Collision tumor composed of primary breast invasive ductal carcinoma and metastatic ovarian high-grade serous carcinoma: a case report

Abstract According to widely-accepted Campbell criteria, TTM requires metastatic tumor deposits growing within the parenchyma of a bona-fide recipient neoplasm, with metastatic foci at least partially rimmed by recipient-tumor tissue. A collision tumor describes two independently-originated neoplasms that abut, interdigitate and intermingle at contact zones, without one tumor population seeding into the inner parenchymal compartments of the other neoplasm. In the present case, ovarian-origin high-grade serous carcinoma and breast invasive ductal carcinoma show extensive interfingering at collision interfaces; nevertheless, definitive nests of HGSOC embedded within IDC parenchyma were not identified morphologically. Therefore, this lesion best fits the definition of a collision tumor. Notably, partial conceptual overlap exists between these two entities: both conditions feature two unrelated malignancies co-localized in one site, and limited tissue sampling may blur morphological boundaries, posing diagnostic challenges for pathologists [1]. A structured literature search was performed across PubMed, Embase and Scopus, covering records from database inception to June 2026. Search terms included “breast collision tumor”, “invasive ductal carcinoma”, “ovarian high-grade serous carcinoma”, “metastasis to breast”, “collision tumour”, “tumor-to-tumor metastasis”, “case report”. Bibliographies of relevant retrieved articles were manually reviewed for additional relevant publications. To the authors’ knowledge, no previously documented case of a collision tumour formed by primary breast invasive ductal carcinoma and metastatic ovarian high-grade serous carcinoma has been described in the English-language literature. Diagnosis was established through immunohistochemical profiling (GATA-3 + breast component; WT-1+/PAX-8 + ovarian component) and next-generation sequencing revealing a somatic TP53 mutation. The patient received platinum-based chemotherapy, interval debulking surgery, and modified radical mastectomy. This case underscores the necessity of integrated pathological and genomic analysis for accurate diagnosis and individualized therapy in such rare scenarios.