Diagnostic limitations of early colorectal cancer and muscularis propria colorectal cancer by FDG-PET/CT

Objectives The objective of this study is to understand the diagnostic limitations of early colorectal cancer and muscularis propria colorectal cancer by FDG-PET/CT.

Methods We performed FDG-PET/CT for 37 lesions [intramucosal cancer (M type): 10 lesions, submucosal invasive cancer (SM type): 14 lesions, and muscularis propria cancer (MP type): 13 lesions] diagnosed as early colorectal cancer by endoscopic histopathological examination. We also investigated the diagnostic ability to detect the presence of comorbid colorectal adenoma.

Results 1. The sensitivity and positive predictive value were 95.7% and 97.8%, respectively, for diagnosis of existing early colorectal cancer. There were 4 false negative cases of two M type cancers (sigmoid colon, 20 mm, IIa + IIc (laterally spreading tumor, non-granular pseudo-depressed type: LST-NG-PD); sigmoid, 18 mm, IIa (laterally spreading tumor, non-granular type: LST-NG)), one SM type cancer (sigmoid colon, 8 mm, Is + IIc), and 1 MP type cancer (transverse colon, 30 mm, type 2 (LST-NG origin)). 2. The positive rates of adenoma larger than 5 mm (n=24), adenoma larger than 10 mm (n=13) and adenoma larger than 20 mm (n=3) were 16.7% (4/24), 30.8% (4/13) and 66.7% (2/3), respectively, for diagnosis of existing co-morbid colorectal adenoma. The PET positive rate increased as the size of colorectal adenoma increased.

Conclusions Among the early colorectal cancers diagnosed by FDG-PET/CT, the LST-NG type requires extra caution as it looks negative on FDG-PET/CT in spite of the large tumor size and the presence of deep invasion