横行結腸に発生し早期癌として発見された髄様型低分化腺癌の1例

Early-stage Medullary Type Poorly Differentiated Adenocarcinoma of the Transverse Colon

要旨　横行結腸に発生し早期癌として発見された髄様型低分化腺癌の1例を報告する．患者は78歳の女性で，貧血の精査の結果，内視鏡的に横行結腸癌が発見され，生検で髄様型低分化腺癌と診断された．切除された腫瘍は病理組織学的に明瞭な核小体をもつ類円形核と豊富な好酸性胞体をもつ細胞から構成され，腺腔形成に乏しい充実型低分化腺癌であった．腫瘍内部にはリンパ球浸潤が目立ち，腫瘍の周囲の腸管壁にはCrohn's-like lymphoid reactionが高度にみられた．免疫組織化学的に，腫瘍細胞はMLH1陰性，MUC5AC陽性を示し，髄様型低分化腺癌と診断された．髄様型低分化腺癌は高齢女性の右側結腸に集積がみられ，比較的予後良好とされる．術前の生検の組織像でも診断可能であり，たとえ低分化腺癌と診断されたとしても，特異な臨床病理像を的確にとらえ，対応することが重要である．

　We presented an early-stage of medullary type poorly differentiated adenocarcinoma in a 78-year-old woman who suffered from anemia. She underwent right hemicolectomy. Grossly, the tumor was located in the transverse colon. Histopathologically, the tumor cells had oval nuclei with prominent nucleoli and abundant eosinophilic cytoplasm. The tumor showed solid-type poorly differentiated adenocarcinoma with prominent lymphocytic infiltration and Crohn's-like lymphoid reaction. Immunohistochemically, the tumor cells demonstrated lack of MLH1 expression and a positive MUC5AC reaction. Thus, the tumor was finally diagnosed as medullary type poorly differentiated adenocarcinoma which was equivalent to medullary carcinoma in the WHO classification. Although poorly differentiated adenocarcinoma was diagnosed pathologically, this type of tumor has a favorable prognosis. Clinicians and pathologists should recognize the characteristic features of this tumor in their daily practice.