277 Cutaneous Collision Tumor with Synchronous Collision Metastasis in Sentinel Lymph Node: A Rare Case

Introduction/Objective Collision tumors are defined as two or more independent neoplasms occurring at the same anatomical location without histological intermixing. While most reported cases involve combinations of benign and malignant tumors, or dual benign lesions, the co-occurrence of two malignant tumors such as melanoma and squamous cell carcinoma is exceedingly rare. Even more extraordinary is the presence of synchronous metastasis of both tumor components within a single sentinel lymph node. Here, we present a unique case of a cutaneous collision tumor composed of melanoma in situ, squamous cell carcinoma in situ (SCCIS), and seborrheic keratosis, with concurrent lymph node metastasis of both melanoma and SCC. Methods/Case Report A 78-year-old male with past medical history including Waldenstrom macroglobulinemia, recurrent papillary urothelial carcinoma, and hypertension presented with a newly diagnosed stage IB melanoma of the left upper back. Shave biopsy revealed superficial spreading melanoma with a Breslow thickness of 1.3 mm, Clark level IV, non-brisk tumor-infiltrating lymphocytes, and no ulceration or microsatellites. The patient subsequently underwent wide local excision and sentinel lymph node biopsy. Gross examination of the excised skin revealed a residual pigmented lesion. Histological examination showed a cutaneous collision tumor composed of melanoma in situ, SCCIS, and seborrheic keratosis. The melanoma component exhibited pagetoid spread and stained positively for Sox-10 and Melan-A, with a focally increased Ki-67 index. The SCCIS displayed full-thickness keratinocyte atypia and PanCK positivity, while the seborrheic keratosis showed basaloid proliferation with pseudohorn cysts. Sentinel lymph node biopsy from the left axilla revealed collision metastasis, with melanoma confined to subcapsular and intraparenchymal areas (positive for Sox-10 and Melan-A) and SCC showing extracapsular extension (positive for PanCK and Cam5.2). These findings confirm metastatic dissemination of both primary malignancies to a single lymph node. Results NA Conclusion This is the first known report of a primary skin collision tumor involving melanoma in situ, SCCIS, and seborrheic keratosis, with synchronous collision metastasis of melanoma and SCC in a sentinel lymph node. This rare entity underscores the importance of detailed histopathologic and immunophenotypic analysis in complex cutaneous lesions.