Depending on your alpha (straight or premult) that edge may or may not show in your picture viewer inside C4D, but may not show in your final comp.
Straight alphas have to generate stuff outside the matte (and that may be what you are seeing) so that the alpha can cut the image off clean and you don't get what amounts to a double matte line - one from the content edge and one from the alpha edge. This extra stuff won't show in the final comp as the alpha cuts it off.
What format of file are you rendering to? What depth?
What color is the object you are using as a catcher? Are you using the built in catcher? or the after market one that we used to use before C4D realized we needed one built in? (R17 I think).