EP817 - Rectal Myeloid Sarcoma - A Rare Cause of Large Bowel Obstruction and Perforation

Purpose/Background: Myeloid sarcoma (MS) is a rare extramedullary manifestation of acute myeloid leukemia (AML), occurring in only 2-8% of patients, and can present in various anatomic locations. Gastrointestinal involvement is present in only 7-15% of cases, and presentation with large bowel obstruction is exceedingly rare. This case aims to highlight the importance of considering leukemia - and specifically myeloid sarcoma - in the differential of rectal masses leading to large bowel obstruction.

Methods/Interventions: Our patient is a 65-year-old male who presented with one month of abdominal pain, diarrhea, hematochezia, and unintentional weight loss. On exam, the patient had mild tenderness of the lower abdomen and a palpable, fixed rectal mass. CT of the abdomen and pelvis demonstrated circumferential wall thickening of the distal sigmoid and rectum with thickening of the mesorectal fat. Flexible sigmoidoscopy demonstrated congested rectal mucosa with circumferential luminal narrowing five to ten centimeters from the dentate line, with nodular mucosal irregularity suspicious for malignancy but no dominant mass. Biopsies were obtained. The patient's hospital course was then complicated by peritonitis for which he was taken emergently to the operating room.

Results/Outcomes: Purulent fluid and succus were immediately encountered upon entering the patient's abdomen. A flexible sigmoidoscopy was performed, demonstrating a negative leak test while the sigmoid colon was clamped. The entire colon was inspected, and a perforation was identified in the mid-ascending colon, for which a right colectomy was performed and an end ileostomy created. Repeat biopsies of the hyperemic rectal mucosa were obtained. The patient's postoperative course was complicated by sepsis requiring admission to the intensive care unit and deep vein thrombosis necessitating therapeutic anticoagulation.

Pathologic evaluation of the right colon demonstrated serositis, vascular congestion and hemorrhage, with ischemic changes in the cecum attributed to increased intraluminal pressure. Pathology from the rectal mass biopsies revealed myeloid sarcoma with malignant cells strongly positive for CD117. Subsequent bone marrow biopsy confirmed a diagnosis of AML.

The patient was initiated on systemic chemotherapy with cytarabine, daunorubicin, and venetoclax, with initial restaging showing a favorable response. Lumbar puncture later confirmed CNS involvement, for which an intraventricular catheter was placed to allow for instillation of targeted intrathecal chemotherapy. The patient is currently undergoing restaging with bone marrow biopsy and PET scan to guide further treatment.

Conclusion/Discussion: Myeloid sarcoma is a form of extramedullary acute leukemia that should be considered in the differential diagnosis of unexplained gastrointestinal masses, especially in patients with current or prior hematologic malignancy. Given this tumor's poor prognosis, prompt diagnosis and treatment via a multidisciplinary approach is essential.

CT Scan Findings
Rectal wall thickening and presacral edema
Endoscopy Images
Erythematous and hyperemic rectal mucosa