"Cancerous" means that it spreads out of bounds and shits up unrelated threads and topics. Furries, for example, always used to show up in discussions they had nothing to do with, and turn it into "hurr fursecution." Bronies would bring up MLP no matter the topic. /pol/ can't shut the fuck up about Jews and blacks no matter the board or topic. That's what cancer is. Cancer can also be shit like, say, Naruto, which attracts underageb& from Gaia or wherever, since it's primarily read by 12-year olds (according to SJ surveys), and 18+ retards who still read it, and then figure they're welcome on /a/ and spread their retardation, thus lowering the quality of discourse.