Adenoma-like Adenocarcinoma of the Colon: Case Report and Diagnostic Pitfalls of an Underrecognized Entity with Favorable Prognosis

Adenoma-like adenocarcinoma (ALAC) of the colon is a recently recognized histological subtype of colorectal adenocarcinoma, characterized by a villous architecture, low-grade cytologic atypia, and deceptive bland morphology despite its invasive potential, which can mimic non-invasive adenomas, leading to underdiagnosis in limited biopsy samples. Herein, we report the case of an 81-year-old male presenting with right-upper-quadrant pain that was found to have a hepatic abscess and a 4 cm villous lesion in the ascending colon. Histopathological examination of the right hemicolectomy specimen revealed a villous adenocarcinoma with invasion of the muscularis propria, consistent with adenoma-like adenocarcinoma. Isolated loss of PMS2 indicated a mismatch repair deficiency. However, adjuvant therapy was not indicated. The patient remained recurrence-free for three years, until he died from unrelated causes in the context of progressive frailty and comorbidities, with no evidence of cancer progression. This case highlights the diagnostic challenges posed by ALAC and underscores the importance of recognizing its distinct morphological features. Awareness of this entity is essential to avoid misclassification and ensure adequate treatment, especially given its typically favorable prognosis with low metastatic potential.