>>could go back to PIE times

Indeed they do, although at Dic.com (see what a well-referenced fellow I am!) you have to look up the related word "canker" to find this out:

[Middle English, from Old English cancer, and from Old French cancre both from Latin cancer, crab, malignant disease. See kar- in Indo-European Roots.]

http://dictionary.reference.com/search?q=canker