180 Rare Intracranial Collision Tumor: Coexisting Metastatic Lung Adenocarcinoma and Clonal Plasmacytic Neoplasm

Introduction/Objective While metastatic carcinomas to the brain are common, collision tumors in the central nervous system are rare. The synchronous occurrence of two distinct metastatic neoplasms within the brain parenchyma has not been previously reported. This case highlights the diagnostic challenges and clinical implications of identifying dual pathology within a single lesion. Methods/Case Report A 68-year-old man with a history of metastatic lung adenocarcinoma presented with acute neurological symptoms. Craniotomy for a left frontoparietal mass revealed two distinct components: (1) metastatic carcinoma expressing CK7, TTF-1, and Napsin A, consistent with a pulmonary origin; and (2) an abundant population of CD138+, CD79a+, Ig kappa-restricted plasmacytic cells, supporting a concurrent metastatic plasma cell neoplasm. PD-L1 testing showed high expression in the carcinoma. Additionally, imaging revealed mixed lytic and blastic lesions throughout the skeleton. Results NA Conclusion Although it remains unclear how two distinct tumors metastasized to the same focus within the brain, this unique case illustrates the importance of comprehensive histopathologic and immunophenotypic evaluation in brain metastases. Recognition of concurrent neoplasms can reveal previously undiagnosed hematologic malignancies and carries significant therapeutic implications, particularly in the era of targeted and immune-based therapies.