Rare undifferentiated colonic adenocarcinoma: A diagnostic challenge

Pathological diagnosis of colorectal adenocarcinoma is typically straightforward. However, undifferentiated colorectal cancers can be rarely encountered in daily practice and definitive diagnosis can be very challenging. In this report, we presented a rare case of ascending colon ulcerated mass from a 60-year old man. The mass showed lobulated solid growth pattern, pushing border, no desmoplasia, no glandular formation, no tumor budding, undifferentiated morphology with pleomorphism and multinucleated giant cells, numerous atypical mitosis (>100/5 mm²) and unusual immunophenotype as follows: pancytokeratin+, CK7−, CK20−, CDX2 focally+, SATB2−, DOG1+, CD138+, mucicarmine+. Molecular test showed classic mutations in APC, TP53, SMAD4 and HNF1A, and wild type in BRAF, KRAS and NRAS. Interestingly, KIT, ERBB4 and PIK3CA showed missense mutation, and PDGFRA showed nonsense mutation, although all of them had uncertain clinical significance. Final diagnosis of undifferentiated adenocarcinoma was rendered after combining the growth pattern, immunostains and mucicarmine stain, molecular test findings, and excluding other poorly differentiated malignancies.