内直筋に認められた胞巣状軟部肉腫

A case of alveolar soft part sarcoma of medial rectus muscle

34歳男性が1年前からの左眼球結膜の腫瘤を主訴として受診した。左眼内直筋に相当する結膜下に12×10mmの境界明瞭な赤色で無痛性の腫瘤があり，周囲の上強膜血管が怒張していた。眼球運動などに異常はなかった。磁気共鳴画像検査（MRI）で腫瘤は境界が不明瞭であり，Gd造影で均一に淡く造影された。腫瘤は内直筋部に被膜で包まれるように存在し，手術時には内直筋と茎で連絡しており，全摘出できた。病理組織学的には，胞巣状構造を呈し，腫瘍細胞は大型類円形ないし多角形で，核には明瞭な核小体があった。PAS染色で，桿状または顆粒状の結晶がみられた。RT-PCRとDNA配列解析で，ASPL-TFE3キメラ遺伝子がみられ，胞巣状軟部肉腫と診断した。本疾患が眼窩に生じた稀有な例であり，遠隔転移が危惧されるが，摘出から2年後の現在まで再発または転移の徴候はない。

A 34-year old male presented with a subconjunctival tumor in his left eye since one year before. His left eye showed a well-demarcated tumor at the site of medial rectus muscle. Its size was 12×10 mm. Ocular motility was intact. Magnetic resonance imaging（MRI）showed a poorly defined homogenous mass. It was enhanced by gadolinium. It was encapsulated and lay in apposition to the medial rectus muscle. Surgically removed tumor showed,histopathologically,an alveolar structure. Tumor cells were large globular or polygonal. Distinct nucleoli were present in the nucleus. PAS staining showed rod-shaped or granular crystals. Reverse transcriptase-polymerase chain reaction and DNA sequencing showed ASPL-TFT3 gene fusion resulting from chromosome translocation. These findings led to the diagnosis of alveolar soft part sarcoma which is known to rarely occur in the orbit. There has been no recurrence or metastasis during 2 years until present.