In one form, the invention is directed to a method for forming a porous implant suitable for a cavity from which tissue has been removed, including mixing soluble alginate and a radiopaque imaging agent with water; incorporating a gas or a pore forming agent into the alginate-water mixture; transferring the alginate-water mixture with the gas or the pore forming agent into a mold to form the mixture into a solid body of desired shape; removing the water from the body; and converting at least part of the soluble alginate to a less soluble alginate. In another form, the invention includes forming a mixture by mixing about 0.5 percent to about 4 percent by weight chitosan into an acidified aqueous solution containing 1 percent to 25 percent by weight acetic acid, along with about 0.5 percent to about 5 percent by weight of a powdered radiopaque imaging agent.