眼窩蔓状神経鞘腫のCT・MRI画像所見

A case report of intraorbital plexiform schwannoma：computed tomography and magnetic resonance imaging findings

要約　目的：眼窩の蔓状神経鞘腫の画像所見の報告。

症例：44歳，男性。2年前からの左眼眼球突出と右方視時の複視のため受診された。上方視での上転障害も認めた。視力は左右眼とも1.5であった。CTで左眼の筋円錘内に紡錘形の腫瘤があり，左眼球を外下方に圧排していた。MRIのT2強調像は，中心やや低，辺縁高信号を示した。Dynamic造影MRIで，中心部のAntoni Aの部位は早期から漸増性に造影され，辺縁のAntoni Bの部位は後期相で淡く造影された。眼窩内の神経鞘腫を疑い，外科的に切除され病理学的に神経鞘腫の所見が得られた。

結論：Antoni AとB型が混合した眼窩蔓状神経鞘腫（plexiform schwannoma）はきわめて頻度が低く希少な疾患であるが，本症例はCTとMRIの画像で典型的なtarget patternを示した。蔓状に連続する小結節の存在と，MRIでの漸増性dynamic造影効果と拡散強調像（ADCマップ）が術前診断に有用な所見であった。

Abstract　Purpose: To report imaging findings in a case of intraorbital schwannoma on CT and MRI.

Case: A 44-year-old male presented with exophthalmos in the right eye and diplopia when looking towards the right since 2 years before.

Findings: Corrected visual acuity was 1.5 in either eye. CT showed a spindle-shaped tumor in the muscle cone in the left orbit. The eyeball was displaced in the temporal lower direction. MRI showed low signal in the center of the tumor and high signal in the cortex by T2-weighted imaging. Dynamic MRI showed gradual enhancement in the Antoni A area and late weak enhancement in Antoni B area. On diffusion weighted image（DWI）, almost mass except central portion showed low signal intensity. These findings were compatible with peripheal nerve plexiform schwannoma. The surgically excised specimen was well correlated with the CT/MRI imaging findings on the pathological examination.

Conclusion: The plexiform schwannoma in the orbital muscle cone manifesting mixed Antoni A and B pattern has been regarded as extremely rare. The present case showed typical dynamic enhanced pattern on CT and MRI. Clinical diagnosis was also facilitated by the findings of almost low intensity on diffusion-weighted MRI.