原発性小腸癌の病理学的および分子生物学的特徴

Pathological and Molecular Features of Primary Small Intestinal Cancer

要旨●原発性小腸癌は消化管癌の3%以下にすぎないが，さらに空・回腸癌はそのうち半数以下とされる極めてまれな腫瘍である．病理組織学的には腸上皮に類似する管状腺癌が多くを占めるが，粘液癌や髄様癌の発生もみられる．免疫組織化学的にサイトケラチンの発現などで非腫瘍小腸上皮と異なる特徴を示すため，原発・転移の鑑別における免疫組織化学染色の利用には注意が必要である．分子生物学的には大腸癌と共通する遺伝子変異が認められるものの，APC変異の頻度は低く，大腸癌とは異なる分子異常を背景とした腫瘍と考えられる．また，ミスマッチ修復異常が20%程度に認められることから，その検索を臨床的な必要性に応じて適切に行うことが望まれる．

　Primary small intestinal cancer constitutes less than 3% of gastrointestinal malignancy, and particularly, jejunal and ileal cancers are exceedingly rare, comprising less than half of all small intestinal cancers. Histologically, the majority of small intestinal cancers are intestinal-type tubular adenocarcinoma but other histological variants, including mucinous adenocarcinoma and medullary carcinoma also occur. Immunohistochemically, small intestinal adenocarcinoma often exhibits a cytokeratin expression pattern different from normal epithelium ; therefore, immunohistochemical examination to distinguish between primary and metastatic tumors requires careful interpretation. Although small intestinal cancer has many genetic alterations common to colorectal cancer, the prevalence of APC mutations is significantly lower, suggesting the distinct molecular background. Also, approximately 20% of small intestinal cancers show mismatch repair deficiency, which may have clinical relevance.