Aggressive appendiceal malignancies mimicking primary colonic tumors: two cases with extramural invasion and atypical clinical presentation.

Appendiceal malignant neoplasms typically present as acute appendicitis. However, certain histological subtypes exhibit highly aggressive biological behavior and may manifest with atypical clinical features, posing significant diagnostic challenges. We report two cases of aggressive appendiceal malignancies characterized by extramural invasion into the distal colon and misleading preoperative presentations. Case 1 involved a 74-year-old woman presenting with progressive weight loss and a palpable right lower quadrant mass without appendicitis-related symptoms. Imaging demonstrated extensive wall thickening from the distal ileum to the ascending colon with surrounding soft-tissue lesions, initially suggesting a primary ileocecal malignancy. Surgical pathology revealed a poorly differentiated appendiceal adenocarcinoma with extensive lymph node metastases. Case 2 was a 76-year-old woman diagnosed with appendiceal large cell neuroendocrine carcinoma, presenting with polyarthralgia, elevated inflammatory markers, and weight loss. Preoperative findings suggested primary cecal cancer with paraneoplastic manifestations, whereas postoperative pathology confirmed an appendiceal origin with extramural colonic invasion and no nodal involvement. These cases illustrate that aggressive appendiceal malignancies may lack typical appendicitis symptoms and instead present with infiltrative growth or systemic inflammatory features. Extramural invasion can obscure the primary site and disease extent, underscoring the importance of considering advanced appendiceal neoplasms in the differential diagnosis of atypical ileocecal tumors.