An implantable device has a body that is substantially rigid and has a predetermined shape. The body is further bioabsorbable and has a density less than or equal to about 1.03 g/cc. When the device is implanted in a resected cavity in soft tissue, it causes the cavity to conform to the predetermined shape. The implantable device is further imageable due to its density being less than that of soft tissue such that the boundaries of the tissue corresponding to the predetermined shape can be determined.