California Cancer Reporting System Standards Volume I: Abstracting and Coding Procedures for Hospitals
Assign codes from the following categories only when the primary site cannot be identified exactly:
Use NOS (not otherwise specified) subcategory when a subsite or tissue of an organ is not specifically listed in ICD-O-3. Do not use NOS if a more descriptive term is available.
Use these codes for diagnoses referring to regions and ill defined sites of the body, such as "head", "thorax", "abdomen", "pelvis", "upper limb," and "lower limb". These sites typically contain several types of tissue (e.g., bone, skin, soft tissue), which might not be specified on the diagnostic statement. If the tissue in which the tumor originated can be identified, use a more specific site code.
Use this code when the primary site is not known and the only information available is the metastatic, or secondary site.