NTRK-positive collision tumor of the gastrointestinal tract: a rare entity case report.

Neurotrophic receptor tyrosine kinase (NTRK) fusion-positive colorectal cancer (CRC) represents a rare molecular subset of CRC. We report an exceptional case of a collision tumor composed of two anatomically adjacent but histologically and genomically distinct primary colorectal carcinomas, each giving rise to a corresponding metastasis. Comprehensive histopathologic and molecular analyses demonstrated that one primary tumor and its matched metastasis consisted predominantly (> 90%) of a solid carcinoma harboring a TPR::NTRK fusion, high microsatellite instability (MSI-H), elevated tumor mutational burden (TMB), and loss of MLH1 and PMS2 expression by immunohistochemistry. In contrast, the second primary tumor and its corresponding metastasis exhibited conventional adenocarcinoma morphology with mucinous differentiation, lacked an NTRK fusion, and carried canonical driver mutations in KRAS, APC, SMAD4, and TP53. This case underscores the importance of integrated histopathologic and molecular evaluation in CRCs with heterogeneous morphology, as the identification of multiple, genomically distinct tumor components may have significant diagnostic, prognostic, and therapeutic implications.