大腸過形成ポリープ―病理診断の立場から

Colorectal Hyperplastic Polyp from the Pathological Point of View

要旨　大腸鋸歯状病変は現在のWHO分類（2010）では，（1）過形成ポリープ（hyperplastic polyp ; HP），（2）鋸歯状腺腫（traditional serrated adenoma ; TSA），（3）広基性鋸歯状腺腫/ポリープ（sessile serrated adenoma/polyp ; SSA/P）に分類され，さらにHPはMVHP（microvesicular HP），GCHP（goblet cell-rich HP），MPHP（mucin-poor HP）の3型に亜分類されている．HPは男性に多く，S状結腸・直腸に好発する5mm以下の扁平隆起であり，癌化の危険はないと考えられているが，MVHPは高率にBRAF遺伝子変異を伴いSSA/Pの前駆病変と考えられ，GCHPはK-ras遺伝子変異をしばしば伴うが腺腫や癌との関連は不明である．MPHPはまれでありMVHPや腺腫に再生性変化が加わったものと考えられている．なお，過形成/鋸歯状ポリポーシスは癌化の高危険群と認識しておく必要がある．

　In the current WHO classification（2010）, colorectal serrated polyps are classified into three types ; （1）HP（Hyperplastic polyp）, （2）TSA（Traditional serrated adenoma）, and （3）Sessile serrated adenoma/polyp（SSA/P）. In addition, HP is further subclassified into three types ; MVHP（microvesicular HP）, GCHP（goblet-cell rich HP）and mucin-poor HP of（MPHP）further.

　HP is seen more common with men, mostly arises in recto-sigmoid colon which is a flatly elevated small lesion（less than 5mm）. It has been thought that HP has no risk of malignant transformation, however, MVHP is suggested to be the precursor of SSA/P with frequent BRAF mutation. Although GCHP is often accompanied by K-ras, its clinicopathological significance is unknown. MPHP is rare and is thought to be due to regenerative change of MVHP or adenoma. In addition, it should be recognized that hyperplastic/serrated polyposis has a high-risk of cancer.