Adenocarcinoma of the colon with syncytiotrophoblastic differentiation: differential diagnosis and implications

A case of adenocarcinoma of the colon is reported, whose metastases exhibited a syncytiotrophoblastic differentiation that gradually increased with advancing distance from the primary. Immunohistochemically, syncytiotrophoblastic giant cells with strong positivity for beta-HCG could be demonstrated within the metastatic tissue. Also, there was beta-HCG positivity in a fraction of the atypical epithelial elements of the primary. The case is discussed with respect to the differential diagnosis, and the cause of the syncytiotrophoblastic differentiation is attributed to the phenomenon of genetic instability. The case shows that by virtue of genetic instability totally new differentiations may be elaborated, so that the phenotypical appearance of a given tumor does not necessarily allow conclusions as to its tissue of origin. Thus, observations such as the one presented here clearly indicate that the histogenetic principle of tumor classifications by far does not apply to all cancers and that, therefore, its value and validity are considerably limited.