Purpose: Efficacy of F-18 fluorodeoxyglucose positron emission tomography combined with computed tomography (FDG-PET/CT) for determining neoadjuvant therapy response in rectal cancer is not well established. We sought to evaluate serial FDG-PET/CT for assessing tumor down-staging, percentage residual tumor, and complete response or microscopic disease with rectal cancer neoadjuvant therapy.
Methods: Patients with rectal cancer undergoing neoadjuvant therapy, definitive surgical resection, and FDG-PET/CT before and 4-6 weeks after neoadjuvant treatment were included. Tumors were evaluated pretreatment and on final pathology for size and stage. FDG-PET/CT parameters assessed were visual response score (VRS), standardized uptake value (SUV), PET-derived tumor volume (PETvol), CT-derived tumor volume (CTvol), and total lesion glycolysis (delta TLG).
Results: Twenty-one rectal cancer patients over 3 years underwent neoadjuvant treatment, serial FDG-PET/CT, and resection. Complete response or microscopic disease (n = 7, 33%) was associated with higher Delta CTvol (AUC = 0.82, p = 0.004) and Delta SUV (AUC = 0.79, p = 0.01). Tumor down-staging (n = 14, 67%) was associated with greater Delta PETvol (AUC = 0.82, p < 0.001) and Delta SUV (AUC = 0.82, p < 0.001). Pathologic lymph node disease (n = 7, 33%) correlated with Delta CTvol (AUC = 0.75, p = 0.03) and Delta PETvol (AUC = 0.70, p = 0.08).
Conclusion: FDG-PET/CT parameters were best for assessing tumor down-staging and percentage of residual tumor after neoadjuvant treatment of rectal cancer and can potentially assist in treatment planning.