Which primary site should overlapping non-invasive tumors of the bladder and ureter be coded to?

For overlapping non-invasive tumors specifically affecting the bladder and ureter, the appropriate coding is to designate it to the bladder, which corresponds to code C678. The reasoning behind this is that the bladder is the primary site where the tumor originates or is primarily localized. When classifying tumors, it is essential to consider the primary site that presents the most significant and direct involvement by the tumor, especially in the case of overlapping lesions.

The designation of overlapping lesions, while a potential choice, is typically reserved for when it is unclear which anatomical structure is primarily affected due to complexity or indeterminate relationships between multiple sites. In this instance, since both sites (bladder and ureter) are involved but the bladder is affected more prominently in terms of coding conventions, it is prioritized.

Tumors that start in the renal pelvis are also important to consider; however, in the case of these specific tumors being non-invasive and overlapping between the bladder and ureter, coding to the bladder is the standardized method.

Thus, choosing the bladder as the primary site underscores the focus on the primary tumor's most direct location while respecting the coding guidelines for various overlapping tumor presentations.