S4525 A Rare Case of Colonic Involvement by Metastatic Endometrial Cancer: Diagnostic Challenges and Immunohistochemical Confirmation

Introduction: Colonic metastasis from endometrial carcinoma is extremely rare, with less than a dozen cases reported in the literature. Malignancies in the colon are generally known as primary tumors or metastasis from the lungs or breast, ovary, prostate, lung, and stomach. Here, we present a case of metastatic endometrial carcinoma presenting as a colonic mass. Case Description/Methods: A 73-year-old woman with a history of recurrent endometrial cancer status post hysterectomy, bilateral salpingo-oophorectomy, and pelvic sentinel lymph node dissection 5 years ago, on immunotherapy came for colonoscopy due to apple core lesion in the transverse colon found on the positron emission tomography (PET) scan. Her oncology history was complicated by peritoneal carcinomatosis, recurrent malignant ascites, recurrent malignant pleural effusion, biopsy-proven cancer recurrence in the vagina, and now with new lesions in the T2 vertebral body and colon. Per the PET computed tomography report, long segment apple-core lesion was noted in the transverse colon concerning for primary adenocarcinoma. Colonoscopy showed a near circumferential fungating and ulcerating mass in the transverse colon. Immunostaining the biopsy sample obtained from the colonic mass supported the diagnosis of a poorly differentiated adenocarcinoma compatible with Mullerian origin. A prior vaginal biopsy and fluid cytology showed adenocarcinoma Mullerian primary origin as well. Given the CK20 and CDX-2 negativity of the colon mass sample, the primary colorectal origin is unlikely. In turn, CK 7 and PAX8 positivity confirmed primary endometrial origin. After these findings, she was started on palliative radiation therapy for thoracic spine metastasis, resumed pembrolizumab, that was on hold due to pneumonitis, and added back Lenvatinib. Discussion: This case highlights a rare and atypical site of metastasis from endometrial carcinoma to the colon. The uncommon and atypical occurrence of colonic metastasis from uterine adenocarcinoma highlights the need to consider metastatic disease when assessing solitary colonic lesions, particularly in patients with a prior history of cancer and highlights the diagnostic value of immunohistochemistry in guiding effective treatment.