Rule B. If a topographic site is modified by a prefix such as peri-, para-, or the like, and is not specifically listed in ICD-O-3, code to the appropriate ill-defined subcategory C76 (ill-defined site), unless the type of tumor indicates origin from a particular tissue.

The prefixes peri-, para-, pre-, supra-, and others are often used with topographic sites and various organs of the body. A few topographic sites modified by such prefixes are listed in ICD-O and given specific code numbers. For example, "periadrenal tissue," "peripancreatic tissue," and "retrocecal tissue" are listed and given the code number C48.0 which is "retroperitoneum." "Para-aortic lymph node" is listed in ICD-O and given the same code number, C77.2, as "aortic lymph node." It is not possible to list all topographic sites that might be modified by these prefixes in ICD-O. In practice, use of such prefixes indicates that the topographic site is ill-defined. Coders should use the C76 rubric for other ill-defined sites not listed in ICD-O. This same rule applies to other imprecise designations such as "in the area of" or "in the region of" a specific topographic site.