FDG PET-CTにて原発巣を同定した転移性小脳腫瘍の1例

Detection of Unknown Primary Tumor in a Patient with Cerebellar Metastasis Using FDG PET-CT： Case Report

Ⅰ．は じ め に

　転移性脳腫瘍の治療方針を決定するうえで原発巣を同定することは非常に重要である^2,4)．しかし，全身検索を行っても原発巣が同定できない例が約16～35％存在し^2,9)，その場合には¹⁸F-flouorodeoxyglucose positron emission tomography（FDG PET）が非常に有用と報告されている^2,4,5,9)．しかしFDG PETは空間分解能が低く，また異常集積は糖代謝の亢進を示しているだけで悪性腫瘍そのものを描出しているわけではない^3,6,7)．すなわち，FDG PETでの異常集積の解釈にはCTやMRI等との対比が必要となる^1,3,6,7)．これらの問題を解決すべく登場したのがPET-CTであり，診断精度のさらなる向上が期待されている^3,6,7)．

　これまで原発巣不明の転移性脳腫瘍例に対しFDG PET-CTで原発巣を同定した報告は，われわれの渉猟し得る範囲ではみられない．今回，転移性小脳腫瘍の原発巣同定にFDG PET-CTが有用であった症例を経験したので報告する．

　Identification of unknown primary tumors in patients with brain metastasis is a continued diagnostic challenge. Several clinical reports have suggested that ¹⁸F-flouorodeoxyglucose positron emission tomography (FDG PET) is useful for detecting them. PET has incomparable abilities to determine the metabolic activity of tissues. But it needs the assistance of higher-resolution,anatomic information. CT is the easiest and highest-resolution tomographic modality to be integrated into PET imaging. Because of this,the market for PET devices has shifted so dramatically toward PET-CT.

　We report a case to show that FDG PET-CT was able to detect an unknown primary tumor. A 75-year-old female underwent resection of a left cerebellar tumor. The histological diagnosis was adenocarcinoma metastasis. Conventional systemic evaluation (chest radiography,chest and abdomen CT,abdominal sonography,and so on) did not show any pathologic image. FDG PET-CT was then carried out. A hypermetabolic focus was revealed in the left hilum.

　In conclusion,from now on,FDG PET-CT will be considered as the first diagnostic process for patients presenting brain metastasis with an unknown primary tumor.