Value of F-18 FDG PET/CT in evaluation for restaging of locally advanced rectal cancer after neoadjuvant concurrent chemoradiation therapy: Correlation with pathology

Objectives The aim of this study was to assess the value of F-18 FDG PET/CT performed at restaging in patients with locally advanced rectal cancer who previously underwent neoadjuvant concurrent chemoradiation therapy (CCRT).

Methods Twenty-three patients with histologically proven rectal cancer who had CCRT and subsequent radical surgery were enrolled. Restaging F-18 FDG PET/CT was performed approximately 4-6 weeks after completion of CCRT. F-18 FDG PET/CT images were interpreted using visual analysis according to the intensity of F-18 FDG uptake in the rectal lesions. We checked pSUV of the lesions if there were positive findings in F-18 FDG PET/CT. Finally, the results of F-18 FDG PET/CT scan were correlated with those of the pathologic evaluation.

Results F-18 FDG PET/CT assessment was considered negative in 15 (65.2%) patients and positive in 8 (34.8%) patients. Of the 23 patients, 7 (30.4%) had a complete pathologic response (pCR) to CCRT. F-18 FDG PET/CT was negative in 4/7 patients with pCR and positive in 12/16 patients with any residual cancer in surgical specimen. Sensitivity, specificity, positive predictive value and negative predictive value were 75.0% (12/16 patients), 57.1% (4/7 patients), 80.0% (12/15 patients) and 50.0% (4/8 patients), respectively. Accuracy was 69.6% (16/23 patients). False negative cases (4/8, PET negative with residual cancer) showed relatively small size (<1.3cm) & depth (pT2) of tumors. False positive cases (3/15, PET positive without residual cancer) had chronic ulcerative inflammation in surgical specimens.

Conclusions Although restaging F-18 FDG PET/CT after CCRT in locally advanced rectal cancer has reasonably high sensitivity, positive predictive value and accuracy, there is some limitation to evaluate complete remission. We expect additional manipulation such as rectal gas distension at the time of PET/CT scanning could increase the detectability for small lesion