能登原 憲司

Kenji NOTOHARA

倉敷中央病院病理診断科

Department of Anatomic Pathology, Kurashiki Central Hospital

キーワード : autoimmune pancreatitis, lymphoplasmacytic sclerosing pancreatitis, idiopathic duct-centric pancreatitis, IgG4-related sclerosing cholangitis, pathology

自己免疫性膵炎（autoimmune pancreatitis：AIP）は，膵癌に類似した腫瘤形成性炎症性病変を特徴とする．1型AIPはIgG4関連疾患の膵病変で，画像診断でも確認できる特異な組織像を示す．膵臓を囲む帯状の肥厚性病変である被膜様構造（capsule-like rim）は，リンパ球，形質細胞の浸潤と線維化からなり，しばしば花筵状線維化（storiform fibrosis）が確認される．膵管，静脈（閉塞性静脈炎），動脈（動脈周囲炎），神経など，さまざまな微小構造に実質障害を伴わない肥厚性病変が観察される．膵小葉はその輪郭を保つが，腺房細胞は減少し，多数の炎症細胞に置き換わる．これらの特徴は，画像上で1型AIPの診断に至り，膵癌との鑑別を可能にする手がかりとなる．一方，2型AIPは，膵管や小葉に生じる上皮中心性の炎症である．上皮や管腔への好中球浸潤（granulocytic epithelial lesion）が特徴的な所見である．炎症による小葉の腫大が膵腫大の原因である．IgG4 関連硬化性胆管炎は，組織学的に1型AIPにおける膵管の肥厚性病変に類似し，炎症による壁肥厚と管腔狭窄が特徴である．上皮は正常であり，この点が胆管癌や，上皮をターゲットとする炎症である原発性硬化性胆管炎との違いになる．1型AIPとIgG4関連硬化性胆管炎の組織所見は特異的ではあるが，これらの疾患の生検診断には限界があり，臨床医はこれを認識する必要がある．

Autoimmune pancreatitis (AIP) is characterized by a tumefactive inflammatory lesion resembling pancreatic carcinoma. Type 1 AIP is a pancreatic manifestation of IgG4-related disease characterized by unique histological features that can be identified on imaging. The capsule-like rim, which is a collar of hypertrophic lesion surrounding the pancreas, consists of lymphoplasmacytic infiltration and fibrosis, and storiform fibrosis is often identified. Hypertrophic lesions of various microscopic architectures such as the ducts, veins (obliterative phlebitis), arteries (periarteritis), and nerves are observed without parenchymal damage. The pancreatic lobules keep their contours, but the acinar cells are diminished and replaced by numerous inflammatory cells. These features provide clues to arrive at a diagnosis of type 1 AIP and to distinguish it from pancreatic carcinoma on imaging. In contrast, type 2 AIP is an epithelium-centered inflammation involving the ducts and lobules. Neutrophilic infiltration in the epithelium and/or lumens (granulocytic epithelial lesion) is a characteristic finding. Lobular swelling due to inflammation is the cause of pancreatic enlargement. IgG4-related sclerosing cholangitis is histologically similar to the hypertrophic ductal lesion in type 1 AIP and characterized by wall thickening due to inflammation and luminal stenosis. The epithelium is intact, which is different from bile duct carcinomas and primary sclerosing cholangitis, the latter of which is characterized by inflammation targeting the epithelium. Although the histological features of type 1 AIP and IgG4-related sclerosing cholangitis are unique, the biopsy diagnosis of these diseases has limitations, which should be recognized by clinicians.