S3855 Invasive Colon Adenocarcinoma Presenting With Retroperitoneal Abscess: A Rare Complication

Introduction: Invasive colonic adenocarcinoma presenting with intra-abdominal or retroperitoneal abscess is rare and often clinically misleading. Abscess formation occurs in only 0.3%–0.4% of colonic cancer cases, and fewer than 1% present with an abscess as the initial finding. Retroperitoneal abscess specifically is an even more uncommon manifestation, with limited case reports in the literature. These atypical presentations frequently lead to delayed diagnosis and treatment due to their nonspecific symptoms and overlap with infectious or inflammatory conditions. Early recognition is critical, particularly in patients with limited access to routine colorectal cancer screening. Case Description/Methods: A 56-year-old man with a history of hypertension, diabetes, and hyperlipidemia presented with progressive generalized weakness and multiple falls over the past year, along with lightheadedness and 2 weeks of intermittent right lower quadrant abdominal pain. He denied fever, gastrointestinal bleeding, or recent illness. On presentation, he was hemodynamically stable; physical examination revealed right lower quadrant tenderness with guarding. Laboratory studies showed profound anemia (hemoglobin 4.8 g/dL), marked leukocytosis (white blood cell count 45 × 10⁹/L), and thrombocytosis (platelets 968 × 10⁹/L). Computed tomography (CT) abdomen/pelvis demonstrated a large, complex pelvic mass with cystic components involving the colon and iliopsoas muscle. Intraoperative findings included a large retroperitoneal abscess and a cecal mass. He underwent urgent drainage of intra-abdominal and retroperitoneal abscesses, followed by right hemicolectomy with ileal anastomosis and bilateral ureteral stent placement. Pathology confirmed a moderately differentiated invasive adenocarcinoma with full-thickness colonic wall perforation. Discussion: This case highlights an unusual and severe initial presentation of colon cancer. Retroperitoneal abscess formation is rare and easily overlooked, especially in patients without classic cancer symptoms. A high index of suspicion, timely imaging, and multidisciplinary coordination are essential. This case also illustrates how social determinants, such as lack of insurance, delay necessary screening and diagnosis. Increased awareness, earlier imaging in atypical cases, and improved access to colorectal screening are critical. Further research is needed to define optimal diagnostic strategies and outcomes for malignancy-associated abscesses.