CT colonography―大腸癌スクリーニングへの応用を目指して

Development of CT Colonography as a Screening Tool for Colorectal Cancer

要旨　大腸のCT三次元表示はCT colonographyと総称され，欧米では大腸スクリーニングへの応用が盛んに研究されてきた．multi-slice CTの登場によるCT三次元画像の高精細化を背景として，既にCTを用いて大腸を診断することは世界的な概念となった．また欧米では，デジタルCT画像データを活用した大腸ポリープのコンピュータ支援検出やデジタル前処置法の研究が進められている．術者の技量に検査の質が大きく依存する大腸内視鏡検査と異なり，CT colonographyのデジタル診断画像には客観性・再現性があり，検査方法として標準化の可能性が高い．multi-slice CTの優れた検査処理能力を生かし，効率よく病変の拾い上げが可能な三次元表示法を開発することで，CT colonographyによる効果的な大腸スクリーニングの構築が期待される．

　With the development of multi-slice CT (MSCT), CT colonography (CTC) has gradually been admitted as an effective option for colorectal cancer screening not only in western countries but in Japan. CTC in preoperative staging of colorectal cancer is now an indispensable modality in our hospital. Using CTC, we can now accurately diagnose primary tumor and its local invasion, as well as small lymph node metastases. CTC is recognized as an effective option for evaluating primary lesions replacing colonoscopy and barium enema study. Virtual endoscopy, volume rendering image, MPR (multi-planar reconstruction), and their combination views are very useful to visualize colorectal lesions more clearly. CTC, which is based on 3D digital volume data, has great potential for computer-aided detection (CAD) of colorectal polyps, and for preparatory measures before treatment. Judging from our past studies, the current CTC-CAD system is not adequate for detection of colorectal flat lesions. However, further studies will surely develop new CAD algorisms for detection of flat lesions, and CTC with CAD can be expected to improve diagnostic performance, especially in colorectal mass screening. On the other hand, digital preparation has also developed, and now is available in CTC. The preparation method will reduce the burden of candidates, and these favorable features in CTC will lead to a standardized and effective colorectal screening method in the very near future.