Colostomy‐site carcinoma with primitive phenotype in a rectal cancer patient after achieving pathological complete response with neoadjuvant chemoradiotherapy.

Herein, we report a rare case of a carcinoma with primitive phenotype (enteroblastic and/or hepatoid differentiation) occurring at a colostomy site. The patient was an elderly male who underwent neoadjuvant chemoradiotherapy for rectal cancer, followed by abdominoperineal resection. A biopsy specimen for the rectal carcinoma before neoadjuvant chemoradiotherapy was conventional tubular adenocarcinoma. Moreover, a pathological complete response was confirmed in the proctectomy specimen. However, a colostomy‐site tumor appeared 6 months after the proctectomy, and it was resected 1 year after the initial proctectomy. The colostomy‐site tumor comprised solid to focal glandular growth of atypical polygonal cells with clear to pale eosinophilic cytoplasm and was immunohistochemically positive for cytokeratin, spalt‐like transcription factor 4, glypican‐3, caudal type homeobox 2, and special AT‐rich sequence‐binding protein 2. Thus, the tumor was diagnosed as poorly differentiated adenocarcinoma with primitive phenotype, with suggested origin from the colorectal epithelium. Additionally, a multilocular cystic lesion comprising various types of epithelia was found adjacent to the tumor, suggestive of metaplasia or heterotopia. Changes in the histology and immunophenotype, and the findings of an adjacent cystic lesion suggest a metachronous tumor rather than a recurrence of the primary tumor. [ABSTRACT FROM AUTHOR]