Well, I am not only interested in etymology, but am also a practising radiation oncologist. There are loads of medical books explaining the etymology of "cancer". "Cancer" or the Greek "Canker" means "crab". Although one is not certain who coined this term, most books agree on Galen (which is sort of ironic, as he was a Roman physician and chose a Greek root; but then doctors have always loved esoteria... :-) ) Anyway, the term comes from the descriptive pathology of a usual malignant tumour (malignant is important, because you cannot/should not term benign lesions as cancer; and more importantly, or relevantly, the ancient ones recognised the entity of malignant & benign, and chose to name only the malignant one as "cancer"!) which spreads its claws in all directions, much like a crab, and has a tenacious tendency to hold on to the parent tissue even when resected, again like the creature.
Hope this helps.
Cheers.

Cogito ergo sum!