Collision Tumors Involving Metastatic Carcinoma and Plasma Cell Myeloma: Report of Two Cases.

Synchronous coexistence of plasma cell neoplasms (PCNs) and metastatic carcinoma is not a common phenomenon and poses a significant diagnostic challenge requiring meticulous histopathological and immunophenotypic analysis for accurate lineage assignment. We report two cases of collision tumors comprising PCN and metastatic lung carcinoma. Case 1 involved a 68-year-old male with metastatic non-small cell lung adenocarcinoma who developed intracranial lesions; resection revealed metastatic adenocarcinoma intermingled with clonal plasma cells. Case 2 was a 73-year-old woman with a lytic iliac crest lesion whose bone marrow and lesion biopsies showed diffuse infiltration by both carcinoma and atypical plasma cells. Comprehensive immunohistochemical profiling, in situ hybridization (ISH), and molecular studies were performed. In both cases, histology demonstrated two distinct, intermingled cell populations without transitional features. IHC was critical for confirmation: the carcinoma cells expressed epithelial markers (AE1/AE3), whereas the plasma cell components were positive for CD138. ISH in Case 1 and flow cytometry in Case 2 confirmed clonality (kappa light-chain restriction). The final diagnoses were collision tumors of metastatic non-small cell lung adenocarcinoma with synchronous PCN (Case 1) and metastatic small cell carcinoma with synchronous PCN (Case 2). There are sporadic reports of PCN coexistent in the same lesion with solid tumors. These cases reveal the critical role of the pathologist in diagnosing collision tumors. IHC panel, including cytokeratins, CD138, and MUM1, can help in distinguishing these dual populations and avoiding diagnostic pitfalls.