Early-Onset Rectal Metastasis From Invasive Lobular Breast Carcinoma Mimicking Primary Rectal Cancer: A Case Report and Literature Review.

Gastrointestinal (GI) metastases from breast cancer are rare and may mimic primary GI malignancies, leading to significant diagnostic challenges. Invasive lobular carcinoma (ILC) has a distinct metastatic pattern with a higher propensity for spread to the GI tract and peritoneum. We report the case of a 59-year-old female patient with stage IV mixed invasive ductal and lobular breast carcinoma who underwent toilet mastectomy at initial presentation (Month 0), followed by systemic chemotherapy. Follow-up PET-CT performed approximately eight months after surgery revealed a new FDG-avid rectal lesion and diffuse peritoneal carcinomatosis. Colonoscopy performed in Month 9 demonstrated a nearly obstructing rectal mass mimicking primary rectal cancer. Histopathological and immunohistochemical analyses confirmed metastatic breast carcinoma with lobular features. Given the near-obstructing rectal lesion and diffuse peritoneal carcinomatosis, curative resection was not considered appropriate. A palliative diverting loop ileostomy was therefore performed in Month 11 to reduce the risk of impending distal large bowel obstruction and to relieve obstructive symptoms; however, the patient died in Month 13 during oncology follow-up. This case highlights the importance of considering metastatic breast carcinoma in the differential diagnosis of new rectal lesions in patients with a history of breast cancer and emphasizes the role of histopathological and immunohistochemical correlation in avoiding misdiagnosis.