大腸鋸歯状腺癌とは何か

What is Colorectal Serrated Adenocarcinoma?

要旨●大腸の鋸歯状腺癌（SAC）は，WHO分類第4版から記載され，第6版（2026年）にも記載があるものの診断基準が不十分であり，本邦の「大腸癌取扱い規約 第9版」には未掲載のため認知度が低い．過去の報告では頻度や遺伝子異常の結果にばらつきがあり，独立した組織型として分類する意義については疑問がある．Mäkinenによる組織学的診断基準で診断・抽出されたSACの検討では，以下のように報告されている．SACは全大腸癌の5.8〜14.5%を占め，60歳前半〜70歳代に多く，近位大腸に発生する傾向がある．予後は通常の大腸癌と同等とするものもあるが，不良とする報告が多い．遺伝子変異に関してはKRAS変異（約40%）がBRAF変異（4.3〜33%）よりも有意に多くみられている．しかしながら，報告によって頻度に大きな幅（5.8〜14.5% vs 筆者らの施設では1.2%）があり，頻度が低い筆者らの施設のSACでは近位大腸発生がより多く，KRAS変異よりBRAF変異が有意である．これらの違いは診断基準の運用に課題があると思われる．筆者らが示した組織学的特徴に加え，MUC5AC陽性であることも診断基準に採用することで，SACは臨床病理学的特徴を有する組織型として独立させる意義があると思われる．

　The diagnostic criteria for serrated adenomas of the colon（SACs）are insufficient, although they are described in the 4th, 5th, and 6th editions of the World Health Organization Classifications. Furthermore, SACs are not listed in the Japanese Colon Cancer guidelines, hindering their recognition. Studies have reported variations in their frequency and genetic features, raising doubts about the significance of their classification as an independent histologic type. A study of patients with SACs diagnosed according to Mäkinen's histologic diagnostic criteria reported the following：SACs, accounted for approximately 5.8-14.5% of all colorectal cancers, were most common in people in their early 60s and 70s and tended to occur in the proximal colon. Although some reports consider SAC prognosis comparable to that of typical colorectal cancer, others suggest that SAC is associated with prognosis. Regarding genetic mutations, KRAS mutations are significantly more common than BRAF mutations（approximately 40% vs. 4.3-33%）. However, there is a wide range in frequency between studies（5.8-14.5% vs. 1.2% at our institution）and in the less common SAC at our institution ; the proximal colon is more frequently involved, and BRAF mutations are more common than KRAS mutations. These differences are considered to pose challenges in the application of diagnostic criteria. In addition to the histological characteristics described in this study, adopting MUC5AC positivity as a diagnostic criterion would significantly contribute to establishing SAC as an independent histologic type with distinct clinicopathologic characteristics.