I'm not aware of any other comics being interpreted as a reference to cancer, but there are a handful of comics directly referencing his wife's cancer. If it is a recurring theme of his, I'd say it's perfectly legitimate for people to consider it a candidate for an otherwise unexplained plot line.
The discussion on the xkcd forums frequently attributed every single comic to be a cancer reference. The forums have since been removed from the main page though. so it's not quite as visible anymore.
