Occult collision tumor of the gastroesophageal junction comprising adenocarcinomas with distinct molecular profiles.

Collision tumors, characterized by the coexistence of two unique neoplasms in close approximation, are rare and pose diagnostic challenges. This is particularly true when the unique neoplasms are of the same histologic type. Here we report such a case where comprehensive tumor profiling by next generation sequencing (NGS) as well as immunohistochemistry revealed two independent adenocarcinomas comprising what was initially diagnosed as a single adenocarcinoma of the gastroesophageal (GEJ) junction. Biopsy of the esophageal portion of the GEJ mass showed a mismatch repair deficient tumor with loss of immunoreactivity for MLH1 and PMS2, while the biopsy taken from the gastric portion of the mass revealed a separate tumor with a discordant, non-overlapping, set of molecular alterations, including an EML4::ALK fusion, as well as intact MMR. This case illustrates one way in which NGS can reveal diagnoses such as collision tumor that are wholly unexpected based on clinical and histological grounds. Such diagnoses can have important implications for patient care, particularly in cases where there is discordance for targetable molecular alterations.