrowed in English.
If someone knows more than I do about the history of the word "benign" in his language or in Latin, concerning cancers, it would be really interesting to know.
It's pretty rude to tell someone they have a strict mother and then tel them you have a kind father xD