I consider it part of the thorax, since the "upper arm" area doesn't include the shoulder, which has it's own set of excision and biopsy codes. I figure if it doesn't include the top of the shoulder, then it wouldn't include the bottom, either. For that matter, since the shoulder area includes the head and neck of the humerus bone, and there's a code for soft tissue excision of a mass in the shoulder area, that one would probably be accurate as well. I'd still use the thorax, myself.