One particular feature that is evaluated is the encouraging presence, or discouraging absence, of a clear layer or plane of fat creating a barrier between the tumor and the vessels. Traditionally, an assessment is made of the tumor's proximity to major venous or arterial vessels, in terms of "abutment" (defined as the tumor touching no more than half a blood vessel's circumference without any fat to separate it), "encasement" (when the tumor encloses most of the vessel's circumference), or full vessel involvement. A resection that includes encased sections of blood vessels may be possible in some cases, particularly if preliminary neoadjuvant therapy is feasible, using chemotherapy and/or radiotherapy.