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Malignant Neoplasm Overlapping Site Boundaries

Rule C. Use subcategory ".8" when a tumor overlaps the boundaries of two or more categories or subcategories and its point of origin cannot be determined.

Categories C00-C76 classify primary malignant neoplasms according to their organ or tissue of origin. Many three-character rubrics are further divided into named parts or subcategories of the organ in question. A single neoplasm that overlaps two or more contiguous sites within a three-character category and whose point of origin cannot be determined should be coded to the subcategory .8, "overlapping lesion," unless the combination is specifically indexed elsewhere. "Overlapping" implies that the sites involved are contiguous (next to each other).

While numerically consecutive subcategories are frequently anatomically contiguous, this is not invariably so (for example bladder, C67). The coder may wish to consult anatomical texts to determine the topographic relationships. For example, "carcinoma of esophagus and stomach" is specifically indexed to C16.0 (cardia), while "carcinoma of the tip and ventral surface of the tongue" should be assigned to C02.8. On the other hand, "carcinoma of the tip of the tongue extending to involve the ventral surface" should be coded to C02.1, as the point of origin, the tip, is known.

Sometimes a neoplasm may involve two or more sites represented by two or more three-character categories within certain systems. Figure 17 lists the subcategories that overlap sites in body systems. For example, "carcinoma of the stomach and small intestine" should be assigned to C26.8, overlapping lesion of digestive system.