An implantable device has a body that is substantially rigid and has an imageable shape. The body is further bioabsorbable and may contain permanent metallic elements to aid in its imaging. When the device is implanted in a resected cavity in soft tissue, it can cause the cavity to conform substantially to a known imageable shape. The implantable device is further imageable due to its attenuation properties being different from those of soft tissue such that the boundaries of the tissue corresponding to the predetermined shape can be determined.