Collision Tumor of Metastatic Lung Adenocarcinoma to Lower Limb Leiomyosarcoma: Case Report and Literature Review

BackgroundCollision tumors, composed of 2 histologically distinct neoplasms at the same site, are rare. They encompass a variety of subtypes, where either lesion may be benign or malignant and where they may "collide" by direct invasion or metastasis. Carcinoma-sarcoma collisions are especially uncommon, and so-called tumor-to-tumor metastasis is even rarer. Tumor-to-tumor metastasis involving lung adenocarcinoma has only been reported within benign mesenchymal tumors; no such report to date has described metastasis into a malignant soft tissue sarcoma.Patient PresentationA 90-year-old man presented with a rapidly growing soft tissue mass on the anterolateral aspect of the right knee. Biopsy revealed high-grade leiomyosarcoma. Imaging also identified a dominant pulmonary nodule. The soft tissue mass was resected, revealing not only leiomyosarcoma but also a distinct glandular proliferation. Subsequent lung biopsy confirmed adenocarcinoma. Immunohistochemistry demonstrated that the glandular component within the sarcoma shared an identical profile with the lung tumor, consistent with metastatic lung adenocarcinoma into the primary sarcoma.DiscussionThis is the first reported example of lung adenocarcinoma metastasizing into a malignant soft-tissue tumor, representing a rare tumor-to-tumor metastasis resulting in a carcinoma-sarcoma collision tumor. A review of the literature identified 12 comparable reports, none involving both a malignant donor and a malignant mesenchymal recipient. This report underscores the importance of distinguishing such lesions from carcinosarcoma using histology and immunophenotyping.ConclusionThis unique example of carcinoma-to-sarcoma metastasis expands the differential diagnosis of biphasic soft-tissue tumors. Accurate classification is critical for guiding treatment, which may require tailored oncologic strategies addressing both tumor types.