―最近注目されている膵の囊胞性腫瘍―膵のsolid and cystic acinar cell tumor (SCAT)―特にその組織発生についての考察

Solid and Cystic Acinar Cell Tumor of the Pancreas

要旨　膵の囊胞性腫瘍の中で，最近本邦で報告の相次いでいるsolid and cystic acinar cell tumor（SCAT）の2症例（35歳女性，28歳女性）を報告し，特にその組織発生について老察した．臨床的には，偶然に腹部腫瘤が発見され，腹部CTにて膵の囊胞性腫瘍と診断された．血液や尿の生化学的検査ではアミラーゼ，糖を含めて異常所見を認めていない．手術材料の肉眼所見では，本2症例は他の報告例と同様に，囊胞変性に伴う著明な壊死傾向を示す腫瘍であった．壊死物質に混じってコレステリン結晶が存在するのも特徴的である．腫瘍実質は周辺にのみ部分的に認められるにすぎない．また，被膜は厚く，広範な石灰化を示している．組織学的には，腫瘍細胞は充実性シート状に配列し，明らかな腺管形成は認められない．均一な明るい胞体を有する腫瘍細胞は異型が少なく核分裂像もほとんどみられない．特記すべきことは，腫瘍細胞の中で多数の粗大赤色顆粒を有するものがあり，これらは電顕的にもzymogen様顆粒である．更にまた酵素抗体法によってα-1-antitrypsin陽性細胞も多数認められる．これらのことから，本腫瘍の主たる分化（major differentiation）は膵acinar cellと判定される．したがって，多くの同義語の中で，SCATが最も適切であるものと考えた．

　A peculiar type of pancreatic tumor has recently been recognized as a clinicopathological entity and termed papillary and solid neoplasm, solid and papillary neoplasm, or solid and cystic acinar cell tumor of the pancreas. In this report, we present two cases of this pancreatic tumor, which were found in one young woman examined because of a large abdominal mass, and in the other during an incidental abdominal examination without detectable functional symptoms. Grossly, the tumors were well circumscribed by a firm, white, fibrous capsule, and their cut surfaces showed mainly cystic degenerative changes with necrotic and hemorrhagic material, with only some solid portions (Fig. 2, 3). The solid portions were composed of uniform cells forming solid and pseudopapillary structures with numerous PAS-positive granules (Fig. 4, 5). Immunohistochemical staining for alpha-1-antitrypsin showed a granular distribution (Fig. 6). Electron microscopic examination showed that the tumor cells had numerous mitochondria and zymogenlike granules (Fig. 7). These findings strongly suggested acinar cell differentiation. We regard this as the “major differentiation” in this tumor, although ducta^l and endocrine differentiation have also been observed by others. Therefore, this type of tumor can best be termed an acinar cell tumor. There was no recurrence in either patient after the operation. The prognosis has been favorable, but reports indicate that this neoplasm should be recognized as a potentially malignant lesion. It seems necessary to accumulate and analyze such cases in detail to clarify the histogenesis and biological behavior of these tumors because this type of tumor is a recently recognized disease which has been under observation for only a short term.