Peritoneal carcinoma metastasis to uterine leiomyoma: A case highlighting the role of the pseudocapsule in tumor-to-tumor metastasis

Introduction: Tumor-to-tumor metastasis represents an uncommon pattern of malignant spread, in which a distinct neoplasm disseminates into another tumor. Instances involving uterine leiomyomas have been rarely documented. Case Report: A 66-year-old nulliparous woman was investigated for a pelvic mass. Magnetic resonance imaging demonstrated a uterine leiomyoma with suspected peritoneal dissemination, while computed tomography revealed nodular lesions in the omentum, right paracolic gutter, and subdiaphragmatic peritoneum, without evidence of extraperitoneal disease. Serum CA-125 levels were mildly elevated. Laparoscopic exploration confirmed peritoneal nodules and disclosed a hemorrhagic uterine mass that was excised. Histopathological examination showed a leiomyoma encompassed by a pseudocapsule containing foci of high-grade serous carcinoma. Immunohistochemical staining demonstrated WT1 positivity, aberrant p53 expression, and strong estrogen receptor reactivity in both uterine and peritoneal components, supporting metastatic high-grade serous carcinoma. Conclusion: This rare case highlights the potential for peritoneal carcinoma to metastasize to a leiomyoma. These findings suggest that the pseudocapsule and its vasculature may provide a favorable microenvironment for metastatic implantation, supporting Paget’s “seed and soil” hypothesis.